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Electrical Hints, Problems, Fixes.
Descriptions of functions, etc.

© Copyright 2014, R. Fleischer

I HIGHLY recommend you FIRST read article 14A.....which begins with basic electricity... lots of HINTS on problem areas in that article!

It will be to your advantage to read this:
ONE of the reasons to read it is because I removed all that multimeter information from the below article!  It contains a lot of information on multimeters (often called VOM's, meaning volt-ohm-milliameter) & test lights.  REALLY worth your time to read & absorb!  If you have always been confused by electrical things, you will have a pretty good working understanding after reading my electrical articles.  But, do the Basics reading first, so you understand electricity.  There are a number of electrical articles on this website,,,,all in one section of the Technical Articles List.

I do not expect you to understand (nor, hardly remember) everything that is in the article, below.  If you read this entire article, slowly, and think about what you are reading, and remember some details, that is probably the best I can hope for.  You can always use the SEARCH function if you need to find something.

I recommend purchase of BOTH a test lamp and a small in-expensive digital multi-meter (aka VOM) that will fit in the tool tray on the bike.  A simple test lamp can be very helpful in tracing down problems.   A multi-meter is NOT a necessity for an on-bike tool. If you decide to not have one on the bike, then get a larger size one to have at home.  If you purchase a test lamp get one with a steel pointy tip, NOT a battery type, & it should be the type that has a longish stranded insulated wire with a decent-sized alligator clip (if not at least 4 feet long, lengthen it).  

***I also recommend that you SAVE your next headlamp bulb (car or motorcycle) that has failed on either high or low beam (where the other beam is OK).    You can use that as a high power lamp to trace short circuits, that are blowing fuses and for other testing....see later in this article.  This is a powerful method!  The headlamp bulb draws a lot more current...(but a SAFE amount)...than the tiny lamp in the 'test lamp with sharp tip'; and thus is quite useful as both an indicator AND for use as a safety fuse that does not burn-out, if testing for short circuits.  Make up such a power-lamp to have a socket with wires attached, although you could use soldered wires onto the lamp.

A test lamp with sharp prod tip, a multimeter, and a power test lamp, will cover 99% of electrical problem analysis!!

I recommend you make up a VR jumper wire; use insulated stranded conductor wire; attach 1 standard 1/4th inch male push-on connectors at each end.   Make this jumper about 4 to 6 inches long. Put it in your ON-BIKE tool kit. Its purpose is to allow bypassing of the Voltage Regulator; for testing, or, if that VR fails. 

Bi-yearly maintenance of your electrical system is a very good idea.   You should disconnect electrical plugs, clean contacts.  Use a contact cleaner, perhaps after mild abrasive techniques, perhaps use a clear silicone grease or spray Caig product to protect against atmospheric effects like corrosion.   There are premium contact oxidation prevention products on the market, the best that are easiest to find are probably made by Caig Laboratories.   Caig's products are highly recommended by me; particularly for sensitive areas, such as K bike computer pins; but they are excellent for any electrical connection that might corrode from atmospheric effects. There is another premium product available.  See my chemicals, etc., article:
Once contacts are clean/shiny, use of common dielectric grease from an autoparts store will protect against atmospheric effects for years. I sometimes use the Caig's protectant & the grease; or, Caig's final special coating products.   I suggest a full electricals check every year or two. Include inspecting the diode board factory solder points, alternator connections for overheating & tightness, brush lengths, 1981+ models have heat sink paste/grease to change on the ignition module (except last versions that use riveted assemblies), etc.  Pay particular attention to the male prongs and mating female connections of the STARTER RELAY. Three hours of time well spent, every other year.

The BMW factory used a contacts/connections protective liquid before shipping the bikes.  The substance used was CRC 5-56. I have no problem if you want to coat connections (AFTER cleaning them to shiny), with that product.

Measurements of voltage can be taken on your Airhead with various items turned on or off, such as the headlight.  Except for the heated grips circuits, voltage drops of over 1/4 volt in any wire or switch or across contacts will indicate that there is a problem.    When measuring voltage drops, for most types of electrical items and circuits, you will often measure directly across the component.  At other times you may want to measure the voltage drops starting at the battery and moving down the electrical system, part by part.   This is best done, and you will get a more accurate reading, by connecting the multi-meter + (positive) lead to the battery + terminal, & using the multi-meter - (negative) lead as the test prod.   

Be sure that your Airhead's SOLID brown wires...grounds, earth, chassis, battery -, are secure, everyplace.   There are places that are known problem areas, such as the front coil mount & grounding wire(s) on the R65 early models....due to cracked mounts.   Many airheads have a master grounding area, located on the left side of the frame, under the fuel tank.  Check the inside of the headlight shell too.

In some repair manuals, it is wrongly said, or wrongly implied, that you can use a voltmeter connected in a series connection, to measure such as current drains.  This is absolutely NOT TRUE.  You use a ammeter, or milliamp-meter function, for current flow.  


1.  VERY FEW OF YOU WILL EVER REPLACE INDIVIDUAL DIODES ON THE STOCK BOARDS. However, the information in this section covers lots more than that.

The large power diodes (which are pressed-in-place) used in the diode boards are equivalent to Motorola 1N3659 (cathode to case) & 1N3659R (anode to case).   You will likely find other manufacturer's numbers that will fit with equivalent specifications. You are UNLIKELY to see those Motorola part numbers on the original parts.  Tracing down part number printed on the diodes can be frustrating, but a good electronics technician MAY have ideas.  Best to just look up 1N3659 and/or 1N3659R, and find equivalents.  Three of each type of these power diodes are used.  No matter whose make of diodes are used, the top row are the same part number & the bottom row are the other part number; the difference is that the diode polarities are reversed from each other.  They are rated at or above 50 volts, and rated at 30 amperes at 100° C.    Some alternator repair shops may stock similar diodes.  Generally any diode rated at 30 amperes or more, that will fit, will work just fine.  AGAIN!...MIND THE POLARITY...all 6 are NOT the same!    You may use higher voltage-rated diodes such as the 1N3660 and 1N3660R (100 volts) or 1N3661 & 1N3661R (200 volts); or, as mentioned, most alternator repair shops have diodes that will work OK.  Be careful pressing them in and out.

The solid wire from these large power diodes SHOULD BE FOLDED OVER & soldered along a length of that FOLD, onto the printed circuit board. 
If need be, scrape away (or use paint remover) the coating over the copper on the board, to gain an adequate soldering area, before soldering.  This folding & soldering was especially critical on the hot running RS/RT faired models. Original Wehrle manufactured boards failed due to a problem of not being folded over.   It is better to use a 50-50 solder, adding rosin flux, rather than 60-40 electronics solder. 50-50 melts at a considerably higher temperature & holds up better in THIS application. Plumbers wire solder is usually 50-50.  Be sure to use rosin flux (plumbers solder usually has no flux core).  If your large diodes on the diode board are showing signs of heat distress at the solder joint of that diode wire to the PC board then they need re-soldering (even if already folded).   IF the wire is NOT folded over, you CAN use 50-50 solder which has worked for me; but, the BEST fix is to drill a tiny hole through the copper printing material, and add a piece of wire, folding it over & soldering it to the PC, & also under the board, have it tightly wrapped & then soldered on the diode lead (clean well first or it will not solder correctly).   That job, for all 6 diodes, is QUITE a bit of a problem for many of you, trying to get the soldering iron underneath the printed circuit board.  I have gotten decent results by cleaning the soldering area pad (to shiny copper-look); using 50-50 solder, on the 'bad' boards that come without folded-over leads.  The reason to fold over the leads is to avoid concentrating the heat from the diode into a tiny hole, covered by solder.

If you are going to test the diode board, a visual inspection, especially the solder joints, is the first thing to do.   Then
use an ohmmeter (or diode test meter function) that produces enough voltage/current to turn on the diodes in the forward direction, & thus check the forward diode function.  Reverse the leads & also check the reverse diode resistance.   The forward resistance or diode tester readout (which may be in voltage) is very low compared to the resistance in the reverse direction.   DO NOT have the battery of the motorcycle connected when testing the diode board in the bike.  All the diodes should be roughly the same.  Anal types can repeat the tests when the board is heated to approximately 200°F.  

The best diode test is with a transformer that supplies ~5 to ~15 volts AC, connected in series with a lamp or turn signal lamp, and the diode tested in this series circuit.  Full brightness is shorted diode, half brightness is proper, no lamp illumination at all is an open diode.   You can modify an old high intensity desk lamp that has a lamp & transformer, or make up something else.  The June 1999 issue of AIRMAIL has a long article on diode boards by OAK.   A separate article on diode boards & their grounding wires, etc. is on this website:   I use a lamp that needs about 1/4 to 3/4 ampere for testing the small diodes on the diode board, and I use an old headlight for testing the 6 large diodes.  Using a headlight for the small diodes could ruin the small diodes.

It is perfectly acceptable to use an application...or two or three....of any common gel type paint remover product...on the outer diode board printed enable removal of that pesky paint coating.  You do not have to re-apply paint later.   Some boards may have a coating that the gel paint remover will not work on, for those you must abrade the coating....I use a sharp Xacto knife on a very flat angle, to avoid removing any copper material.  If using paint remover, use a water dampened cloth to remove the excess & then clean the board soldering area to shiny condition, before doing any soldering.  I use 50-50 plumber's solder, adding soldering rosin (soldering rosin is available in all hardware stores in a small can, it helps clean & prepare the joint as you solder), to make a stronger joint.  DO NOT use common electronics solder, which is typically 60-40.  Removing the paint coating will enable easy soldering of the diodes.  One of the indications of problems is when the solder joints at the 6 large diodes is discoloring & otherwise deteriorating upon a simple visual inspection ...the board need not even be removed to see is right in front of you with the aluminum cover removed.  FIX those joints!  Be sure the battery is disconnected!   Use an adequate sized soldering iron. Even a low power iron can work OK, if the iron tip is massive.

2.  CRITICAL ITEM!..........Unless you have an aftermarket EnDuralast Permanent Magnet Alternator:
remove the outer timing chest cover without first disconnecting any and all leads at the battery negative terminal.  IF you have ONLY the large diameter black battery negative lead at the battery negative post, then you CAN disconnect at the speedometer cable hollow bolt, THEN there is no need to unfasten the single large wire at the battery negative.   If you have other wires at the battery negative post, you should disconnect all of them at the battery negative, so as to be SURE there is no grounding going on, such as through a power jack, etc.  Safest ground for 'things' is back to the frame, not the battery negative; so keep that in mind when installing accessories.  Failure to heed this hint can result in ruining a diode board during removal of the outer timing chest cover.  

The accessory jack may or may not be grounded in its mounting, depends on jack model. Anything that provides an alternative grounding for the battery is suspect. T
he original plastic cover for the BMW accessory jack was 1 piece rubber or plastic & eventually broke from flexing.  You can replace it with the pricier, but better one from a K model, & the part is:  61-12-2-303-574.  There are other jacks, of course, that work fine.  Just be aware of the possibility of a JACK grounding problem. There is NO problem if the jack's black or brown grounding wire is connected to THE FRAME.

At the hollow speedometer cable bolt, disconnecting the large battery negative cable is made easier by clipping, filing, or cutting the lug, so it JUST fits OVER the screw, so the screw does not need removal, only loosening.  Be sure the washers are there.  That hollow bolt is modestly fragile, don't over-tighten; be sure it has the the two flat washers.  If one is a waverly washer or lock washer, that will ensure there is no need to excessively tighten; it should be snug with a 10 mm short wrench...that is all the tightness that is needed here.

3.   Starter circuitry:   the /5 bikes, and ONLY the /5 bikes, have a different type of starter relay (and an additional function) from all of the rest of the later Airheads.  In the Slash 5, once the engine starts, at some RPM near idle, the rising alternator output is used to prevent the starter relay from being accidentally operated. The original stock  /5 relay had a problem, that could cause one to think the battery was bad or failing.   The /5 relay has a very simple transistor circuit inside.  It was a faulty design.  In cold weather, and/or sometimes with just a slightly weak battery, the transistor fails to operate correctly (its 'gain', or amplification factor, is greatly reduced).  The relay will vibrate & chatter rather than stay pulled-in when the starter button is actuated.  This results in a noise called 'the cricket'.  "Oak" Okleshen coined that name, Cricket, in the early 70's, and, of course, the fix is called 'The Cricket Fix'.   The sound of the Cricket is sort of a clackety-clackety at a fairly fast rate.  Both that starter relay & usually (but not always) the large solenoid relay on the starter motor,... are doing the clackety dance together.   The result is usually no starter cranking function, just the noise, & is very similar to what one might expect with a near dead or very low charge in a battery in a car.  The proper fix is to remove the  /5 relay (located in a metal can, left side of frame backbone under the fuel tank, and is the furthest forward item), open it up, & make a few simple changes.    There is a complete illustrated article on the  permanent fix for the relay on this website at  which is article 37.   I still see them once in a great while that were never modified.  I HIGHLY recommend this fix be done; even if you have not yet experienced any problems.

It is possible to use a common ordinary auto-parts store relay to substitute for the /5 starter relay, if you understand what to do.  The safety anti-start function is gone however....but the /5 will operate like the later bikes (but no special neutral switch nor clutch switch functions...see below).

After the /5, the starter circuitry was changed, the safety anti-start function in the /5 was eliminated.  In the later models (/6 onwards) the starter relay is used strictly as a low power relay to power the starter motor solenoid coil.  Later models (/6 onwards) had a neutral switch that did more than illuminate a lamp (that is all the /5 neutral switch did). BMW added starting complexities tied to that /6 and later neutral switch & also added a clutch switch on the handlebars that tied into the new circuitry.  There are variations on the actual circuitry used on these later models.  After the /5, the starter relay circuitry is arranged such that the starter cannot be engaged if the bike is in a gear and the clutch lever is out.   That does not prevent the damge that can occur if you accidentally press the starter button in neutral.

In this article, below, it is assumed that the KILL SWITCH is in the RUN position.  If it is not, no power goes to the starter relay engaging circuitry.   

The starter relay on the /6 and later can be substituted by commonly available relays, but some relays in the BMW have a diode, so I don't recommend substitutions unless you understand what is going on with YOUR relay & bike.  Use of the proper BMW relay allows the proper function of the neutral lamp.   The starter relay is often repairable, often by simple cleaning/burnishing of the contacts inside it.  Sometimes all it takes is plugging and unplugging the starter relay because the problem is the exterior spades of the relay and/or its mating plug (socket). 

BMW never explained the purpose of ONE of the diodes in late model starter relays.  It is my belief that the current drain & other characteristics of the Valeo starter & its solenoid switch are such that REVERSE current effects upon the DE-energization of the PULL-IN coil of the Solenoid, are such that the starter relay points could be damaged, and the diode prevents that.   Possibly an interim the relay was modified for just the Valeo starter, later on, with a Service Bulletin being published about it....this is described several paragraphs down this article.

NOTE that the physical position of the starter relay varies by model and year.  Rather than make a listing of all the variations, here is a simple way of determining which is the starter relay (there are other ways).   If the right side of the top frame tube has TWO relays side by side (left to right) at the rearmost, then the starter relay is the one closest to the tube, and the headlight relay is the one to its right.  If the top frame tube has ONE relay at the rear most, then that relay is the headlight relay and the relay in front of it is the flasher, and the relay to the right of the flasher is the starter relay.

There is a peculiarity of the starter relay on the /6 and later models.  I think it was done for some sort of electrical disconnect function during factory checkout following production of the motorcycle.  The starter relay, under the tank (but can now be in several positions on either side of the frame backbone), has been known to get corroded male-female plug-in connections.  The peculiarity is that 100% of all electrical energy for the motorcycle (except for the large gauge wire that feeds the starter motor solenoid/starter motor) goes through a JUMPER built INSIDE the starter relay.  You can lose part or all of the electrical energy for the bike if there is corrosion at the relay or its socket.  Wiggling the relay or unplugging & re-plugging it will usually return power.  To fix properly, clean the male & female spades carefully, THEN add silicone grease; or, a high quality protectant from such as CAIG Labs; before pushing the relay back into its socket.  NOTE that if the starter relay is warped by using excessive force during removal, it may twist the relay internal parts just enough to actually close the internal relay contacts, causing unwanted starter motor action!

Here is another way of stating some of this...and more...

Beginning with the /6 airheads, BMW changed the wiring at the starter relay.   There are two wires leading from the battery + terminal into the airhead electrical system.  As previously, the big heavy gauge wire goes from the battery + terminal directly to the starter solenoid on the starter motor assembly.   That supplies the very large current demand when the starter is operated.  The alternator diode board also output feeds that point.   The starter can not operate from just that wire, it needs its solenoid energized, which is done via the starter relay.   A small gauge red wire at the battery + terminal goes to the starter relay under the gas tank.  Do not mistake that wire for a possible third wire to an accessory socket or other added accessories.   That smaller gauge red wire, that connects to the starter relay, connects to one of that relay's internal contacts, supplying power to the starter solenoid via its mating contact, when you press the starter button (key switch on). 

But, there are one or two other red wires at the relay.  Usually two heavy gauge red wires & one lighter gauge red wire are what is seen at the relay plug.  There is no connection between the two larger gauge of the three red wires at the relay socket, unless the relay is plugged-in.  The relay does NOT have to be energized by the starter button for is a design feature of the relay, metal is molded inside, into the relay base, that jumpers one terminal 87 to another terminal 87.  There are some minor variations over the years and models, but this is basically what happens to the two larger gauge red wires at that relay's socket....they are connected when the relay is plugged-in.  My suspicion is that BMW did this for some production line testing purpose.     If corrosion or poor contacts are present at the plug/relay terminals, there will be some voltage drop at those connections.  It HAS (rather often) happened that a total electrical failure has occurred from such corrosion.    This is NOT uncommon!

In the past, as above,  I have recommended that the relay be at least unplugged & plugged-in, several times, and silicone-greased on the connections to help with some weather protection.  Other recommendations from me have included doing a painstaking cleaning of the contacts.  "Painstaking" because the female socket connections are hard to clean.  My thinking changed some years ago on this subject.   As the Airheads age, more electrical problems are being seen from years of exposure to atmospheric smog, etc.   I still suggest cleaning and contact treatment, but for those wanting a permanent solution, all the Starter Relay RED wires can be joined permanently the next time the tank is off.   For safety, you must disconnect the wires at the battery negative first.    To do this wire joining, use a fairly sharp knife on a very flat angle, remove about half an inch (or bit more) of insulation from each of the red wires close to the relay this so the removed insulation is opposite each other.  DO NOT nick or cut the wires.  Cleaning the insulation without nicking the copper wires is not as easy as it sounds.    The internal wires are hopefully still shiny copper, otherwise you must abrade/clean them.  Find an old piece of CLEAN SHINY stranded wire around the garage, remove the insulation, and wrap the shiny copper wires around the bare wire areas.  Join them tightly.  Use a soldering iron with a substantial size (in mass) tip to carry the heat well; use 60-40 electronics type solder, multi-core type. Add some soldering rosin if you need to.  Solder the connection neatly & thoroughly.  You MUST use a soldering iron with a lot of heat ability.  Tape up the connection properly.   Do NOT fail to do a neat job!!!  If your system had been somewhat corroded, even invisibly, this modification MIGHT cause the charging system to INcrease its voltage to the battery; so do check it with a digital meter in the usual fashion, after the battery is fully charged from riding or a charger, at a goodly rpm.  Check the voltage AT the battery POSTS, themselves.  I mention this because if you already have your voltage regulator adjusted towards the high end of specifications with slightly poor connections (at the relay socket), you might have to make a small downward adjustment.   ~~13.9 to ~~14.4 volts is a good range.

On early Airheads, the starter relay & the horn relay look nearly identical, except for the numbers on them, & the BMW parts numbers (usually not on them).  If the relays are reversed, the bike likely will not start, AND, the battery will slowly run down.  The starter relay is BMW part 61-31-1-243-207; the part will have the Bosch number on it  0 332 014 118.    The horn relay is BMW part 61-31-1-354-393; the part will have the Bosch number on it   0 332 014 406.

When you install a Valeo starter in a bike that came originally with a Bosch, there can be problems with the stock starter relay.  The problem occurs, or can, because the motorcycles 1985-1988 had a weaker starter relay.  Thus, the contacts could weld, etc.  The updated relay has a stiffer springs & contacts that handle the increased current flow of the SOLENOID coil of the Valeo, ETC.  ONLY the later model Airheads with starter relays containing a diode are affected.  See the next paragraph.

Some later model Airheads use a starter relay with a diode inside.  Some have substituted a 0-332-014-118 relay (03-32-014-118);  some HAVE used a DF005 'Blazer' relay from AutoZone stores (which has two 87 terminals and no 87a terminal).  The Bosch starter relay uses two #87 terminals, and may sub to Bosch 03 32 019 150 for 1977+ bikes.  That is a common Bosch accessory use relay.  Connector, if you need one, is 0 334 485 007, while the spring loaded terminals are 1 901 355 917.  It is certainly possible to substitute almost any common 20 or 30 ampere 12 volt relay for the starter relay on Airheads, but there are some complications, such as with the /5; and with late models with diodes in the relay, so ask ON THE AIRHEADS LIST about it.  Complications, if any, are rather easy to deal with, so don't be discouraged with my remarks here.  BMW has an SI on retrofitting the Valeo starter to certain bikes that came with Bosch starters (1985-1988 bikes).  BMW thinks that the old starter relay contacts are not up to the larger current draw of the Valeo SOLENOID COIL.  Better contacts and a stiffer spring also helps the contacts to break cleanly.  It is my belief that the original relay will usually be OK, but if you want to install one with larger current capacity, see this document, which I have here in pdf format for you: Valeo Starter-2383.pdf         
Note what I said, earlier: 
BMW never explained the real purpose of ONE of the diodes in late model starter relays. It is my belief that the current drain & other characteristics of the Valeo starter & its solenoid switch, are that REVERSE current effects upon the DE-energization of the PULL-IN coil of the Solenoid, are such that the starter relay points could be damaged (they could even stick closed, and the starter continue to run!).  The diode absorbs the kick-back current, helping to prevent that:  That article ALSO contains a very complete description of the starter circuitry on all BMW bikes. See also, 4. and 5., below.

4.  Various problems, descriptions, etc., for 'funny things' with neutral lights, clutch switch functions; etc.     Pesky Diode Problems!!

A.  This applies to models after the /5, that:   Have a neutral light ON when the transmission is in any gear, & that light is OFF in neutral.   A common cause is poor contacts in the umbilical cable plug that fits into the back of the instrument pod.  Clean them.   Just why this can happen, which is fairly rare, is unimportant to try to explain here.  If you have installed the WRONG transmission-mounted neutral switch, that is another possibility.

B.  There is a lot of fairly complicated information in the red box, just below.  While you could read/glance through just to get some ideas; unless you have a fair amount of prior experience, the information may not make a lot of sense, and certainly not retained as usable information.  If, however, you are more anal ...then...I suggest you obtain a printed copy of a large-scale schematic diagram for your bike (some are on this website), & follow-through on the schematic, as you study the below information.  If you have the Chitech Electrics Manual, that is just as good, perhaps easier to understand, start on page 29, where the /6/7 sketch is located.   If you follow along carefully, & go on to page can become competent-enough to understand AND analyze most any starting &/or strange neutral light & other problems, for most Airheads.  You may just get one of those moments when the light goes off in your head!

Models from 1974 through 1984 (except the R45 & R65 & Monolever bikes) have a diode mounted on the UNDERSIDE of the board inside the headlight shell.

PRE-1981 R45 and R65 have this diode plugged into the wiring below the starter relay.

On Monolever bikes, the diode is INside the starter relay (do NOT install wrong type of relay!)

 Symptoms of a shorted diode in those places:
 Pulling the clutch lever towards the handlebars will cause the neutral lamp to illuminate. 

 Symptoms of an open diode in those places:
 The starter motor will not operate if the transmission is in neutral.

The diode must, in some conditions, pass the starter relay coil current; and absorb any high-voltage ‘kickback’ from that starter relay coil. 

I recommend a diode rated at 400 volts or higher; & at 3 amperes. 3 ampere diodes have considerably more reliability in this usage, than 1, 2, or even 2-1/2 amp diodes, due to the internal construction of the diode. Be SURE to install the new diode so that the band-marked end (silver stripe) is in the original direction. I have seen these diodes installed wrongly, that is, backwards. For the diode when mounted on the underside of headlight bucket wiring board, the banded-end (silver stripe) of the diode is connected to terminal LKK.

On a /6 & later bikes, if the neutral lamp seems to function normally for neutral & normally for any gear other than neutral...BUT...if you are in any gear & the neutral lamp comes on when the clutch bar lever is pulled backwards, then a diode has shorted.    If that diode opens, there will be no starting in neutral, UNLESS the bar clutch lever is pulled back. 

Turn on the ignition at the key switch & have the emergency kill switch in the RUN position. Put transmission in NEUTRAL (be SURE it is!).  The starter should work from the push switch.  This tests the starter relay, starter solenoid, neutral switch at the transmission underside, & handlebar push switch.

Turn off the ignition, then turn it back on.  Put your Airhead into ANY gear (be sure it is in a gear!). Now, pull back the clutch lever on the left handlebar, & while pulled back, check that the starter button will, again, cause the starter to operate.  This adds the testing of the switch at the clutch lever at the handlebars. 
To finish the testing, repeat the above paragraph, but the clutch lever should be OUT.  Hold the brake. Tap the starter button, it should NOT work.

Will the bike start if you pull in the clutch lever, but the NEUTRAL LAMP lights up as you pull in the clutch lever? If SO, the diode is SHORTED.  If that diode opens, there will be no starting in neutral, UNLESS the bar clutch lever is pulled back. 

The purpose of this diode is to prevent the neutral light from being on when in a gear & the clutch is pulled in.  

If the neutral light is ON, & it is not the diode or switch on the bars that is at fault, perhaps you simply installed the wrong switch at the transmission!  Yes, there are two different switches, they work opposite to each other. See:                

The starter circuits vary with year & model. In ALL models, the starter solenoid, a huge relay-switch itself, is wired directly via a BIG wire to the Battery + terminal. A much smaller 'starter-relay' is under the tank; the purpose of the small starter relay is to allow a small switch, the on-bars push-button, to operate a powerful & current demanding relay (the solenoid relay). There are differences in the various airheads are in how the starter relay & its circuitry are connected & wired.  These differences can be important if you have a problem.

In a /6, assuming it is stock, the power to run the starter relay begins at the battery + terminal, goes to the ignition switch, then to the KILL switch on the bars, & then to TWO different but somewhat connected circuits.  The #1 circuit is the starter relay COIL terminal 86 side.  The power goes through the coil and comes out at terminal 85, then to the starter button, and through the starter button, at which point it must be, somehow, grounded, to have the starter relay coil to be energized. 

Just how that is done:
That point is connected to the clutch-operated switch on the handlebars. If the bars lever is pulled backwards, the switch closes, and if the starter button is also pressed, the starter motor will operate.  This is why a bike should always start if the clutch lever is pulled backwards. If a bike does not have any starter function at that time, then the bars clutch switch or connections are faulty.  

The circuit has a second part.  The junction of the clutch operated switch and the starter button switch are also connected to the anode of a diode. This is the diode I talked about in its various locations, including under the board in the headlight shell.  The cathode of that diode connects to the neutral switch at the transmission, which, on both styles of that switch, are electrically closed in neutral.  Thus, assuming the diode is OK, there is no need for pulling in the bars lever to start the bike IF in neutral at the transmission.

The junction of the diode and neutral switch is also connected to the neutral light. Anytime the ignition and KILL switch are in the ON and RUN positions, and the neutral switch CLOSED (which it is supposed to be, in the N position ONLY of the transmission), the lamp lights up.  

I previously explained that + 12 volt power went through the starter relay coil &  then to the clutch switch.  At the clutch switch connection (or, - side of the relay coil, which is the same thing), a diode is connected.  That diode is connected to the transmission neutral switch. The actual purpose of the diode is to BLOCK flow of current from the N indicator lamp to the clutch switch.  You don't want the neutral lamp turning on just because you are pulling in the clutch, rather, you want the lamp on only in neutral.

A diode is a one-way device when it works correctly. TWO things happen if this diode fails.  If it "opens", then the neutral switch has no effect on starter operation, and the neutral switch does NOTHING but illuminate the lamp in neutral. There will be NO starter motor operation withOUT the clutch pulled in.  The bike is supposed to be startable in any gear, or neutral, if the clutch is pulled in; it is supposed to be startable anytime the transmission is in neutral.  If the diode shorts, the neutral lamp will illuminate EVERY time the clutch lever is pulled backwards.  The bike will also start in neutral.

I will ASSUME here that you have a failed diode from whatever the fault indication is.  What to do?  The diode on a /6 bike should be located UNDERNEATH the circuit board in the headlight shell.  It is not easy to get to.  If shorted, it MUST be disconnected, although you can install the new diode above the board.

What type of diode?

You can use just about any silicon power diode rated at 1 ampere, but I prefer to use one rated at 3 amperes as that size of diode has MUCH improved longevity for your bike.  A diode rated at 100, 200, or even 400+ volts is perfectly acceptable, while one rated at 50 volts (almost non-existent these days) does not have an adequate safety factor.  Radio Shack sells small power diodes in small packages of 2 or 4 rather cheaply.  Keep the other diode(s) to experiment with; perhaps with your ohmmeter or multimeter with diode test function, to see how it tests diodes; what good diodes look like on that meter.

You MUST install the new diode with the LINE marking on one end of the diode in the SAME direction/position as originally installed. I have described an anode and cathode, well above, and the LINE marking is the CATHODE end. Some diodes have a line with an arrow going into it...same thing, line end is cathode.  You won't burn anything out if you do not, but the proper functions will not be had. 
Nerdy comment:    + current applied to the NON-line end, will pass THROUGH the diode.

 Be sure to read #5 below!

C.   There is a peculiarity with the 1978-80 models, which have a front brakes master cylinder under the fuel tank (ATE brakes).  These incorporate a float switch, whose purpose is to illuminate the brake failure light if the fluid is low.  The lamp gets tested each time you start the bike, via a diodeIf the diode shorts, & you are also low on fluid, the starter could energize!    This is extremely rare.  The anode of the diode connects to the brake switch; the cathode of the diode connects to terminal 85 on the board and also to the starter relay coil.  The 1977 bikes do not have the diode in the brake warning lamp circuit.   None of this exists after 1980. 

NOTE:  BMW's production year is not from January 1st to Dec 31st.   The factory shuts down in August for the annual vacation; all bikes manufactured afterwards are the following year's motorcycles.   This was just about universally true for the Airheads, with a few later peculiarities (& the K bikes in mid-1992 were ID'd officially as 1993 models...ETC).

5.   In the 1985 & later (but NOT including the GS & the 1985 R65), the starter BUTTON & starter relay COIL circuitry was CHANGED.   The starter button is on the PLUS (+)  side of the relay coil instead of the minus side. Thus, BMW could now mount the diode mentioned in item 4., INside the relay.   This can make a difference in troubleshooting! It also means the relay is a different type.  

The physical position of the starter relay varies by model & year. Here is a simple way of determining which is the starter relay (there are other ways). If the right side of the top frame tube has TWO relays side by side (left to right) at the rearmost, then the starter relay is the one closest to the tube, and the headlight relay is the one to its right.  If the top frame tube has ONE relay at the rear most, then that relay is the headlight relay and the relay in front of it is the turn signal flasher, and the relay to the right of the flasher is the starter relay.

Here is a circuit description for these later models, since the basic operation is somewhat similar, no matter the circuitry:  
When you turn on your ignition switch, the battery positive (+) is connected to both the neutral light & to the starter button.  Pressing the starter button sends electricity to the starter relay coil. The other side of that relay coil DOES NOT go directly to ground, rather, it connects to TWO places. It connects to the CLUTCH switch (at the lever at the bars).
This applies to earlier models too!....if they have that switch (all after the /5 do).   It also connects through the diode (inside of the starter relay) to the NEUTRAL switch (underside of the transmission). BOTH of these two switches, other pole, connect to ground. Thus, if EITHER the clutch switch or neutral switch are activated (grounded), the button will operate the starter (starter relay will send power to the solenoid on the starter motor, and activate it).    NOTE:  if you try to substitute a relay without  the diode, things won't work as intended!

The neutral LAMP is wired such that it will ONLY turn on if the NEUTRAL switch is closed (closed internally, due to the transmission being in neutral).

IF the diode fails by OPENING, then the neutral switch will still operate the neutral LAMP OK, but you CANNOT operate the starter UNLESS the clutch is pulled in.

IF your neutral lamp itself is OK, and if the circuitry to the lamp is OK, but the LAMP is NOT being turned on & off by the transmission being in neutral or not...then the transmission neutral switch is probably faulty. Easy to determine, just short across the connections at the switch on the bottom of the transmission. Typically with a faulty switch the bike will NOT be startable, unless the clutch lever is pulled backwards.

Thus you need to determine if the lamp is operating normally, in order to decide if you have a bad neutral switch, or a bad diode. Be advised that you might simply have a wire pulled off the neutral switch or corrosion in the connector that wire goes to (near the battery).  The neutral switches are NOT the same, early & late models...they LOOK somewhat the same, but a change was made in 1976.   If you have a bad one, be SURE to get the correct one.   If you have a 4 speed transmission, the neutral switch is totally different, is on the rear, not bottom, of the transmission, & only activates the lamp.  On the 5 speed transmissions, neutral switch functions are, as you have seen, more complex, and quite complex on later models.  Shift-kit transmissions use the later type switch.  
The 1974-5 neutral switches have a shorter stem.  If you install the wrong switch, things do not work correctly!    Neutral switches are CLOSED in neutral, turning on the green neutral lamp, enabling the starter function if the starter button is pressed.   But if the diode shorts, then the lamp is ON if the lever at the bars is pulled.

If you do not install the correct starter relay, you can have an open diode indication, as relays you might find someplace other than the BMW bike dealer, will not have the diode.   This can happen if someone swapped with the headlight relay, it is not the same relay internally. 

There is an additional problem if you substitute the starter relay that requires 2 diodes, with one that has one diode.   You can also have the problem of the engine not wanting to turn off!

Many have also mixed up the horn & starter relays...see much earlier in this article.

If you have a 1987+ Monolever bike & the lights come on with the starter in operation, there is a faulty diode inside the starter relay (2 diodes are in these models, inside the relay box).  This lights-on thing will then also happen in the PARKING lights position.

Headlight relay & Load Shed (Load Relief) relay, all about them:

Headlight relays were not used on all models; there were none up until 1975.  From 1975 to 1977 the headlight relay was added as a pure headlight relay, used THEN to reduce current through the headlight switch.  AFTER 1977, the relay was kept, but had an entirely different function.  It had a load-shedding function (but was not called a load-shedding relay at that time).  After 1977, the headlight relay is wired so that when the starter motor is energized the headlight relay coil de-energizes, the relay contacts OPEN, thus the headlight goes OFF during cranking.  In 1978,  BMW eliminated the light switch. 

The 1978+ relays have an additional function:
The headlight relay on 1978+ models has a diode between connection #86, the green or green/violet wire, & connection 87b, a gray wire. The CATHODE of the diode connects to 87b, & sustains the tail & instrument lamps DURING starting, probably a German safety requirement. If the diode shorts, the ignition will stay on; even the key will not shut off the engine, unless you disconnect a battery wire;  which removes the problem instantly...but only until the next start.    

Pin 85, the black wire, is the "to-be-grounded" side of this relay's coil, & connects to the STARTER MOTOR. During cranking, BOTH sides of the relay coil connect to the nominal 12 volts, & the relay is NOT energized.  A problem with an open starter motor, which can be intermittently open, can cause this relay to be inoperative, thus, no headlight unless you use the high beam "passing" push button, which bypasses the relay; it has its own separate wiring.   NOTE!  This PASSING switch that turns on the high beam, is spring loaded to return when you remove your finger, AND it bypasses the headlight relay, AND it is also NOT fused!     THUS if you have a problem of NO headlight in High or Low beam selection switch setting, but you push that switch all the way down, for passing mode, and the headlight comes on, check the headlight relay. 

Here is a different approach to explaining how the headlight relay works (/6 & later models but not the 1975-1977 models):
The headlight relay is not what you may think it is.  The headlight relay is NOT a relay that simply selects between high & low beams.  In fact, it does not do that at all!  The purpose of the headlight relay is to turn off the headlight when the engine is being cranked by the starter motor.  The coil of the relay is so wired that one side goes to power, the other side to the starter motor.  When the starter motor is NOT energized, the relay is ON (assuming ignition is ON), and the lights operate normally.  When the starter is energized, the headlight relay's coil has battery voltage at BOTH coil wires, so the relay turns OFF.   Due to how the relay contacts are arranged, that turns off the headlight.   But, a DIODE built into the relay keeps the tail light & the instrument lights ON during cranking. The BARS hi/lo switch passes full current to the headlight directly.  THAT is why a relay should be added if larger than stock headlamps are being used, as the extra power they require would otherwise wear the switch excessively.  The Eastern Beaver relay kit is very convenient for this, as it is plug and play (except to install a fused red wire to the battery).  The High Beam Flasher part of the bars switch is a separate circuit to the high beam. 

If the relay is marked 1.244.411 then it has TWO diodes inside.  I am not 100% certain of the extra diode's function.   I THINK that the extra diode's function is to prevent
current in the opposing direction from operating the relay.  That could come about with a stuck starter solenoid (which contains two contacts).   This other diode is connected so that the cathode connects to pin 85, & the anode is to the coil.  From 1987, Monolever bikes had two diodes diodes in that relay.  If the lights all come on when the starter is operated .... or Park position of the switch is selected.... then one of those two diodes has shorted.

The method of de-energizing for the headlight relay from 1978 is that the starter is at + 12 volts when cranking, and so is the input side of the headlight relay coil; but the coil is also connected to the starter... so with the same voltage on both sides, the relay coil is NOT energized.  The headlight relay coil is energized all the time normally when the key is ignition-on, except in cranking. There are normally-closed & normally-open   contacts inside that relay, and the appropriate ones are used for the functions intended, which include the headlight lamp, but BMW wanted the tail light & instrument lights left ON during cranking, so that requires the other connections inside the relay can.

The headlight relay is wired to the headlight switch & momentary Hi Beam switch, depending on model & year, & is not wired the same for all models & years, &, further not the same on even the same model in the same year as shipped to the same country!!!  I've run into this, & found wire colors not on schematics....& combinations of Euro & USA.  One example:  on some Airheads, the Hi beam flasher button will operate withOUT the key being on.

The "Load Shed" relay, in the later bikes, is used for the same idea: turn off the headlight when the starter motor is in use; but,  the Load Shed (Load Relief) relay works with more items.  For instance, the Load Shed relay may, during cranking, DE-energize the horn, the turn signal flasher, the right bars switch assembly, & the heated grips (if those exist).  There are differences between Euro & USA bikes in the Load Shed circuit, & the circuit may actually be split into two sections, one of which is fused.  I think I remember the R100GS using Fuse #4 for that.  The effective operation of the Load Shed relay is similar to the headlight relay, but with added items being shed of power during cranking; in some instances with the key off.  The Load Shed/Relief relay does more (in the bikes with it), than in the bikes with just the headlight relay. BMW incorporated the Load Relief relay into all bikes eventually.  Load shed relays have their own problems, in bikes with early ABS systems.

All these headlight relay & load shed relay circuit changes are confusing.....the complexity has foiled many an owner in trying to identify a problem.

The stock /6 and later headlight lamp uses 55 watts on low beam.  For 55 watts, the current drain is only ~4.4 amperes. That is not much, compared to the starter motor, which draws as much as 100 amperes (or even more on a cold day cold start). Even if you add heated grips, the amount is still relatively low. Thus, for the Airheads (& to MY way of thinking), the complexity of the headlight relay or load shed relay & circuitry was not worth BMW's efforts (there is a reason for it for ABS bikes, but could be done MUCH more simply), complexity; and, in particular, the peculiarity of how BMW de-energized the relay (by wiring it to the STARTER, so the starter is the normal GROUND for the relay coil).
If you think about all this, you may get the idea that if the starter motor has a lot of carbon brush dust in it, that peculiar things can happen, depending on the particular bike...and you'd be correct!  You also may get the idea that the current reduction during starting is probably of little or no real consequence to the starter electrical needs.  Just in case someone who really knows the circuitry wants to send me commentary, yes, I am aware that some of the bikes don't shed electrical loads exactly how I describe.

BMW makes complicated vehicle electrics. BMW has a hazard light system that was available on some bikes, and as an add-on is a complicated MESS.  I have the schematics on this website for anyone wanting to look at them, but it surely is WAY over-complicated, IMO. Sometimes I think BMW has a round-table discussion with sales...[ and engineers (???)].....and the discussion is "no matter how complicated it gets, what would you like to see in electrical functions....?".

BMW is noted for such electrical complexity. The first time I delved deeply into a K bike turn signal circuit, & the complications allowing monitoring of certain lamps, & then a function that shuts off such as the turn signal lamps after so many feet or so many seconds,... gads!  It also (and this is PRE-CanBus!) makes it complicated to add more lamps, except for the headlight....).  Just to be fair, some of the complexity MIGHT come from Euro safety requirements.

I have advised those doing 'café' conversions to eliminate the headlight relay instruments and rear tail ON function, and maybe a few other things.  I am NOT recommending anyone modify the stock system or remove it on anything but a Café type bike, which usually does not have the stock type instrument pod, etc.   The stock setup works fine.....until it doesn't. Then you get to be confused in trying to figure it out.  Perhaps you will ask questions on the Airheads LIST.   BMW is NOT the only company having complex electrical's....they ALL do these days.

There is much more about the headlight relays here:

If you have the folding-out fog/driving lamps in your RT fairing in place of the air vents, there is a diode associated with them, and you might want to read the notes here:

7. The charging lamp (GEN) MUST be brightly lit when the ignition is on, engine not running. This lamp provides the initial alternator rotor energizing current. DO NOT depend on the alternator to self-energize at 5000 + rpm.   IF the lamp does not illuminate properly at key-on, or, perhaps there is no charging when the engine is running (especially at say over 1500 or somewhat more RPM), the regulator can be checked with a little jumper wire with spades attached...see at the top of this article.   Remove the tank, UNplug the voltage regulator, and jumper D+ blue to DF black in the plug.  DO NOT jumper to a brown wire!!   The proper jumpering bypasses the socket for the removed regulator, and the light should go out around 1000 to 2000 rpm.  Strong charging as rpm rises, where it did not show charging previously, indicates a bad voltage regulator. If the lamp was not lit at any time, and the lamp itself and its printed circuit connection (and no hidden cracks!) tested OK, and if the regulator was bypassed by the jumper, and the lamp now lights up, you probably had a faulty regulator. 

A rare situation is for the voltage at the battery to be WAY excessively high while riding your Airhead down the road.  This condition can come from a battery that has high internal resistance and/or poor grounding or other connections to the battery.  It can come from a bad voltage regulator.  It can come from a bad ground (brown wire) coming from the voltage regulator socket. That wire MUST be grounded.   Another possible cause is a bad SMALL diode in the diode board.

Regulator problems in general:
If you need to delve further, you might start by measuring the voltage at the rotor terminal...or, better yet, the slip ring itself (brushes have been known to not make contact!).  If you have a bad regulator, you can limp home at lower than normal rpm (to avoid overcharging the battery), with the mentioned jumper in place.   Most any three terminal regulator used in modern cars also works.  

After the /5 (rotors about 7 to 8 ohms), rotors measure about 3 to 4 ohms, & about 2.7-3 ohms for the last production version.   If your rotor measures very considerably higher, the rotor IS BAD (be sure it is not bad brush contacts...measure AT the slip rings, then at the brushes).  The brushes, being carbon, typically add nearly 1 ohm to the rotor reading.  Rotors tend to OPEN circuit, sometimes this happens only at some rpm, or when hot or cold.  When a snail spring bottoms-out, the brushes get intermittent.  If the snail spring has insufficient pressure, there will be poor contact of the brushes...particularly important on the later low ohms rotors, and this can sometimes cause very intermittent GEN lamp operation/indication.  It is rare, but a rotor coil can short-circuit to the steel body of the rotor. I have seen this mostly happen on re-wound rotors.  An ohmmeter will tell the story on a rotor already in the motorcycle; measure between either slip ring and the engine case. The resistance should be close to infinite...millions of ohms.

Due to slight run-out on the rotors (call it wobble, or slight miss-machining), the GEN lamp can illuminate weirdly as rpm rises.   See 8.

There is information on this site on problems with the diode board connections that can influence charging from tiny voltage drops:

Information on modifying the GEN lamp circuit so that the alternator will still operate if the lamp burns out, is filed here:

If you have been tardy about inspecting & replacing your alternator brushes, & it is way-short brushes that are the charging problem, you can do a 'field fix' that works very well:  SLIGHTLY lift the tip of the snail spring off its brush, & insert a tiny piece of thick paper.  Cut the paper to fit the brush properly.  A thickness of a matchbook cover is usually adequate enough.  This will fix the problem of a snail spring that is bottoming-out in the slot on the side of the plastic brush holder.  The field-fix can last a long time.

8.  If you have a GEN lamp that lights up faintly while riding along (sometimes it is faint enough to be seen only at night)... this is almost always due to a build up of small amounts of poor contacts and/or corrosion, at various places in the electrical system.  It IS ALSO POSSIBLE for a badly sulfated battery, with one or more shorted cells, or nearly so, to cause the GEN lamp to come on, often dimly.  Poor electrical connections can cause a dim lamp, as can some brush problems. If this gets bad enough for the light to shine fairly bright, you have serious connection problems OR possibly so-so brush contacting of the slip rings.   Brushes USUALLY last about 70-80K. They will last less on bikes ridden in dirty air or with higher electrical loads. Brushes are best changed with the stator assembly removed.  When at least one brush is worn far enough that its snail spring will start to bottom out on the plastic brush housing, a tiny bit of wobble (slip ring runout) of the rotor will cause the GEN lamp to light up dimly...and possibly to increase that brightness as rpm is raised.  Article on this website:

NOTE 1:  When a GEN lamp does NOT light up at ignition key turn-on & engine not running (or at idle rpm), the lamp is faulty, or the lamp socket, wiring, regulator, brushes, or rotor is open circuited.  This is almost always very easy to moderately easy to figure out.

NOTE 2:  The GEN lamp is connected at the output of some small diodes in the diode board, and the other side of the lamp connects to the battery, but through the key switch.  As the alternator begins to produce current, those small diodes are SUPPOSED to have the same exact output voltage as the large power diodes that feed the battery more directly.  Anything in either of these two connections circuits that allows for enough voltage difference to light the lamp, even faintly, will do so.  Thus, a difference could occur with a problem in the diode board, but also in any voltage drop that was excessive at several places, including the switch, the starter relay internal jumpering, etc. 

It is actually possible to measure the voltage drop down the system, by using a simple setup.  You connect your digital meter positive (red or + ) lead to the battery + terminal, & the meter negative lead connects to various test points you are interested in.  That allows directly measuring the drop itself.  You could also measure the voltage drop across items.  Be SURE the battery connections are very good, have a KNOWN GOOD battery, & known good connections from battery to transmission & to the starter motor & to the large terminal on the right side of the diode board, facing from the front.  A known good battery is NOT just one that will start the bike.  You can measure the battery terminal voltage, under load, such as the headlight or during starting, that will give you some idea....but a formal load testing is vastly better.  This can be done IN BIKE, with such as the inexpensive Harbor Freight battery Load-Tester.

NOTE 3.  Alternator brushes are about 16-1/2 mm long when new, measured from the middle of the concave to the top of the brush.  While there have been minor changes in the brush holders over the years, a common problem is to let the brushes get too short, & then the coiled spring pressing on the brush will bottom out on the brush holder, the brush will then have no, or hardly any pressure against the rotor, & you will have strange charging problems. This often includes a glow from the GEN lamp, at riding speeds at night, sometimes even visible in daytime. The rotor is not perfectly concentric in operation, thus as it rotates at various rpm, it can cause the brushes to very slightly move in & out when the snail spring is touching the holder.  This can make for a brightening or other irregularity of the lamp as rpm rises.   It is best to replace the brushes when they are around half worn, but you can let them go until the snail is almost bottoming.

NOTE 4: Do NOT overly clean the slip rings, which are soft copper. Never use coarse abrasives.   A pink lead pencil type eraser is OK.  Use of fine grade kitchen cleaning sponge/plastic light scouring pad, is OK...BUT NOT the heavy duty type.  Carbon on the slip rings does not cause problems, so a bit of most any drying solvent & a rag is actually enough cleaning.  You could even use a rag with some 'home rubbing alcohol'. You do NOT have to have shiny copper slip rings. It is NORMAL for a black carbon 'track'.

NOTE 5:   If you have to remove & replace a rotor, heed this warning!...USE ONLY the one-piece factory tool, or a homemade tool of HARDENED steel.  Do NOT use soft steel.   The BMW factory tool is not expensive, & it is a good idea for one of these tools to be in your on-bike tool kit!  A ONE-PIECE aftermarket tool, of hardened steel, is also OK.  See my tools article: 
OR, for just a photo of just these particular tools:   MUCH GRIEF has come from using the wrong rotor removal tool!

Brushes connect to terminals with casting stamping identification of D- and DF.  D- is the grounded terminal, even though it appears insulated on earlier alternators.  Df is insulated (be sure you do not mix-up the insulating washers during brush replacements), and D- must go to the BROWN wire.  Do NOT goof....many have!   Do NOT fail to properly assemble the white plastic brush holder insulating washers at the correct Df terminal...IF you have disassembled this area.  If you mix them up, you will ground the Df wire, the D- wire is grounded elsewhere's the GEN lamp will light up, but you get no charging.  This is unlikely on the later stator cases which have a slightly changed nib for the brush holder mounting at the D- terminal area...but it is still possible to ground the Df, and then no charging.

NOTE 6:  When brushes get worn enough, & the snail spring will touch its own side-notch in the white plastic brush holder, and even very slight and acceptable run-out (wobble) on the rotor can cause intermittent contacting of the brushes.   This can vary with rpm, making you think that you have a rotor that might be opening at higher rpm.    

9.  If you have to remove a Diode Board or Stator: 
a.  When replacing either, note that the black wire of the diode board goes to the starter solenoid; the blue diode board wire goes to either of the D+ terminals (D+ terminal also goes to starter relay D+ terminal).  

b.  Alternator STATOR housings have three terminals next to each other, the three mating wires may go to them in any order.  They MUST fit tightly, as must the larger gauge RED wire at the RIGHT LOWER SIDE of the diode board, facing from the front. 

c.  A /6 and later diode board has an extra terminal on the left side (facing from in front); a wire from that terminal goes to a terminal on the stator housing.

d.  Diode boards of all models can interchange, but a /5 board will give lower charging in a /6 system, since the /5 board does NOT have a terminal for the center-tap of the diode board (left side of diode board, facing from the front).

10.  The headlight shell may contain a parking light lamp socket (in the headlight reflector unit) that may be unused....such as on RS & RT models. You may find it has a lamp in it, and it is unconnected to the bike.   I am NOT talking about the separate eyebrow lamp!   A 10 (I can't recommend the 20) watt quartz lamp can be installed in this unused lamp socket.  Phillips bulb 12452 is a 10 watt quartz, has PLENTY of light output.  The stock lamp in those sockets, when being used, was only 4 watts and an incandescent.  The 10 watt quartz, which is HUGELY more bright, can be used instead of the headlamp, saving some power.   

You can make a simple modification, if desired, so that the ignition is ON in the PARK position.

Although I do NOT recommend it, if you are installing an oversize headlight lamp on a /6 or later (the /5 already has a high temperature lamp socket), you should install a high temperature socket for the lamp. Here is ONE source; scan down this site to find the ceramic one:

11.  The early spark plug caps were 1000 or 1200 ohms, not the later 5000. It does not USUALLY make a difference on a stock 1980 & earlier points bike if you use 5000, but 1000/1200 caps MAY give slightly better performance if something in the ignition is marginal.  NGK makes nice spark plug caps.  LB05F are 5000 ohms, LB01F are 1000 ohms.    Check them with a meter before purchase, be SURE they are they make similar ones that are 10,000 ohms: DO NOT use LB10F.  Do NOT use anything but 5000 ohm caps on the electronic ignition models (1981+).  If you have a POINTS model, without any aftermarket electronic ignition module, etc., & you are replacing the spark plug caps, use 1000 ohm spark plug caps.  NEVER use the carbon type of resistance wire.  I recommend AGAINST resistor spark plugs.

12.  Ignition cutout problems after some modest miles on the road (1981 & later models) can be due to overheating of the ignition module under the tank.  It is NORMAL service to remove the module & clean & replace the electronic heat-sink grease every couple of years.  That is not done on the very last versions of the module, that are RIVETED to the heat sink.

13.  The /5 bikes had a 180 watt alternator.  They can be updated to a specific version of the early 280 watt alternator used in 1974 & maybe early 1975.  You can identify which /6 alternator stator you are looking at, by the stator size....if the one to fit a /5, it is 105 mm; all the rest are 107 & won't fit a /5.  You can use the /6 diode board which has the Y connection diodes for the stator, or just use the /5 diode board, BUT, with a modest reduction of the 280 watts.    Later rotors & electronic regulators CAN USUALLY be used, & require no maintenance & fit perfectly.     Bosch stators have a Bosch number on the HOUSING.  The numbers ending in -001 and -002 are the early type, and are 105 mm.  -003 used a R90S stator (which could have been changed), & if original will result in slightly reduced output.  -004 and -005 are the 107 mm stators.  Rotors for the earliest bikes were 73.4 mm in diameter.  For the 1975 /6 & later, the rotors are 73.0 mm in diameter. NOTE:  trying to use an early higher resistance rotor with the last generation of higher resistance STATORS is NOT a good idea.

You can get a later stator & in a kit from Motorrad Elektrik.  Similar items from such as Ted Porter's Beemershop are recommended.  They also have a higher power alternator conversions.

There is an EnduraLast permanent magnet alternator conversion kit available that will fit all models of Airheads.    

There is some testing that seems to show that the latest low ohm rotors do slightly better in all models for lower rpm charging.  NOTE that these lowest ohm rotors work best with the latest model VR and the later stator.

BMW modified the stator windings & rotor resistance of the last of the Airheads; the tradeoff was that while the TOTAL watts output was reduced some (to around 240), but the charging began earlier. I'm unconvinced this was an improvement, over-all.

There is a detailed article on this website about aftermarket alternators:

14.  Electronic regulators WILL substitute for the earlier mechanical regulator, the plug is the same & it will fit in the same mounting holes.  The BOSCH metal can electronic regulator, which looks like the Bosch mechanical metal can regulator, but is a bit shorter, works OK...but the very last plastic-cased VR's handled the lower ohm rotors with better reliability.

15. Wiring up heated grips...& other situations; series-parallel switching, ETC:
Heated grips need a method to be able to reduce the high heat that would occur with both grips having full power on them all the time.  BMW uses a resistor & hi/lo switch.  That works OK; but the resistor is built into the harness and gets fairly warm.   Use of a power rheostat is not practical, as its physical size would be fairly substantial and finding a mounting place may be a problem.   In order to reduce the heat from grips, there are three 'practical' methods:

a.  Sometimes I am asked how to wire up a single switch so that something, such as heated grips, can be operated, via a single switch, for both as parallel & as series connection.  Here is how to do that.  You need ONE switch, & the switch is called "Double Pole, Double Throw", or, commonly, DPDT.  The switch has 6 terminals.   You can purchase these switches commonly in 2 or 3 position types. The 3 position type, which I usually recommend, has a center off position; in that instance it is called "DPDT-Center Off".   Turn the switch upside down so you have a vertical row of three terminals on the right, & three on the left.  The center terminal on either the left or the right is always the moving contact for that side.  In the sketch, below, that means
Oc & Od.    On the LEFT side, connect the center terminal to +12 volts, probably via a fuse & possibly via your key switch.  Connect the UPPER terminal on the LEFT side to Load  #1.  Connect the CENTER terminal of the RIGHT side to this SAME upper left terminal.  Connect Load #2 to the lower terminal on the LEFT side.  Connect  the not yet connected Load #1 wire to this same point.  Connect Load #2 unconnected wire to GROUND.  The last connection is the LOWER one of the RIGHT switch side, ground it.   NO connection is to the upper right side of the switch.

Here it is, in much much more depth, and, a bit later, a graphical explanation:

Ground one side of one heated grip...let us just say this is one of the Right heated-grip wires.  Connect the remaining right grip wire to one of the left grip wires.  On the switch, connect the center (arm internally) of one section to the +battery after the ignition switch, but via a 4 ampere fuse.  On that same section of switch, connect one terminal to the left grip wire that had no connection. On that same section of switch, connect the other terminal to the JUNCTION of the two grips wires made previously.  You now must wire in the second switch section. On the same end of the switch that connects to the ONE left grip wire, run a short jumper to the CENTER (arm) of the other switch section.  The same end of the switch, opposite, other section, has no connection. The remaining connection of the second section is connected to ground.    When switch is in center position, no power goes anyplace.  When switch is in one position, +12 goes to arm of first section of switch, then to the left grip & to the arm of the second section. +12 travels through the left grip, through the right grip, & then to ground. Thus, this is the LOW power, or series connection.  When the switch is in the other position, the first section of the switch applies +12 to the junction of the two grips. One grip, the Right one, then goes to ground. The other is grounded via the second section of the switch.

Below is a sort of  graphical view of the rear of a DPDT switch, with the O meaning a terminal, & my adding of alphabet letters next to each one. It is critical that the switch be wired EXACTLY as described just a bit below this sketch:

Oa  Ob

Oc  Od

Oe  Of

Connect two grip wires together. Ground one right grip wire. No connection is made to terminal b.  Connect +12 to c, via 4 ampere fuse & to output of the ignition switch.  Connect a & d together with a jumper wire. Connect e to the junction of the grips wires previously joined.  Connect the a-d connection ALSO to the left grip wire.  Ground f and ground the Right grip.

NOTE that the wattage (heat) in a circuit follows Ohms Law.  Because of this, when grips normally in parallel mode are changed to series mode, the heat (wattage) produced, assuming both grips are similar in resistance (which they always are), is reduced to a fourth of the parallel amount.  This is a low value, but may be acceptable if the parallel connection is WAY too hot...which is not at all uncommon with aftermarket heated grips.   The Ohms Law I mentioned is: voltage squared, divided by resistance, equals watts.   What this means on a practical basis, is that (using a typical 14 volt level while cruising), the power is 196 divided by the resistance.  If, for an example, the grips are 15 ohms EACH, then in normal high heat 'parallel mode', EACH has 13 watts of heat.  BUT:  If the grips are in series, then the resistances ADD, and you have 30 ohms.  Now the heat is a TOTAL of 6.53 watts.   3-1/4th watts is in each grip, on low heat.  This is rather low, still, it is feelable.  THUS, using a series/parallel switching method is not what I normally recommend; but may be usable on LOW for YOU.  You can temporarily simple-wire it for series connection, and see if that will be enough for you.

b.  Left and Right heated grips are normally wired in parallel, & a 3 position switch used to turn them to High, Off, Low.  In the low switch position, a resistor, which might also be a a short length wire of a special resistance type, lowers the power to the grips, but that resistor itself can get warm to hot.  This actually works well.....if you like the particular Low position, & the resistance wire is routed safely/properly, or if you use a resistor it is mounted properly. 

c.  This is my suggested & preferred method:  PURCHASE a very small control box of the type that is for motorcyclist's heated clothing. Versions that control via pulsing or duty-cycle are the best, as they hardly get warm themselves & are very efficient. NO resistance wiring nor resistor to get hot, just the grips! ...and you get any amount of heat you want at any time.

I use tank bags, & mount my clothing controller on the rear, facing me; makes it very convenient to adjust when needed. These controllers are also available in versions that can be mounted on/in bike dashes, on handlebars, ETC.  For my tank-bag version I have the power plug going into the usual BMW power outlet. Typically these controllers have a type of 'transistor'  inside that is used as a time-controlled switch, the time of 100% ON is controlled by the adjustment knob.  At a very low heat level, the transistor is turned on 100% for a fraction of a second, then turned off for a much longer period of time.  As the heat control is adjusted higher, the ratio of on to off time increases, until at full heat the clothing/grips are on all the time.   Because there is hardly any 'resistance' in the semiconductor no matter what the adjustment is set at (providing a pulse circuit, as described, is used),  heat developed in the controller is quite low & efficiency is very high.   Light dimmer controls in your house work pretty much the same way.  Basically these devices, to describe them another way, act like you are manually operating a switch, for either a short period of time, or a long period of time.  Unless you are very good at electronic design work, and don't care about the labor involved, it is better to purchase one of these little boxes.   There are cheaper controls, even dimmer controls, available as kits.  I recommend you just purchase something from a heated clothing maker.

The cheapest way to go is the special switch wiring, for parallel or series connection, a., above.  Some might be OK with that.  The next type is hardly much more expensive and adds a single resistor in method b., which actually is quite decent...but you need to find the right value of resistance...& mount it in a place where the heat developed in it is no problem.   A 6 ohm resistor was used by BMW on many models for heated grips for the LOW heat switch position, and I have found that to work on MANY aftermarket grip heaters.  A 10 or 25 watt electronics resistor can also be installed on a small heatsink someplace, away from wiring.  Method c. is the most expensive, but gives you full variability and no heated resistances nor heating wiring.

There is an article on this website with the schematic diagram of the factory heated grips circuitry....see

16.  Craig Vetter's Windjammer fairings have on-line help available, including the wiring diagrams, information on saddlebag & windshield mountings, etc.

17.  Any stock airhead electrical system, (including the /5,180 watt system), will maintain the battery in a charged condition, from something like 2200 certainly not needed over 3500 rpm....ONCE the battery is fully charged.   The later model Airheads have some slightly higher power usages, & almost always will require ~2800 minimum.  Adding an electric vest will raise the rpm required a bit....perhaps a FEW hundred rpm.  Continuous riding below 2800 is not good for the engine, so you shouldn't be doing that anyway.    Any really large electrical load COULD require 4000+ rpm continuously; which is a good RPM on Airheads anyway.     On a practical basis, considering sometimes poorly maintained electrical systems, old components, etc., don't plan on over 80% of the rated watts output of the alternator.   Commuters (stop and go, heavy traffic) will have more problems keeping the battery charged. 

18.  Folks sometimes ask about the wiring diagrams for such as the 4 way flasher, various versions of headlight switches, etc.    Some of this information is in some factory manuals, rarely in owners manuals to any degree, & almost nothing in such as Haynes or Clymers.   Reference can be made to information on the R65 4 way flasher (hazard flasher), which has application to other models, & on other items mentioned here in the 1985 service fiche #4, section 61.  That will also provide information on all the 1979 models for adding a headlight on off switch, applicable to others, ETC.   There are also other sources.  I have most of the information.  There is a separate article on this website for these things:

19.  The earliest /5 model was not fused.  The later /5 model fusing should be added.  Don't let  your wiring burn up!  The /5 & early /6 bikes did NOT have any short circuit protection for the headlight flasher circuitry.  I recommend fuses be adde; or at least install a fuse in the main power lead from the battery (not in the starter wire). Just adding a 30 ampere fuse in the battery circuit will be of some help on any of these models, if you don't want to add the proper fuses at the proper place in the headlight shell.  In fact, installing one 30 ampere fuse (you can use 20 or 25 for additional safety, depending on alternator size of output) is a somewhat better over-all solution than adding the later /5 fusing system.  I think converting to the /5 fusing AND adding the 30 ampere fuse is the very best thing to do.   The later blade type fuses are more reliable than the tubular types. 

20.  BMW has two bulletins out on certain models built from 1985 into 1989, regarding the brake light switch on the handlebar, and the switch at the rear brake lever.  The information will be found on this website at

21.  Here is the URL for the Chicago Region BMW Club, the source for their various repair manuals.
The Electrics manual is still priced at $30 & is simply THE best electrics manual for airheads; Oak was primarily responsible for that manual.  HIGHLY recommended.  
That website also has some information on the background of that Club, & the background on Oak, etc., and those various manuals.     My CRITIQUE of the electrics manual is on this site:    Once you obtain your Chitech electrics manual, use my CRITIQUE to pencil-in the corrections!

22.   The R100R models PRIOR to serial 0280227 (the last 7 characters in the VIN number) have a problem, and you should consider adding a grounding wire, on silver-painted frames models.   Failure to have this added ground wire added WILL  (or MAY)... show up as misfiring...or...poor alternator output.   ADD a wire, neatly, between the left starter motor cover bolt, and the ignition module bracket. Use a 14 gauge wire, SOLID BROWN in color, so it follows factory color coding for wiring.  You could fashion something from INside the starter area, to the cable outlet at the top of the timing chest, if you wanted it more hidden.

23.  Turn Signal Flashers/Relays:
The /5 bikes use a 2 terminal bi-metal heater type flasher relay unit.   It is load & voltage supply dependent, so flashing speed varies with alternator output (rpm), especially if you use different from stock recommended lamps, etc.   Some BMW's, including the /5, have ONE indicator lamp, and that lamp is electrically connected to BOTH left and right side turn signal lamps.  How this works has confused some folks.  It is important to use the proper wattage of lamps, and, assuming this is so, the following will explain how this single indication lamp works:
The flasher output goes to the bars left/right flashing selector switch. I will call that connection to be the +12 volt (nominal) output to the turn signal lamps for one side of the motorcycle.  Let us, for this example, assume that the switch is set for the left lamps to be flashing.  The flasher, being loaded by the two left side lamps, now operates and the turn signal lamps will blink.   The single indicator lamp also connects to the left side lamps.  the GROUNDING for the single indicator lamp connects to the OTHER SIDE lamps (in this instance, the RIGHT side two lamps).  Those two lamps are in parallel connection, and provide a quite low resistance, so the INDICATOR lamp will flash in unison with the LEFT side lamps, yet the INDICATOR lamp, a low wattage type, does not allow the RIGHT side lamps to light up, as the current flow through the INDICATOR lamp is too low.

NOTE that the BMW's with the more complicated later flasher units may have the indicator lamp connected to a specific flasher output terminal.  More on this later in this section.

The following is an acceptable substitute for the /5 flasher, although you can use almost any of the two prong bi-metal heater types:

Tridon Stant Electronic Extended Life Flasher 12 Volt, 2 Terminal  EL12.   Actually, almost any of two-terminal flasher will work OK.

After the /5, the stock $$$ electronic flashers are a bit complicated inside.  They have a dual relay function when any of the directional signal lamps burn out.  You usually lose that special indication for a burned-out lamp with the aftermarket flasher, but still can have the turn signal indicator lamp function like in a car, & including for a burnt out lamp. There are different types of these flashers. They can be differently marked.  Terminal 49G and 49a are same function.  KBL and C are the same function.  
ALL of the flashers from 1974 can be replaced very similarly with such as the Tridon EL13 or HD13 and similar.  If your motorcycle has TWO indicator lamps (one for left, one for right), then there is NO connection to "C" or "KBL", & you need NOT be concerned about making that connection when substituting an aftermarket flasher relay.   Do NOT use the solid brown wire that BMW had in its connection to the original flasher relay (1974+).  Connect the old wire that went to KBL or C to the flasher P terminal.  The BMW wire for this is probably Black/White or green.   The aftermarket flasher relay probably has a terminal marked X, that goes to the +12 power, the old wire connected to flasher terminal 49, likely was green/black.   The turn signal output of the new flasher relay is L, for LOAD, and is likely green/yellow.

Connect the P (which stands for PILOT lamp) terminal to the indicator lamp (usually a green wire); the X terminal to the +12 power (old terminal 49 wire, green-black); and the L terminal to the load (old terminal 49a, green-yellow).  If you have two INDICATOR lamps, don't connect to the P terminal.

NOTE:  aftermarket flashers don't use the brown ground wire that BMW's flasher relay does.

A recently recommended substitute is the Blazer (or, Tridon/Stant) flasher.  The model used for that recommendation was 550, which is a thermal flasher, but handles the BMW lamps just fine, & would even handle extra lamps, such as on a trailer or hack.  An electronic unit could also be used. 

For those of you that intend to use LED lamps for flasher (turn signals, trafficators), you may have problems such as all 4 flashing, or some flashing dim, on the wrong side of the bike, indicators flashing wrongly, too fast flashing, etc.  This is all due to the LED lamps not having the current draw of the original incandescent lamps.   Here is a place where you can not only purchase lamps, but they have full electronic flashers that do not need load resistors. They also have quality load resistors if you need them:

For those installing extra incandescent lamps, or have specific reasons to replace an existing flasher
unit, ETC, there is a heavy duty flasher unit available at auto-parts stores, under the SignalStat brand, model 263.   Mechanical/electrical.  Flash rate is 60 to 120 per minute, has 3 each 1/4" male spades, is 1.33" round, 1.35" high, works on 11-15 volts, and from well below freezing to damned hot.  It will handle 20 ampere loads!!

NOTE!    The early /6 bikes had poorly grounded front turn signals.  You can run into all sorts of intermittents & strange indications from the pod turn-signal indicator lamp.  While other causes, including a bad flasher relay, poor contacts & bad lamps, etc., can cause some similar problems, DO try improving the grounding. Test for the problem, & if you have it, fashion some grounds from the headlight bucket to each turn-signal pod.  Please use conventional color code, so that means solid brown insulation.  BMW fixed the problem in ~1976.

NOTE!   Some of the BMW flasher relays are PRICEY.  You might be able to fix yours....and here is one particular problem that is easily fixable.  This happens with the later model bikes where the flasher relay is mounted on the backbone frame, under the fuel tank.  These models are 1981 & later GS, G/S, ST, R65LS, &, in general, all the other models from 1988.  Here is a typical complaint and fix:
"On a recent ride, my turn signals quit…left and right no flashing, same for the lamp"

a.  Remove the fuel tank & test the wires (by slight pulling up & pushing down to the flasher relay socket). No problem with the factory crimping should show up electrically.

b.  Consider SLIGHT tightening of the 3 female connectors just a bit where the relay male spades fit. Use a pointy tool.  ONLY SLIGHT bending.

c.  You will probably find the fault is inside the relay, often (not always) you can prove it to be a SIMPLE FIX, by slightly moving the relay, left-right…just a bit of pressure both ways.  That MAY cause the turn signals to stop & then restart.   In most instances the problem is inside the GREEN relay made by Wehrle.

.  The relay contains an integrated circuit & a small electrolytic capacitor, etc.  The capacitor USUALLY dries out & causes flashing speed problems only after many years, so you can probably disregard trying to test it, unless the complaint is flashing speed. It is LIKELY that 1, 2, or all 3 of the relay's male spade solder joints inside are cracked.  To get inside, you must pry at the side places with a very thin tool.  This can further damage the solder joints, but not by much & you have to fix them anyway.  Remove the printed circuit board with the mechanical relay, as one unit, just as built.  Mount into a vice, with LIGHT pressure, so the male prongs & circuit board are at the proper 90 degrees.  RESOLDER the three male solder joints. Install the relay cover, being careful that the printed circuit board fits into the inside cover groove especially for the printed board.  Be careful to install the relay into its socket squarely, with even force on the whole case, & not too much force.  You just saved $$$.


It is possible to rather easily add a piezzo tone unit to any of the flasher setups.

24.  If you have a 1979-1980 R65 (maybe R45 too??), check the front ignition coil mounting bracket.  They tend to crack & the ground wire there becomes disconnected.   You will find funny electrical problems if that happens (like maybe a crazy tachometer, lights doing weird things....).

25.  A RARE, but super-annoying problem, because you likely will go CRAZY before you find the answer, is one of the 1981+ electronic ignition airheads, that typically will idle OK, but won't raise its rpm up properly.    This problem acts somewhat similarly to a hole in the carburetor diaphragms.  The actual problem is a poor ignition kill switch at the bars.   Cleaning that switch may not fix the problem.  Bypassing the switch will show whether or not the problem is that switch.

26.  Many blown fuses??  Problem finding; different fuse types:   
Sometimes a fuse has been irregularly or consistently blowing.  Perhaps that fuse is used to protect more than one item (fuses on BMW airheads & most fuses on Classic-K bikes ARE like that).   Once in awhile someone has installed a glass fuse with pointy metal tips, that is the American style of replacement fuses for the 8 ampere German-style fuse....which is an open type on a tiny piece of ceramic, often reddish or white in color...with the fuse element also being the formed-tip.  The American style (whether or not actually made in America) uses a metal cap bonded to the glass tube, & the internal fuse element is bonded to the caps.  That type is NOT as reliable as the German type, & the American type has been known to 'open', withOUT it looking 'blown'.  This is easy to check on your ohmmeter if the fuse is removed....or, I simply use a voltmeter across the fuse, or at one terminal, then the other...or, a test light across the fuse. 

When a fuse is blowing relatively often, or perhaps even immediately upon the ignition being turned on or power otherwise applied, it is sometimes a Pain In The Ass to determine WHAT is wrong.  Is it a shorted wire...from frayed insulation or a pinched wire perhaps?  A shorted horn or horn that needs internal service?  Bad wiring at a tail lamp?.....whatever.

Here is an old Snowbum & technician's TRICK, that can usually GREATLY speed up the analysis process when an initial look-see makes you think things will be difficult:

Keep an old headlight lamp on hand, with a cheap headlight socket (from any autoparts store) attached and at the ends of the wires put insulated alligator clips. Most of the time only ONE section of a hi/lo headlamp has blown, so the lamp is discarded. Use the section that has not blown.    Remove the fuse that has been blowing, substitute the headlamp wires.  A stock headlamp bulb of 55 or 60 watts will only pass ~5 amperes, even into a direct short circuit.  If the lamp glows brightly, start tracing down the various wires & devices connected to that fuse holder.   When you find the problem, the lamp goes OUT, or greatly dims.

You can make various types of these lamp testing tools.  I have one that is made from a blown common thin rectangular blade type fuse.  These types of tools can be VERY useful.  My article #14 has a lot more information; of both the above type of test lamp, PLUS the commercial lower power type with sharp test prong; how to use multimeters, etc.  Article #14:


This is how an H4, 9003, etc., lamp is internally connected.  In this view, you are facing the base of the lamp from the rear.  This sketch applies to just about all similar 3 terminal lamps.

27.  Ignition points for 1970-1979 Airheads:   BMW has shipped wrongly made points sets, made in China.  The rubbing block is too long; you cannot get proper timing, etc.   I suggest the Noris points from such as Beemershop, etc.  This problem MAY be fixed by now.

28.   High idle RPM after full warmup? This is actually a mechanical problem from wrongly advanced ignition timing caused by sticking ATU parts.  Almost all of the time this problem is found only in the CANISTER ignition models (1979 and later). RARELY it has happened from bad springs in the ATU of earlier models (non-canister).  That is easily seen by moving the weights by hand.

For the canister models: T
here are several TESTS for proving that the canister ATU is at fault & not, say, a vacuum leak at the intake rubber hoses, or no free play in the throttle cables; or a mal-adjusted idle mixture screw.   In some instances, just turning off the engine & restarting the engine after it was already hot & exhibiting the very high idle, is enough to reset a stuck ATU, so try that.  If that now shows a normal idle, it is likely a ATU problem.   Repeat to be sure, and maybe do the next step.

Get a friend's help for a more accurate test.  First take the bike for a ride, & if the idle rpm went quite high after a FULL WARMUP of the ENGINE CASE, then pull the bike up to a nice big object, like a brick building.  You could also just use the front brake.  With the bike in gear, let out the clutch very slowly, loading the engine & allowing the engine to slow down to about normal idle, perhaps 900-1100 rpm.   Have a friend use a timing light, triggered from the left spark plug, point the light at the timing hole. If the timing is well-advanced, then the ATU IS THE PROBLEM.  Prove it by pulling in the have a high idle rpm again, right?

29.  Information on changing to Euro switch gear....and in general, replacing /6 switch gear, with the 'problem' of extra wires, etc....see article 38B.  I will eventually put step by step information there, to cover all models.
Article 38B:

30.  Once in awhile I hear of someone who has a bike that is in known good state of tune...valves and timing set correctly, good spark plugs, good coil(s), carburetors carefully checked & are good.....ETC....but the bike is difficult to start, AND RUNS ON ONLY ONE CYLINDER FOR AWHILE.  This may be the ignition module.   I describe this problem in my article, but you can also find a version of it in Oak's column of November 2011 AIRMAIL.

31.  Sometimes someone wants to modify a bike's wiring so that the engine will run in PARK or HEADLIGHT position.   This allows the headlight to be turned off in one position.  In the early stock airheads where this is possible, the 30 terminal of the ignition switch is red, for + battery power. Terminal 56 has a white-yellow wire for the headlight.  Terminal 58 has a gray wire for the parking lights function, and terminal 15 has a green wire for the ignition.   MOVE the green wire to terminal 58. DO NOTE THE FOLLOWING:  Many German motorcycles have a PARKING position of the ignition switch, in which the key can be removed in that position.  It is possible that almost any key that can be pushed into the lock, will turn the switch to PARK position, so test for that.

33.  Versatile relays that can work fine for most functions in your motorcycle, such as switching lamps, running horns, starting, etc., is the Bosch (now Tyco) 330-073, rated at 30/40 amperes & 12 volts, SPDT, 5 pin, with tab for screw (tab area can be removed); or the Blazer DF005 or DF005W which also has a tab/screw mounting.  Beware of using such universal relays in place of BMW relays that have diodes inside....unless you know what you are doing....including adding the diode(s).

When a battery is getting old OR getting closer to failure (even not very old batteries can start dying), if your stock BMW dash voltmeter is wildly swinging during use of the directional's flasher, & you have already checked the wiring & connections at various places, the battery may need replacing.  LOAD TEST the battery on a proper instrument.

35.  VOLTMETERS (and clocks):  
The dash voltmeters (as on RS/RT)  themselves are almost always quite accurate...but they are not connected to the battery at the battery terminals.  They are connected much further down the line, so-to-speak. If the SYSTEM is in good condition, the dash voltmeter on a RT will read approximately 0.3 volt +- 0.1 lower than the reading taken at the battery terminals. The problems with the readings, & wildly swinging needles, etc., can come from several places:  Wear & somewhat poor connections at such as the ignition switch, connections at the starter relay (small relay under the tank), & in the headlight shell. When the connections get just a bit too much resistance, you may well NOT see any problems with anything but a swinging voltmeter, & they can swing wildly. I have fixed voltmeter internals where the meter is actually at fault, but it is not worth all the effort to take them apart. There are some things that make the voltmeter even more wonky. The alternator stator connections may be somewhat overheated & providing iffy connection. Same for the + output (large red wire) at the left end of the stock diode board. These wires MUST fit TIGHTLY to make good connection, considering the large current flow. If the wires do not fit tightly & make good connection, the output of the alternator system will have A.C. spikes on it, plus somewhat fast happening D.C. spiking, & that WILL affect the VOLTMETER!

Damping of the voltmeters can be poor. Several possible reasons for that.   Perhaps a CAR voltmeter that looks identical was installed. Another reason could be that damping of the correct voltmeter CAN decrease...&  considerably, due to aging of the internal parts. A massive capacitor external can help, but is not the best answer. A series resistor and a modest capacitor works better, but, still, I don't like that method. Another reason is that the pivot bearing of the voltmeter will age/wear, & as it does it gets grabby, & can thereby get jumpy, & can look rather similarly to a voltmeter with bad damping.  If the pivots are poor, tapping on the voltmeter with a finger will show up that problem most of the time.

The turn signals require about 4 amperes or so from the electrical system (2 amperes +- for each of the two lamps on either left or right sides of the bike). The electrical connection comes from close to the same place the voltmeter is connected to. That means that the voltage to the voltmeter could have a fair change in value, & if resistances in the system are high enough, the meter can really swing A LOT!

Most of the digital meters that are designed for vehicle dashboard use are damped electronically quite well, & there is a sampling rate situation going on with those digital meters that also ... USUALLY...tends to ACT as if the meter was much more highly damped. Not going to get into that further here.

Unless you need absolute originality, I would suggest a 2-1/16" round DIGITAL meter, as it fits in the original dash hole. They are available reasonably from such as Summit Racing. You won't then need the lamp either.   For those who want a high quality totally sealed meter, & are OK with one that is rectangular, the one's made by DATEL are VERY good.

Substituting an accurate DIGITAL meter for the existing BMW in-dash round meter...or, just adding a voltmeter someplace:

There are digital voltmeters on the market that require a separate power source. There can be problems with them! Some of these require...or MAY require, a separate supply, perhaps 9 volts.  The worst part is often that the supply negative is NOT common to the measuring side of the meter....UGGGH!     These are difficult to use, & some use them with an external small 9 volt battery.   I HIGHLY recommend you do NOT use this type; AND, there is NO NEED TO.  The type of digital meter I recommend you install in your bike (Airhead, K bike, ETC.) is a TWO WIRE METER.  These meters provide their own power from what they are measuring.  They also need NO lamp as they usually self-illuminate.  They are LOW DRAIN.   These meters are available in LED & LCD versions.  The LED version is much more visible in all conditions of lighting (day, night, etc.), & pulls only a small bit more current; that drain is totally negligible on your bike if you wire it so the meter is powered AFTER the ignition switch.  Drain is UNDER 0.020 ampere for the Datel (Murata) unit, ...that is under 0.28 watt!  It is BRIGHT!!  There are numerous types of digital meters available. 
a. To install a ROUND faced digital voltmeter in the Airheads, in place of the existing round voltmeter, I use a 2-1/16" round faced type from such as Summit Racing, in the USA.   See also, they have BOTH round AND rectangular digital meters.

b. To install a rectangular type digital voltmeter:  My favorite meters came from, now .  Good quality, rugged, reliable, accurate, red or green or blue digits, all of which light up themselves; work fine at any temperature.  I recommend RED digits, easy to see; easy on your eyes at night.   See also or   (Retail outlets primarily, Datel type meters, with information, so snoop around).   

I recommend Datel's ( model number DMS-20PC-1-DCM-C.  These draw only about 13 ma when powered.     If you want a panel bezel, that is DMS-BZL4-C, with gasket.   The meter will read accurately from +8 to +50 volts D.C.     These are two wire, self-powered, NEED NO LAMPS.  They are fully encapsulated in a polycarbonate rectangular body, & are VERY rugged.  RED digits for the above number.  If you want blue digits the number is DMS-20PC-1-DCM-B-C; green digits is DMS-20-PC-1-DCM-G-C.    The LED's are 0.37" high.  Do NOT purchase the 0.01 volt resolution type (has an extra digit), as it is WAY overkill; and the always changing indication on the right-most digit will drive you crazy.  You need only 0.1 volt resolution even for critical work on your bike; and the only such work is in setting the voltage regulator for the alternator using the meter with leads attached as a test instrument for setting that Voltage Regulator or measuring circuit drops.  There are cheap "equivalents" to the Datel unit, sold on Ebay.  They SEEM to be OK.

Back around 1999 or so, I designed a special 2-1/16" round faced digital voltmeter, made a few by hand & then had a company run off a batch of them.  The Company was originally called Intellitronix (C.R. Industries) & became Nordskog in the USA.    May still be at   I sold all of them to Airhead owners except one production version was put on one of my own bikes, and I kept my original homemade one in my 1983 R100RT.  These became popular, so, a few years later, I wanted a much larger batch made & the company refused. I could not find another company to make them inexpensively, so dropped the project.  Later, I found nearly an identical meter was being sold by them to various hot-rod shops, such as Summit Racing, who resold them to the public. Guess who developed them?  Their plastic-cased meters are actually rather good.  Various types are available, some with push-buttons on the face to enable storing peak voltages, ETC. I recommend against non-weather-proof push-button versions.  The advantage of these round meters is that they fit in the existing voltmeter hole in the BMW dash.  However, you can also make a mounting plate and mount one of the Datel's (or, Ebay available versions) to fit there too. 

All electrical connections should be cleaned, particularly at the two fuses in the headlight shell. Be sure the starter motor solenoid connections are TIGHT. Be sure the STARTER RELAY has no corrosion at its spades...even unplugging and re-plugging in that relay can help, often a lot.  That relay is a KNOWN TROUBLESPOT. You can modify the wiring there, by JOINING ALL RED WIRES externally at the female plug. You MUST do it neatly. When corrosion gets bad enough at that relay, the entire electrical system will shut down, COMPLETELY.

NOTE!!.....Some later versions of the stock BMW dash voltmeters can be calibrated to match actual voltage at the battery (which is typically a wee bit higher), or, to just calibrate the voltmeter for wherever you might reconnect it to....or, just for the stock wiring.   Remove the LAMP and use a flashlight and look down the hole.   If there is a small slotted place, that is the adjustment.  
If the stock meter damping fails, the meters can go crazy.  Usually happens with the turn signals on.  That can also happen from poor connection joints.

NOTE that BMW bike voltmeters are dampened against some types of vibration; CAR voltmeters will swing more wildly; a CAR type from a junkyard may not be a good choice.


VDO instruments:   It is possible to substitute VDO instruments, such as:
Voltmeter:  #332-103 (or  332103).   This fits the original 2-1/16" round hole, is well-dampened; has black face with WHITE LETTERING & red pointer.  It uses a spin-on clamp.  Has through-dial lighting; & using diffusers, can have red,  green, or white lighting.  It has a 8 to 16 volt scale, just like the original one in your Airhead.  Was MUCH cheaper, last time I checked, than buying the BMW voltmeter.

VDO  #370-152   Those are black face & black bezels; have a threaded part to mount them, rather than a bracket. That was discontinued;  the new number is 370-100 and 370-100B.   These are way too pricey for most to consider.

You can also use VDO Vision Marine voltmeters and clocks.

06/29/2003:  Total revision of entire article, combine electrical hints & electrical problems pages, moving some items to appropriate places on the website.
07/20/2003:  add #20
09/05/2003:  add information to #2; modify #3,  #7, #13 and #19 all slightly
09/21/2003:  add note 2 to item #8; move #18 information to #8, add new #18 information, modify #19.
01/05/2003:  add #21
03/15/2004:  Corrections to item #6...several actually.
04/11/2004:  Expand #3 with all the red information
07/05/2004:  Expand #8
10/15/2004:  Redo and greatly expand #15 for maximum clarity
03/21/2005:  expand on neutral switches
03/28/2005:  update #3 for hyperlink for cricket fix; revise #4 slightly
04/09/2005:  add hyperlink on 21
11/27/2005:  In entire article, go through and clarify things.  Add R100R information too.
11/23/2006:  Clarify Monolever 1987+ diode information inside starter relay
04/15/2007:  expand #4
01/06/2008:  fix chicago club's hyperlink and description
01/07/2008:  fix vetter's URL and url
06/28/2008:  minor clarification about monolever diode problems
10/28/2008:  Add #23
02/23/2009:  Clarify #13
06/08/2009:  Expand #23.
11/04/2009:  Add #25.
11/21/2009:  Move #26 information from end of #3 and make it #26.  Go over entire article for clarity.
12/05/2009:  Add Signal Stat information to #23
02/11/2010:  Add information to #27
03/15/2010:  Expand a bit at #22.
06/20/2010:  Expand a bit at #23.
11/20/2010:  Add #28, previously in another area of the website
02/16/2011:  Add #29
03/29/2011:  Expand upon #6
05/09/2011:  Expand #10
06/07/2011:  Add #30
08/29/2011:  Add #31
09/09/2011:  Edited #24
03/24/2012:  Add 33
07/04/2012:  Expand 4B and a bit of cleanup (tons more needed!!)
08/01/2012:  Clarify #23
08/13/2012:  remove link to relay article; it no longer is there
09/27/2012:  Add QR code; add language button; update Google code; clean up article a bit, but no technical information changes (the language button was removed in 2013)
03/30/2013:  Add some to 4B about the diode.
08/03/2013:  Revise 6 entirely for clarity, added information.  Prompted by an article I posted to the Airheads LIST.  Also revise 3, 4 and 5, strictly for clarity.
02/17/2014   Revise #33; minor other changes.
02/23/2014:  Revise 4B extensively, and slightly in 4D.
04/27/2014:  Add 35.
07/31/2014:  Add note on wrongly interchanging the horn and starter relays, and add note on use of CRC-5-56.
05/19/2015:  Incorporate much more information on voltmeters and clocks
01/18/2016:  Clarify details on the diode involved in the neutral switch and clutch switch circuitry.
02/15/2016:  Review entire article; re-arrange considerably; clarify details a/r.  Justify left, fix horizontal lines, update metacodes, increase font size, ETC.
06/08/2016:  Final updating for layout, metacodes, scripts, ETC. 
09/01/2016:  Modify information on LED turn signals and flashers, to show an additional source for the electronic flashers. Add word Incandescent to the 263 info.
04/17/2017:  Modify #23 for clarity on how the indicator lamp functions with the /5 style of single indicator lamp and two-terminal flasher relays.

© Copyright, 2014, R. Fleischer

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Last check/edit: Monday, April 17, 2017