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Troubleshooting the BMW Airhead Motorcycle Alternator/Charging Systems
Copyright, 2014, R. Fleischer

FIRSTLY, and IMPORTANTLY, this article is NOT a substitute for Articles #14, #15, #15-A.   This article is to be used in addition to those articles.    I have not placed lots of hyperlinks in this article, just a few.  I WANT YOU to read those three articles.

You may also want to refer to one of the articles on the Airhead Voltage Regulators.

Troubleshooting a dead GEN lamp:

Because a common problem is a GEN lamp that does not illuminate at all, this troubleshooting section is first.

The problem may not be a bad lamp. The GEN lamp SELDOM burns out. More often it is the rotor has opened electrically.  Sometimes there is a crack in the connection where the lamp fits.  A bad connection for the lamp can include at the lamp base, the pod plug connection, or inside the external pod plug itself. Other places for problems are occasionally the Voltage Regulator located under the fuel tank, on the right side of the frame backbone.  Worn brushes frequently cause problems. 
You can easily test the rotor without any instruments.
Assuming that the lamp does not illuminate when the ignition key is ON; recheck that first:

Turn on Ignition....GEN lamp should still NOT be lighted.
For a /6 or later motorcycle, wiggle, a bit, the umbilical cord that goes into the back of the instrument pod.  If the GEN lamp lights up, the problem is in the plug, or, the mating male parts in the pod. NOTE that this is NOT a 100% perfect test.
Assuming the lamp does NOT light up:  Turn off the ignition. Disconnect battery (removing all the wires to the negative post will do). Remove front cover. Reconnect battery.

Turn on ignition.  GEN lamp should be the same as before: NOT be lighted.

Connect a jumper wire from the brush holder Df terminal to the case.  You can also use a screwdriver to do the jumpering.  If the lamp DOES light now, remove the jumper or screwdriver, and, with the lamp now not lit, short the Df and D- connections.  You should get the SAME results, lamp lights up.
Conclusion:  Bad rotor, bad brush(es), or bad brush wire(s).

Assuming the lamp does NOT light up:  You have a bad voltage regulator, or bad wiring or connections in the lamp circuit. 
How to determine if the voltage regulator is probably bad:

Turn off the ignition.
Remove the fuel tank.
Be sure you have not left the D- or Df wires disconnected at the alternator.
UNplug the voltage regulator.  It has is a three female connection plug. You may have to press at a place to allow the harness plug to be removed.
Turn on the ignition.  
Ground the solid BLUE female connection wire in the voltage regulator's harness socket.
If the lamp does NOT light up, the wiring from the VR to the pod or its large rear plug, or internal wiring or lamp or lamp connection in the pod is at fault.
Assuming the lamp DOES light up: 
Remove the grounding wire you just added.
Jumper the OPPOSING female connections in the plug.  Usually this is blue/black and blue, but do NOT jumper to the brown wire.
If the light now shines, you have a bad regulator. 
Note that this test should probably be the last test for the lamp circuit, as you want to be sure what is going on with the rotor and brushes and Df wire at the alternator first.

Do NOT forget to disconnect the battery before replacing the front cover.

The GEN lamp, necessary in the stock system to initiate charging, does not fail often.  There is a modification that allows the system to start up even if the lamp fails (and makes a slight improvement on rpm at which charging begins).....that is on this is article #19.


Troubleshooting a constantly on GEN lamp:

If, when your ignition key is ON, the GEN lamp goes ON brightly, but it never goes out, or only fades a bit, when the engine is revv'd up, it usually indicates NO charging, or very little charging.  A number of things can cause this, and there are various approaches to figuring out the problem. While you can use a test lamp, if you know how, to analyze things, use of a multimeter is probably easier, and you need to know how to use one.  Read my article on them:

That article covers both multimeters AND test lamps usage.

You can obtain one for FREE from Harbor Freight Company...they often give out quite adequate and accurate ones for FREE, upon ANY sale.  Otherwise, they sell them for a variety of prices, depending on a lot of factors, always not costly.

1.  The brushes may be well-worn.  Usually when the brushes are quite worn, one of the brushes will be seen to wear faster than the other.  The snail spring
     on the brush will be seen to be bottoming on the white plastic brush holder.  You can put a tiny piece of paper or other insulating item under the tip of the
     spring, which will work until you can get around to installing new brushe(s).
2.  Something in the diode board has failed.  Bad connection, bad diode(s).  This is not all that common, but happens.  SOMETIMES the problem is the
     owner, removing the front cover, before FIRST disconnecting the battery!....and shorts the diode board in the cover removal operation.  SOMETIMES
     the motorcycle is one with the RUBBER MOUNTS, with a mount failure and shorting (Rubber mounts are abominable, replace them with METAL mounts).
3.  The voltage regulator has failed.  To test for that, either install another one; or, UNPLUG the VR, and in the PLUG, connect a paper clip or other item,
     between the opposing female connections.  DO NOT connect to the solid BROWN wire!    If the charging is now good, and voltage keeps rising with
     increased RPM (do NOT let it go over ~14.6 or so), then the VR is almost for sure bad.
4.  The rotor has OPENED (open connection internally in the rotor).  This is VERY common.  To test for that, and to eliminate that the brushes might be the
     problem, put a piece of paper under both brushes. Use an ohmmeter between the slip rings, should be very low resistance, a few ohms.  As a second
     check, check between either brush and the chassis.  If you get a low resistance reading, the rotor is shorted to its steel structure.
5.  You replaced brushes, and now you get constantly on GEN lamp, was OK before the brush replacement.   You have probably taken the brush holder apart,
     and misplaced the correct order and installation of the insulating washers in the Df brushholder section.  Testing with an ohmmeter, D- and Df wires OFF,
     and paper under the brushes so they do not contact the slip rings of the rotor, are a quick method of determining things.  ONLY the D- connection may be
6.  You have mixed up the two wires going to D- and Df of the brushholder.  The BROWN wire goes to the D- spade.  Also see #5, above.
7.  It is very rare for a stator to be bad.  Usually that happens, if very rarely, when someone removes the stator, using metal tools improperly, and shorts
     some of the wound-wires.  Note that the stator metal is made of thin special steel laminations, do NOT put scrape marks across those laminations, that
     can REDUCE the maximum output of the alternator, and also can cause the lower rpm output to be lower than normal.

Poor charging:

Poor charging can come from quite a few reasons.  Sometimes more than one reason at the same time, which makes finding the problem(s) a bit more involved. 

One of the first things to do is to make sure that connections are clean, tight, no strands of wire are broken.  When you have the front cover of the engine removed (disconnect the battery negative leads before trying to remove the cover), you can eyeball the stator connections, those push-on connections need to fit tightly.  You can check at the RIGHT larger spade terminal of the diode board to be sure the fairly large red wire and its female connector are tight, not burned, and that the stranded wires into that connector are all intact and crimped into the connector...wiggle to make sure.  
It is important that the battery be good, and the best test is by an instrument called a load tester, on the battery.  You could also just charge the battery fully, then try to crank the engine for 10 seconds or so, without starting the engine, and monitoring the battery terminals (NOT the connections) with a digital meter.  This is not as good as a formal load testing, but is usually adequate.  With the float bowls empty, and gas off, the engine will not start, which is what you want for a load test.  Do not remove the spark want engine compression to make the starter motor work reasonably hard for this test.
Be sure the connections to the starter solenoid, located on the body of the starter motor, are TIGHT.  If they are even a bit loose, that can limit proper charging.
There are a lot of small details areas too, but the above should suffice for a beginning checkout.

Description of the Charging System Parts:

The BMW Airhead charging system is the same; with only some modest variations, on ALL models from the end of 1969 (with the introduction of the /5 series), to the end of any production in 1996.  Changes over the years included a larger stator and changes to the rotor electrical resistance;  an added diode board connection in the 1974-1975 transition period, and some stator winding resistance changes, and assorted variances of voltage regulators.

The Authorities models have varied somewhat; but, in general, they begin charging at a lower engine rpm; but the TOTAL MAXIMUM output is REDUCED; but the voltage regulator is set to a higher voltage.   This is, or can be, depending on other factors, also the effect of later lower resistance rotors; with some late versions rated at ~238 watts.   These details are more for the nerdy (except the /5 information, see next paragraph), as rotors get changed (brushes and rotors are the main failure points, with occasionally a diode board or voltage regulator), and few pay much, if any, attention to the rotor diameter nor resistance when replacing one. I believe it SHOULD be considered, as charging can be quite marginal with the wrong combination, particularly a last version with an earlier rotor.  Re-stating:  the last of the Airheads did NOT have 280 watt alternators, but the charging began earlier, just like the Authorities models, and note also that charging can be poor if the associated ROTOR is not the correct model. 
NOTE:  Some testing seems to show that the latest low ohm rotors do slightly better in all models for lower rpm charging.  

The Airhead charging system consists of a three-phase alternator, a diode (rectification) board, a voltage regulator, the GEN lamp, and the battery, and the various connections, including the ignition switch, etc. 

Since the GEN lamp is fed by current coming through the ignition switch, I include that switch.   Models after the /5 had a KILL SWITCH, and that switch is involved with sending current to the GEN lamp, but it would be unusual for the switch to fail as far as passing current to that lamp.  SOME riders never use that kill switch, so if you are having some problems, I suggest you move the kill switch back and forth half a dozen times and then see if your GEN lamp is working more properly.  When you do testing on the system, you need the KILL switch centered ("ON" or "RUN" position).

As noted, the maximum wattage output varied by model and year and what is in any specific bike, since many parts interchange physically and for many parts, electrically (fully or partially).  NOTE too, that substituting for the last version rotor in a system with an earlier stator, is NOT always a good idea.

The stock /5 had a 180 watt alternator, which can be upgraded to the 280 watt alternator by use of a very specific stock early /6 alternator STATOR; only one version of which will physically fit.  That is the 1974 and some early 1975 production year, but you MUST use the 105 mm stator, and some late 1974 and early 1975 models did have a 107 mm stator, which will NOT fit the stock /5 timing case.  All later stators are 107 mm and will NOT FIT the /5.  BMW apparently phased-in the change to the timing chest casting and incorporating the larger stator at irregular times, perhaps until the stock of early cases was used up?

You will need the /6 or later style of diode board in order to realize the full benefit. 
Variations over the years included adding the mentioned extra wire terminal, which is a center-tap to the stator winding.  That is the extra wire terminal that is on the /6 and later alternators.  To use that connection, BMW added some small diodes to the diode board.  This was done to the 105 mm /6 alternator and later 107 mm.  The stator diameter is the measurement of the stator PORTION that fits a short distance INTO the timing chest cavity.

BMW mounted the diode board on rubber mounts on some models. This was a bad idea, and was never done on all models, see later in this article.  The following models come with cast-in metal mounts, and do NOT need to have 'solid diode board mounts' installed:  /5; /6; 1978-1987 R65 and R80.

BMW also has used a variety of battery ampere-hour sizes (with two basic sizes of case over the years); and, as noted well above, changed the windings on the rotor (resulting in 3 different resistances), and there are several versions of the voltage regulator.   

Nerdy point: There was a rotor diameter change during production, early being 73.4 mm, later 73 mm. It is unclear if this was the R90S change in rotor diameter, which was carried through to the end of production, but it is likely. 

Whilst the tendency is to simply think of the alternators as 180 or 280 watts of output, in truth, outputs are specified at 180, 238 (R90S and Authorities), 250, 280; and the last of the Airheads were probably 240, but it is unclear, as literature is not all that consistent.   The R90S STATOR inside diameter was changed due to the rpm attainable, to avoid the rotor from striking the stator laminations, due to crankshaft whipping. The Rotor was, I think, the smaller one, see the above paragraph.  The larger diameter 73.4 rotor could be used, however.   I believe that the idea of rotor interference was mostly unfounded, perhaps in racing it was seen; or maybe some instances of tolerances accumulation. The Authorities model has the same 238 watt rating as the R90S; but, is designed to produce usable output at a lower rpm than all the others.   Almost all alternator parts are physically interchangeable (the 105/107 mm stators being the exception)....and one might find almost any combination of parts when examining an Airhead charging system on any one bike.  NOT all combinations are wise or will work properly.

The /5 model diode board did not have the extra diodes for the center-tap of the stator winding, as did the /6 and later, and thus this is just one other reason the /5 had a lower output, besides the stator windings themselves (the /5 had a higher resistance rotor too, 6.9 ohms, often just called a 7 ohm rotor).  All the diode boards interchange physically, as far as MOUNTING is concerned, but use of a /6 or later board (with its Y connection to the center-tap on the stator) is mandatory to obtain full rated alternator output from a /6 and later stator. 

The first voltage regulators (there are no current, that is, amperes regulators used on the Airheads, primarily because the alternators cannot produce more than a certain number of amperes) were mechanically operated, in a metal can.  These had a mechanically adjustable relay, the contacts of which fed electrical current to the rotor, and the contacts began to vibrate or open, as the battery voltage (actually, alternator output) reached the adjusted-for value.  The next regulators were similar, the metal can less tall, and inside were electronics for the regulation.  These electronic regulators were also adjustable, the adjustment being sealed by a drop of paint on them, but the regulators could be rather easily modified for full adjustability by removing the paint or replacing the tiny control.  The last of the regulators were in flat plastic boxes, fully transistorized, not made to be adjustable (although possible).  All sorts of information on these various regulators are in separate articles on this website.

In the early 1980's, BMW had many problems with Wehrle brand diode boards.  These would severely overheat at the short-length soldered wires at the 6 large power diodes, due to failure of them to be bent-over and soldered over a wide area.  This is fixable.   Aftermarket high power diode boards are also available commercially.  The stock board is certainly adequate contrary to a lot of false ideas and some misleading advertising.  That does not mean that improved boards are not available.

The electronic regulator should be used on the 1981 and later bikes, as they have need for a smoother, less spikey (electrical noise) output due to the electronic ignition.  The mechanical regulator slowly deteriorates, and an electronic type can be substituted, whether the Bosch, or Wehrle, or even a car type.  Most any VR from a car that has the same three prongs and same case mounting, will work OK, if not perfectly.  I prefer modifying the early Bosch electronic regulator in the metal can, so the adjustment is easier to do (on this website in detail); or, to use an aftermarket adjustable regulator, from such as or   They have them quite reasonably priced.

There is some indication that the last version of the regulators will handle the increased rotor current drawn by the lower ohm rotors a bit more reliably.

The GEN lamp, necessary in the stock system to initiate charging, does not fail often.  There is a modification that allows the system to start up even if the lamp fails (and makes a slight improvement on rpm at which charging begins).....that is on this is article #19.

Description of how it all works:

When the ignition key is turned ON (or pushed, in the case of a /5), a small amount of electricity flows from the battery, through the switch, then through the GEN lamp, and then to the D+ terminal of the voltage regulator (blue wire). In a motorcycle with a KILL switch, the electricity also flows through it.
 The lamp internal resistance acts to limit the current and gives you an indication by illuminating, that the lamp is OK. 
This small current then travels further, through the voltage regulator via its Df terminal (Blue-black); and then to the Df terminal at the brush holder for the rotor.  The electricity goes through one carbon brush into the rotor and then out the rotor via the D- terminal, to the engine ground, which is the same as the battery negative (-) terminal, electrically-speaking.  The connection to ground is made doubly, by the mechanical connection of the D- terminal (it does look insulated), and the brown wire going to it, which goes to the voltage regulator.

This makes a "complete circuit" in electrical-speak, and the small current will produce a small magnetic field in the rotor.   The GEN lamp is LIT, brightly.
When the engine is started (and thus the rotor is rotating as it is fastened to the end of the crankshaft), the rotor magnetic field produces a magnetic field in the non-moving STATOR windings.  Thus, rotor magnetic field is transformed into Alternating Current electricity in the non-moving stator windings.  Small  diodes on the diode board are used to change the electricity into Direct Current, and the electricity is applied to the same input side of the voltage regulator that the GEN lamp feeds.   When enough rpm is reached to have about +12 volts supplied at that point, the lamp extinguishes, as it has approximately +12 volts on one side of the lamp, and approximately same +12 volts on the other side of the lamp, and thus no voltage DROP across the lamp.   These small rpm increases...produce the MUCH larger current (than the lamp could supply), through the regulator, that is needed to fully power the rotor.  Thus, the system can be though of as a merry-go-round, supplying itself....once initiated by the battery-fed lamp current and enough rpm. 

There are six large high power diodes mounted on the diode board.  They connect to the stator main output windings, the same as do the above smaller diodes.  As the rpm (and thus the voltage in the alternator stator) rises just a bit more than for lamp-extinguishing, these large six diodes will begin to pass a fair amount of current, but their connection is to the battery.  These 6 large diodes are 3 positive and 3 negative as far as the internals of the diodes are concerned.  More on those 6 diodes in the next paragraph.

The diode board connections (via printed copper 'wires') have 3 large diodes to the chassis (negative side of battery) and 3 large diodes to the battery + terminal output of the diode board. The + battery charging output of the diode board is the larger spade terminal on the right side of the board as you face it from the front of the motorcycle.  That connection is NOT FUSED and depends on the extremely high reverse resistance of the diodes to prevent the battery from discharging back into the alternator when the engine is not running (key on or off).   The same system is used in most cars and trucks.

If the rpm is high enough, or system needs low enough, the alternator could produce too high a voltage into the battery and the rest of the bike.  The voltage regulator has an internal voltage reference (it is called a Zener diode), and the regulator receives the output of the mentioned six small diodes, as previously described, and compares that to the internal reference zener diode (mechanical regulator does this with spring and coil).  When the voltage rises excessively, the voltage regulator reduces the current flow into the rotor, which reduces the stator output. 

The mechanical regulator regulates by separating two contacts in the regulator to reduce the charging.  Separation of those contacts occurs as voltage rises and increases the mechanical relay's magnetic field in its coil.  Contacts will actually open and close rather rapidly once the regulation set voltage is reached, thus they spark gently during actual voltage regulation.  This produces some electrical noise into the well as deteriorating the regulator's points, although slowly.   The later low ohms rotors are really a bit much for the mechanical regulator (which was not used after the 1970's), as the rotor could draw more current than the mechanical regulator points were designed for.

Performance and failures:

The system in any of the Airheads is adequate, particularly if you keep the rpm above 3000.  Those with much higher system usage...such as larger wattage headlights, many extra other lights, heated clothing, ETC., may need to make accommodation for those things.   Ask on the Airheads LIST; and, read the various articles on this website; particularly the one that compares the Bosch alternator output with the aftermarket alternators.   The /5 180 watt system is adequate for the later 55/60 watt headlight (original was 40/45 watts).....and maybe a few smaller lights; it can also just barely handle a 50 watt heated vest....but this is the limit for practical purposes; but NOT adequate for in-city stop and go traffic.    

What fails?  Anything and everything has failed at one time or the other:

The GEN lamp, necessary in the stock system to initiate charging, does not fail often.  There is a modification that allows the system to start up even if the lamp fails (and makes a slight improvement on rpm at which charging begins).....that is on this is article #19.

Rotors fail due to aging of various types, and rotors are either rewound or discarded for new ones.  Without measurements, you may not know what rotor you have, and you may need to know what you should have.   For the most part, rotors are being rewound with the later lower resistances..  These work better than earlier 7 ohm rotors.  It is not a great idea to install a 3.4 or 2.8 ohm rotor into a mechanical voltage regulator bike, without changing to the later electronic regulator in the plastic case; but, it will work.    

NOTE that the 2.8 ohm rotor may not produce enough charging voltage from an earlier stator.  

Stator failures are NOT all that common; rarely one fails from age failed insulation.   Some have failed from owners changing rotors, and nicking the stator windings in the process. NOTE that the stator windings are done on multiple thin special sheets of steel (generally called laminations), and those thin sheets are coated with a varnish insulation. If you scratch across these sheets accidentally by using metal tools, you can have a stator that seems to work fine, but has somewhat REDUCED output.

Rotor failures, diode board and/or rubber mount failures, wiring failures, voltage regulator failures....and ignition switch failures....all are seen now and then.  

Over and over we technicians find that the MOST common failures are a bad battery and/or the starter relay connections or internally in that relay. The reason for the small starter relay causing problems is that all the power in the motorcycle travels through an INternal jumper connection INSIDE that relay (from 1976).  While the rare internal connection has failed, often annoyingly an intermittent failure because of a loose internal rivet or some such, MORE often the problem is one or two poor connections through the SPADE CONNECTORS at that relay and its socket connectors.  When the jumpering or corroded (even slightly) connections at this relay are at fault, you may have a completely dead (electrically) motorcycle. UNPLUG and re-plug in the relay.  There is a permanent fix, a few paragraphs downwards.

Failure to disconnect the battery before removing the outer cover of the timing chest, has caused electrical sparks and failures of the diode board.   Failure of the voltage regulator is indicated by poor or no charging or wrong voltage.  VR failure is hardly the only failure to exhibit these problems.   Failure of the diode board is usually indicated by lower charging (lower watts) capability, or lower voltage output under load of such as the headlight.  ONE "open" large diode will so indicate by vastly lowered charging with the headlight turned on, yet charging may be OK, headlight OFF.  Problems with the small diodes, while rarer, do occur, and can be seen by poor charging.   In fact, almost anything that goes wrong with the charging system components can result in lower voltage, or lower output/charging.   Overheated or slightly dirty or corroded connections ARE COMMON.   GEN lamps that can be seen glowing dimly at night while cruising are usually an indication of poor connections someplace, or many places.  I usually see such a glowing lamp at night and my first assumption is that electrical wiring has never been cleaned, etc.

GEN lamps that do not light up, and thus you have no charging (or, maybe only at VERY high rpm), are usually a bad lamp, a bad lamp socket, a bad regulator, or, commonly, an open rotor or excessively worn brushes.  Rotors can fail at cruising and higher rpm, and be OK at lower rpm, and the reverse, too!

After the /5 (which had no printed circuit board because it had no instrument POD), the printed circuit board that the GEN lamp fit into can get microscopic cracks, and the lamp connection to the board can fail.  You can carefully repair the board....or replace it (IF you can find a replacement! ones NLA from BMW).  Do NOT yank lamps out on the printed material with a fingertip, rock the lamp out carefully.

From 1976, many models have a connection of the battery into the electrical system located at the starter relay under the fuel tank.   The red wires there can cause problems if there is corrosion at the connections of the relay prongs (spade connections) and socket.  A similar problem occurs if the INSIDE of the relay box has corroded connections, from such as brake fluid or moisture, and there have been instances of a slightly loose rivet causing problems.   The problem can be as slight as just a small voltage drop, or as major as a total complete lack of electrical power.    This is because the INTERNALS of the relay, even if the relay is NOT being actuated for the starter motor, have a 'jumper function'.  These larger gauge red wires, involved in the internal connection, can have the insulation to the wires bared-away......NEATLY!!...and joined permanently, which fixes the problem forever....unless there is an INternal relay corrosion problem.    There are NO problems caused by the external jumpering, if done neatly.  If the battery has voltage on it WHILE the lights are turned-on (but they do not light), you may have a relay plug/socket problem, or inside the relay....probing with a voltmeter will tell you if power is at all red wires at the starter relay, or not.  A temporary fix, if the external spade connections are the problem, is to simply UNplug the relay, and plug it back in.   I again caution, that if you plan to jumper all the red wires at that starter relay, to do it slowly, do NOT break any wire strands, join the bared wires NEATLY, soldering with a hot large well-tinned iron and plenty of rosin flux, then insulate your joint carefully.  I usually use soldering braid or de-soldering braid, to join the wires, but a wrap of common lamp cord (insulation removed!) will do.  Do your cutting, soldering, and insulating, NEATLY.
***If the system seems to be working (Gen light is ON with ignition on, engine off) and there is some charging, perhaps not enough, and you have checked out the rotor, brushes, diode board, etc....check the Starter Relay connections...unplug that relay, plug it back in...which wipes the contact male and female connections a bit...if charging voltage improves, consider doing a careful cleaning job on the contacts.


If something fails, what are the USUAL indications?, and how to determine what is going on:
1. Batteries:  A battery failure can cause an enormous amount of problems and indications.   It is QUITE possible, and, frankly the norm, for a battery to simply lose capacity over time.  That in itself is usually not a problem, as we normally do not operate many hours of accessories with the engine off.  Of course, if severely enough discharged, the starter motor will not have sufficient current to rotate the engine.   One common failure mode is a full separation....or part separation...of a connection inside the battery.  The battery will act as if there is a resistance inserted in series, and may even charge up to the correct voltage at the battery terminals, yet be unable to properly crank the engine.  This is easily seen by measuring the battery terminal voltage, ignition off, then ignition on, then when attempting to crank the engine.   The battery may not crank at all, or you hear just a simple clunk if quite bad in this respect, yet the headlight MAY seem to operate normally...until the starter button is pressed.  

The headlight MIGHT dim to nearly or completely OUT, when cranking.   In a few instances, once the engine IS started and run, the battery MAY seem to act OK....until the next cold startup. It is somewhat rare, but I have seen batteries that were temperature sensitive, regarding the opening and closing of an internal connection.    Another common failure mode is a shorted cell, usually only one fully shorts, but, it can be partial.   The battery MAY never charge up to proper voltage, and if this is seen when using an EXternal charger with a voltmeter attached, the battery can be considered worthless, and must be replaced.  IF partial, the battery might charge up to the correct voltage, yet after standing a half hour or hour, have its 'open terminal' voltage fall towards 12, rather than maybe 12.5 to 12.7.  That battery should be discarded too.  There are dynamic battery testers at most repair shops, that can USUALLY determine if a battery has the proper cranking power.   These are usually called Battery Load Testers.  You can get your own at a quite reasonable price from Harbor Freight Company; which often has them on sale.   I test all my vehicles' batteries once or twice a year, and, yes, with a Harbor Freight Company Load tester.

It is possible for bad starter motor to 'pull' or 'draw' so much current from the battery, that the battery appears faulty.  This is commonly seen with the early Valeo starters in which the pole pieces, which were glued in place, come loose and lock up the starter.  The Bosch starters are FAR better constructed than early Valeo starters, and the Bosch are easily re-buildable anyplace...they are nearly 100% interchangeable, but the Bosch needs the forward end support plate, and the early versions were 8 teeth, later versions 9 teeth, to match a change in flywheels...and you MUST use the correct number of teeth version.    Late model Valeo starters are OK, and draw less current than the Bosch, and has them reasonably priced....see my references page.  They are a GREAT source for electrical parts for your bike.

A load test at about 90 amperes on the 28 or 30 AH batteries (and 70 on the smaller batteries), while watching the battery voltage, is about correct for the Airhead batteries.  The load test is done by reading the voltage accurately at 15 seconds of loading time.   There is a chart available for the lower voltage limit, versus temperature....and usually the load tester comes with the chart.  

2.  Diode board and rubber mounts and brush-holders, etc:   On those models with rubber diode board mounts, the mounts will eventually deteriorate, and then allow the diode board to fall downward and cause shorting or other problems.   In MY OPINION ALL rubber mounted diode board models should have the diode board mounts changed to all-metal.  Not only is reliability improved, but grounding and output and regulation tightness of the voltage is improved too!   There is some evidence that the diode boards stay cooler and more reliable.  Rubber mounts are a BAD IDEA!   Diode board failures can cause everything from low output and charging to no output and charging.  

The Airheads that already should have the solid metal diode board mounts cast into the timing chest (check your bike, no matter the model) are:
/5 models; /6 models; 1978-1987 R65 and R80 models.

NOTE that the RS and RT models are particularly prone to diode board problems...PLEASE do yourself a favor and install these cheap diode board solid mounts.   Euromotoelectrics; Motorrad Electrik; Thunderchild.  It is a good idea to add openings/louvres in the front lower plastic cover that is in front of the engine on models using it.  Early ones had no louvres; later ones did, and that improved cooling air to the timing chest (where the alternator is).

Once in awhile I hear of someone installing solid mounts; or, for some other reason, they have had the diode board out of the bike, and when replaced, there is no charging.  They have probably mis-wired at the rear of the diode board. 

No charging, and a bright GEN lamp, can come from improperly re-assembling the rotor connections' white-colored brush connection block....the insulating washers must be properly assembled at the Df terminal!!!

NOTE:  Over the years there have been various problems with diode boards, grounding, the rubber mounts, etc.  PLEASE read the article on the diode boards.  PLEASE read the other electrical articles on this website....#14, #15, #15-A, etc.  Some of the problems were also caused by extra heating due to fairings on some models, and the change in 1981 (most countries) to the square air filter...which included a change to the cooling air flow through/past the diode board....into the starter area.

NOTE:  Oak Okleshen published an extensive article on testing the diode board using a transformer and lamp, in the Club publication, AIRMAIL.  June 1999 was a most comprehensive article, but there are many others.  Buy the Airtech Index  from Oak:

Information on testing using the transformer and lamp method is also on this website, but see the next paragraph...

Diodes are usually tested (disconnect the bike battery first!!!) by using an ohmmeter, first with the leads in one direction, and then reversing the ohmmeter leads, so that forward conducting resistance is measured (ohmmeters have batteries and pass a small current through the probes) in one direction, and hopefully extremely high resistance in the other (NON-conductive) direction.  A test with the diode board OUT OF THE BIKE using a 6 or 12 volt brake or turn signal light bulb and a 5 to 15 volt AC transformer is a VERY good test, and HIGHLY recommended, as it IS a better test.  A series circuit is used.  Shorting the leads gives full lamp brightness.  Connecting across a diode gives roughly half-brightness.  Anything else and the diode is faulty.  Both tests, ohmmeter and lamp/transformer; are a good idea.   Many multi-meters now incorporate a Diode Test function.  The function typically reads the forward voltage drop of the diode, with a small current applied.  It is roughly as good as the ohmmeter test (properly interpreted), and the AC transformer and lamp test is MUCH better than either.  Yes, you CAN use an ohmmeter on the diode board, with it fully installed and connected...but, you MUST disconnect the battery first.

3.  Voltage Regulator:  Usually a failure here means an opening of the series pass transistor (electronic versions of the regulator) and no output/charging.  Other failures have been seen. Rarely the internal regulator series pass transistor short-circuits, and the indication is always a vastly too high charging voltage at higher rpm. 

The mechanical regulators usually fail by slowly deteriorating the output voltage.   A failure to charge, in which the regulator is suspected, can be PROVEN, with the mechanical regulators, in the SAME way as discussed very early in this article, by bypassing the regulator.  This is done with a jumper wire with male spades in each end.  Remove the regulator plug, and insert the jumper into the opposing (NOT BROWN WIRE!!!) plug connections.  If the system now charges, replace the VR.  
Best sources for voltage regulators:

NOTE:  a somewhat rare condition, that can drive you nuts, is when the ground wire (brown) from the voltage regulator is OPEN, either from a faulty wire or connection; OR, from corrosion or other problem with the three pin plug at the VR.   An open ground connection will generally result in a WAY high charging voltage....upwards of 20 volts is possible, depending on the condition of the battery.

4.  Stators:   Stators seldom fail unless abused.   Abuse is usually nicking or otherwise injuring the wires during stator/housing removal. Nicking the steel laminations can reduce output.    Due to the low resistance windings, ohmmeter tests on the three phases themselves are not usually or always indicative.  HOWEVER:  There must be no continuity to ground from any stator terminal (stator disconnected from anything else including, of course, the diode board).   There is a difference in the resistance of the windings after 1990, but this is mostly a point of nerdy discussion, unless you are trying to use an earlier higher resistance rotor with it, where charging can be poor in voltage output.    Stator failure means low output/charging.  The group of three connection wires to the one area of the stator can be in any order.   Some models have a molded plug for these three.     Typically, a large failure involves a shorted turn or a grounded stator.  The shorted-turn failure can be very tricky to analyze, because while an openly visible shorted turn is easy to see (typically from those using screwdrivers to pry the stator from the engine, when changing brushes, etc....), the resistance measurement may not show any difference between phases, on common ohmmeters.
Some types of tests would be Ohmmeter tests; A.C. connection to a transformer and then read voltages or use an oscilloscope; A.C. ammeter tests, and several other types of tests can be done.
One of the better and easier to do set of tests would be the following; all can be done with the system on the bike, and are done with the phases and centertap (if present) wires disconnected from the harnesses to the stator (all simply pull off the stator spade lugs):

    a. Ohmmeter tests, between phases, and to ground.  Short the ohmmeter leads together, and subtract
        the meter and its leads indication from the readings you get from connecting the ohmmeter test leads
        to every combination of the three output spades of the stator.  The stators are typically 0.62 ohms
        per phase connection.  If you test the centertap terminal, located on the other side of the stator case,
        the values between that center tap and any of the three main phases will be ~HALF the phase to
        phase value.   NONE of the a. tests are the best testing method, the resistances are hard to read
    b. With Df harness wire disconnected, and D- grounded and Df connected to battery +; measure
        phases with an A.C. Voltmeter, at ~2000 rpm.  The voltage should be about the same between any
        connection type of the three terminals stator output group at the SAME rpm.
    c. Repeat, but with A.C. Ammeter....properly connected!  Few of you have an A.C. ammeter, so this
        test is seldom done by anyone.
    d. The tests can also be run with a common headlight lamp, for equality in brightness, instead of meters.
        Simply put the lamp across any phase connection. Don't raise the rpm too high.

        FAST test for stators on /6 and later types:  disconnect the 3 phases and centertap connections. 
        GROUND the centertap.  Use a test light, at just a bit over idle rpm, on each of the three stator
        terminals, the other end of the test lamp to chassis.  All should light up the same.  The lamp tests
        are really quite good ones.

        A shorted turn makes a rather gross difference in output from a phase.

5.  Rotors:    Rotors rather commonly fail, they rotate fast and can have large centripetal forces on the windings when up and down shifting as rpm change can be abrupt.  I have theorized that some types of clutch and shifting of gears can cause very high reversing forces on the rotors, particularly susceptible will be the old non-epoxy impregnated ones.    Rotors are in an area that gets hot, and the current flowing into the rotor plus the transformation of the magnetic field adds more heat....and thus there are heat-cycling effects.  Same can be said for the diode board, which is susceptible over the long term to this type of heat cycling effect.   In the rotor, its wires are soldered to the slip rings. Solder joints can fail.   Original rotors were sealed with a type of varnish, and are not as good as later rotors that might have been sealed and protected with high temperature epoxy (hopefully by vacuum impregnation)....and thus the wires can move about from vibration and centripetal forces on original old rotors and be damaged.  Rotors usually OPEN, and a simple ohmmeter test across the slip rings will identify that.  An open rotor or brush will usually not allow the GEN lamp to light up.  A rotor can work (or sort-of) at low or high rpm, and then not at the other rpm.  GEN lamp indications can be erratic with rpm too, signifying an opening and closing of the rotor internal connections. It is possible to use an ohmmeter on a rotor with the engine running over a range of rpm, with the rotor D- and Df connections disconnected, but a better method may be to monitor rotor VOLTAGE as rpm is raised.  A smooth moderate change is OK, but a sudden instantaneous drop is not.

Once in a while a rotor will short circuit, and this may or may not be such that an ohmmeter can show a short.  The above voltage check is usually telling. 

Another form of rotor shorting is slightly different:  one wire passes through the rotor steel core through a small hole, a place for short circuits if the rotor is not very carefully assembled during the winding process. The ohmmeter WILL then show a short to the rotor steel frame.  To properly test for this whilst the rotor is in place on the motorcycle, you must slide a piece of paper under both brushes. I have seen rewound rotors improperly tested before sale, that were shorted.   

Rotors are easy to remove, but you MUST use a hardened factory, or HARDENED home-made tool. 
DO NOT USE A COMMON SOFT BOLT.  8.8 is the minimum specification!!   Under NO circumstances should you use a common automotive 'legs' type of tool, nor a NON-hardened bolt!!!

>>>>>DO NOT fail to use a hardened tool. I VASTLY prefer the ONE PIECE tool.

The BMW-sold tool is 88-88-6-123-600.
You can get the equivalent tool from such as, etc.  They are NOT expensive.  You can also make a tool from 8.8 bolts....ask on the Airheads LIST. I am NOT all that much in favor of this multiple piece tool method....due to the possibility of wrong use, and canting of the inner part, causing you HUGE problems.

RARE situation:  a magnetized rotor


If you get no output from the alternator, and you have checked the diode board, checked for proper and functional connections, checked the function of the GEN lamp, checked the stator, checked the rotor resistance (and for any resistance to ground from either slip ring), could have a RARE instance of a pre-magnetized rotor.  This very rare situation, typically happening with a replacement rotor.   NOTE that the D- terminal is the same as engine case ground, and that the Df terminal is insulated from the case, and that the D- terminal is the one that should have the SOLID BROWN WIRE.

This section is somewhat technical.   The fix is to have everything installed and properly wired to start with.  Then, unplug the connector going to the brush holder Df terminal.    Do an ohmmeter check of the D- terminal to the case, it should be ~zero (after subtracting the meter leads if they are shorted together).  Then do an ohmmeter check of the Df terminal of the brush holder, to the case.  You should read the resistance of the rotor (same as if across the slip rings) PLUS a modest fraction of one ohm, to account for the two carbon brushes.  As a final measurement, slide a piece of paper under the brushes so the brushes do not contact the slip rings.  Recheck your measurements...and add a measurement from either slip ring TO THE CHASSIS/ should be in the millions of ohms, if anything is shown.  NO SHORTS or near shorts.

To properly magnetize the rotor, remove those pieces of paper.  With the Df connector still removed, connect a piece of wire (any jumper wire from 14 to 22 gauge will work OK) from battery + to the Df brush holder terminal.  The battery must be connected to the frame and engine wiring, so if you disconnected the battery before removing the front aluminum cover, reconnect it for this.  Connect the wire for only a second or three.  That will properly polarize and magnetize the rotor. 


The stock Bosch rotor used on any of the Airheads, is not designed, necessarily, and certainly not specifically, to work as a residual magnet. By that, I mean that while it certainly is an electro-magnet, its design was not specifically to keep some sort of residual magnetism in its pole-pieces. Saying this differently, the rotor structure is not like a permanent magnet you use to post notes on your steel refrigerator.

It may well be that a very tiny residual magnetism is left in the rotor structure, most, I think, do have that, and it is THE reason that if the GEN lamp fails, you can sometimes get the alternator to start producing current, typically only by beginning at a very high rpm.

If it was possible, at reasonable cost, WAY long ago, to have the rotor structure be a strong permanent magnet, which could be enhanced during rotation by the stock method (described later herein), Bosch might have done that, ....I do not know. High performance magnetic materials HAVE been available for some time now, at considerable cost (China is purchasing as many rare earth mines as it can, rare earths are needed), but the original rotor was never re-designed, ...and here I again speculate: as there was no specific need. As a side-note, I have thought that such an upgraded structure might well produce usable current from the otherwise-stock alternator system at a much lower RPM than the stock one does. THAT would be nice for commuters, that is, city riding, stop and go folks. NO proof, and to my knowledge, no one has produced such a rotor for the stock Bosch system.

Certainly, 100% permanent magnet type rotors (NO rotor windings) for alternators have been designed, with huge numbers produced on stock bikes, which have some interesting characteristics. There is an aftermarket alternator for the Airheads, the original EnDuraLast, it produces a fair amount of current at lower than stock rpm. A description of its performance, and for the original Omega, versus the stock Bosch, is in an article on this website.

I do NOT know any specifics on the stock Bosch rotor steel, nor for any aftermarket-made rotors. It could be that the magnetic properties has been different on them. For this reason, rotors MIGHT vary in residual magnetism. I have never done formal testing to find out, as I was not curious enough.

Back to the stock Bosch:
The alternator requires a rotating magnetic field for charging/output to begin. In the stock system, the INITIALIZING magnetizing current is supplied by the battery, through the ignition switch, through the GEN lamp, and through the VR, then through the rotor. This initializing current is small, with the main current limiting item being the GEN LAMP. Thus, the GEN lamp allows only a quite small amount of current to flow through the rotor, which magnetizes the rotor (at a minimal amount). The rotor has a very low resistance (amongst other characteristics nor conducible to being magnetized much), compared to the lamp, particularly when the lamp is lit (lamp resistance rises when lit), so the magnetic field really is miniscule.

When the rotor rpm rises, its rotating magnetic field induces a modest amount of current in the STATOR windings, first very tiny, then a bit more as RPM rises, and if the rpm is enough, and the system needs the electricity, several amperes is produced. That AC current is rectified to DC by the SMALL diodes on the diode board. This DC current is then applied to the Rotor (via the Voltage Regulator), to GREATLY ENHANCE the rotor current that existed (just from the lamp current).

If you are getting the idea that the alternator is SELF energizing above a modest RPM, you'd be absolutely correct. The rpm for this varies somewhat with system condition and with the model of rotor and stator, but typically is strongly evident at a few hundred rpm above idle. The point of transition is approximately when the GEN lamp EXtinguishes.

Note that the LAMP-allowed initializing current is quite low. Any residual magnetism in the rotor, without the lamp current, is non-existent or extremely low.

IF the rotor happens to have its quite low magnetism in the reversed direction, then the lamp current must overcome that, to cause the proper direction magnetic field. If the lamp current can not overcome it, then there is no output, and the lamp would likely just stay lit. The rotor needs to be magnetized in the correct direction. THAT is what the re-magnetization does...>>>one hopes.
Just how the rotor might become magnetized in the first place in the wrong direction, is a matter for speculation. Since the D- brush connection is always grounded (one hopes), it is unlikely to have a rotor wrongly magnetized from reversing the two connections. Wrong magnetization could easily come from rotor TESTING, if DC current was used, or, wrongly used AC current. It is possible for a rotor to have a default during winding, which could cause the problem.....any sort of grounding would especially be suspect. Note again, that it is possible that some rotors are more susceptible, due to the structure and materials.


6.  Brushes:  Brush failure is common (there is an article on this website about replacing the brushes).  Brushes are a hard conductive carbon material and wear slowly.  When the brush is short enough, the snail spring that presses the brush against the rotor slip ring will bottom out on the plastic brush holder, and thus there is no longer any...or only slight...brush pressure against the rotor.  You may have strange indications on the GEN may seem to light up properly, it may not light up properly, it may not light up at all.   It may be rpm sensitive.  If the slip rings show approximately the proper resistance, yet the resistance measurement at the brushes themselves (Df to D-) is not approximately 3/4 of an ohm higher, the brushes are likely not contacting properly...and a physical inspection is needed.  Brushes last ~80,000 miles....depending on the dust in the atmosphere, amount of wattage normally used, etc.   See the brush replacement article. 

As noted above, as the brushes wear to the usable limits, the snail spring starts bottoming in its slot.   At just the right amount of wear, the very very slight sideways wobble of the rotor or irregularity of non-concentricity of the slip rings themselves, while the engine is running (at some particular rpm usually), will allow the brushes to either contact.....or not contact....causing very irregular GEN lamp and charging.  Be aware of this phenomena.
SEE article #17.

NOTE!   It is rare, but replaces the brushes, and now there is zero charging!  You probably removed the brush holder assembly, and failed to reassemble it correctly.  The D- terminal is grounded to the case, the Df is insulated from the case, even though a first glance shows them identical.  You ALSO MIGHT have put the two push-on wires on the wrong terminals.  BROWN is ALWAYS ground (D-, in this instance).   The BROWN wire MUST be to the brush D- terminal!...if you reverse the brush terminal wires, the alternator will not work!  You do NOT have to disassemble the brush holder mounting for a brush job.

BE SURE that if you measure the resistance of the rotor at the slip rings, you also measure the slightly higher resistance you should normally have at the brush holder output terminals D- and Df. 

The brushes rarely have had the flexible wire they connect to the holder terminals, break, sometimes invisibly.
   You can pull a bit on the wire, as if to pull the brush out of the holder, slightly, to see if the wire is intact. 


7.  Miscellaneous items:  
NOTE!....if you had the diode board out, and you have no charging, you probably failed to connect one wire at the rear of the diode board!
Key switches may develop intermittent and excessive resistance, which can confuse the voltage regulator operation. 
Wires can get pinched. 
A rare short circuit in some /7 and later bikes... at the two-wire connector near the top front of the engine...the blue wire.... may fray and short to the frame. 
Another wire problem may be the blue-black wire from the Df brush holder to the voltage regulator Df terminal.  
If the POSITIVE heat sink of the diode board is grounded (the board has insulators for JUST that top section) from such as wrongly placed wire or failed insulator, etc., then the board output will be shorted to ground.  The board output connects via a heavy red wire to the battery!!  Other causes for a ground at that diode board output (large spade, right side, as you face from front of the bike) are shorted diodes, maybe with a shorted stator. 
Wires (well, the end connections) can overheat and therefore have essentially too much resistance.  This is seen at two places commonly:  The alternator stator terminals (the group of three)...but is not seen at the center tap of the stator windings (/6 and later).  It is seen at the other end of the group of three wires....behind the diode board. 
Sometimes one sees overheating at the larger gauge RED wire at the right lower side of the diode board (as you face the board from in front of the bike)....that is the battery direct connection.   The female spade connectors can be carefully squeezed to fit much tighter.
There is no need for the insulating black covers over the leads to be perfect, if yours are cracked and partly missing, which is commonly seen.

It is rather common to find hairline cracks in the flexible printed circuit board material inside the instrument pod, which, if in the GEN lamp circuit, can cause the GEN lamp to not work, and thus NO charging.   Rarely, but not all that rare, there is a problem with the pod CABLE PLUG, or the pod side of that.
There is an article on this website about modifying the lamp circuit so even if a lamp fails, charging will occur.

Fuses:  Fuses are generally not used in the charging system, certainly not in the output from the diode board to the battery.  There is one exception, and it is the later /5 model that has fuses in the headlight shell, and some /6 models.  On these, one of the fuses opening will shut down the GEN lamp.

Front Covers:  /5 covers had poor ventilation.  Using a later cover will improve reliability.


The following graphic came from:
I have modified the sketch with some identifications of wire colors and small diodes.
There is information on that page, but there ARE ERRORS.  I am listing it here just because some know about it.  Please do not of 11/05/2014 errors were STILL not fixed, after YEARS of him knowing.

A very complete book, written by OAK, on the entire electrical system of all the models from somewhat before the /5, to the end of production, is obtainable from the Chicago Airheads Club.  This contains all the schematic diagrams of the entire bike systems, as well as very detailed individual parts and systems drawings, schematics, descriptions and operations, etc.  This book is really a training manual, and does contain some troubleshooting information.  I consider it a must-have item.     See my URL page on this book, and there is a critique of it on this website too!

A smaller book on the airhead charging system, but quite adequate, especially for troubleshooting, with illustrations and practical advice, is available from Motorrad Elektrik.    (256) 442-8886

Final release:  01-03-2004
11-14-2005:  add hyperlink for additional schematic
02/23/2009:  added some clarification to rotors and stators
05/09/2009:  recheck, fix minor typos and clarify a few details.
11/21/2009:  review and update entire article.
08/13/2010:  Review and minor updates, mostly for clarity
01/15/2011:  Add the Thunderchild gif, modify information about buchanan's page.
09/18/2012:  Greatly expand section on testing the stators. Add QR code.  Modify Google code. 
                     Go over entire article, and add material and update things.
05/13/2013:  Go over article.  No substantive changes, but eliminate some redundancies and add some
                    clarity here and there.
06/15/2013:  Add first section, troubleshooting the dead GEN lamp remove language button later, due to
                    problems on some browsers
09/08/2014:  Clarify a few details, clean up for narrower screens.
11/05/2014:  Go over entire article.  Simplify some testing for ease in understanding and speed.  Edit
                    entire article as needed for clarity.  Update sources, links, etc.
01/18/2015:  Add section on rotor magnetization; which came from a response I did on the Airheads LIST.
07/07/2015:  Add new section on gen lamp always on, clean up some of the rest of the article (more should be done, when I get to it....).

Copyright, 2014, R. Fleischer

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