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Troubleshooting the Starting System.
Starter doesn't engage or otherwise function properly.
Starts, or barely, won't stay running, enrichener problems.
You seem to need the throttle to be twisted 'on' some, when starting.
Faulty or otherwise noisy Valeo starters.
Load testing the battery, etc.
Wrong relays?

Copyright 2023, R. Fleischer

It is important that the reader has already read section 16-A. To do that now, click on

I also suggest reading

How to properly start an Airhead engine:

This section was written because quite a few folks don't understand a few things about starting an Airhead engine, sometimes it is because they are new to Airheads.  Sometimes it is because the owner's booklets are not correct, or not correct enough.  Sometimes it is because the new owner has been using an outdated copy of Clymers.   ETC ETC.

1.  One or two versions of Clymers says (my words here) that the CHOKE is to have its lever pointed horizontally (towards the rear) for starting a cold engine, and then the choke is to be turned downwards for running.  Firstly, that is backwards.  Secondly, one Clymers version does not tell you that the wrong or the correct methods...both..... apply only to the EARLY Airheads, ...those have the choke lever mounted on the left air cleaner housing!  Later models have the choke lever mounted on the left side handlebars and operate as you might expect.    If the engine is warm, you may not need any choke, or, only a bit.   You may find that after starting, you need to reduce the choke for smooth running, perhaps even in two or three steps.

2.  The Bing CV carbs do not have specific accelerator pumps, so moving, multiple times, the throttle on and off before or during starting, will do nothing but speed the WEARING-OUT of your throttle assembly gear and cam gear.

3.  If the engine is cool/cold, I recommend you leave the throttle closed (OFF), use about half-choke (more if bitterly cold), and then push the start button.     Reduce the choke setting as needed after the start.  ONE of the reasons I have recommended setting the hot-engine idle rpm to 1025 rpm is because if the setting is too low, a slight amount of throttle might be nearly always needed for a good start-up....because a too low idle (I mean maybe 800-900, engine hot/warm) will not provide exactly the best starting situation. 

4.  Sometimes there are starter motor problems.  Perhaps yours is in need of an overhaul or replacement.  Perhaps it is an early Valeo brand...which has glued magnets, well known to fail, causing the starter to jam, and have quite the battery loading.

5.  There are instances when voltage drops in the electrical system will not allow a good spark to be developed in the ignition coil(s).  While there can be more than one reason.....a quick test is to connect a voltmeter to the coil wire that goes to the ignition switch.  You should have at least 10.5 volts DURING CRANKING.

Engine will not start.  Possible strange battery discharging situation.   Wrong relays installed.

On early models, the starter relay and the horn relay may look identical, but mixing them up will cause a multitude of problems.   See SI 61 002 77 (1035R) of BMW Munich, Sept. 1977.
Starter relay, BMW number 61 31 1 243 207, Bosch 0 332 014 118.
Horn relay, BMW number 61 31 1 354 393, Bosch 1 332 014 406.

Do not mix up the relays. If mixed up, you can test for the situation thusly:
1.  With battery negative lead disconnected, repeatedly touch that lead to the battery post. If a click is heard, the relays are BOTH starter relays.
2.  If relays are interchanged, the engine will not start.
3.  If 2 horn relays are installed, the engine will not start.
4.  It is possible that the battery will be slowly discharged, even if the ignition switch is OFF!

Starter motor rotates the engine OK, but is difficult to fire up the engine, or to keep the engine running, particularly after ~10 or 15 seconds:

Always check valve clearances and if you have ignition points, check their gap, then check ignition timing, ...always do those things first, unless known to be OK. I usually also test for spark (properly!).  If those items are OK, then the problem is almost always in one or both carburetors.  Assuming it is a carburetor problem, there are only a few causes; these hints are for Bing CV carbs:

1.  If the engine will hardly fire up, and if it does so only one cylinder seems to be working, but the other cylinder might 'catch' after awhile ...and maybe you need to use a lot of choke (enrichener) ...then the likely cause is a blocked jet in the carburetor float bowl.  This is a tiny jet in the bottom of a Well in one corner of the carburetor.  Clean that jet, carefully, if clogged.

If the tiny diameter tube leading downwards from the carburetor body, that fits into that Well when the float bowl is assembled to the carburetor body, has a crack on its side, from freezing water in the well area at some time in the past, that needs to be fixed.

2.  Be sure that the idle jet, which screws upwards into the carburetor body,  is not clogged, & that the carburetors are in some sort of reasonable adjustment.

3.  Enricheners were wrongly assembled, such as left and right carburetor parts exchanged; or, the more rare instance of a factory wrongly marked enrichener shaft (lots of photos and information in the Bing CV carburetor articles on this website).

The enrichener cover at the carburetor may have a sucked-in gasket or otherwise gasket failure.

4.  Check enrichener rotating disc to be sure the jets are clean.

Starter motor load testing, including battery condition, load & connecting wires:

Check that + and - battery terminals and wire connectors are clean & shiny on all sides.  There is a long-term tendency for the battery red + wire to corrode the internal copper wires, for up to 2 inches from the + post, so check carefully.Reattach those cables.  Check that the ground wire is in good appearance and tightly fastened at the battery terminal (-) and grounded to the transmission case reasonably tightly (DO NOT over-tighten the hollow bolt) with the proper washer(s). 

The battery must be fully charged.  Disconnect the charger.  If you have a formal load-testing instrument (Harbor Freight's TWO meter version is excellent, but the ONE meter version is OK), then use it.   You can then forget (or, just read) the rest of this testing section; ...otherwise:

Connect a digital voltmeter to the battery's own posts/terminals themselves (not to any other place).   You will be doing TWO tests, within a few seconds of each other. In both tests, the voltmeter black (-) lead will be connected to the battery negative (-) post.   In the second test the voltmeter positive (red) lead will be moved to the starter motor itself (terminal 30). In order to do that test you must have access to the starter motor, so remove the top cover of the engine.  It is a good idea to temporarily disconnect the battery before removing the starter area cover, then reconnect the battery.   The connections must be made where I said.  Do not use a frame or engine as a grounding place instead of the battery negative terminal, as doing so could mask a grounding lead problem.

Put the transmission into 3rd gear (4 speed transmission) or 4th gear (5 speed transmission).  Engage the rear brake strongly, and it is a good idea to engage the front brake, and keep the brakes ON.   On the 5 speed models, you will have to first short across the clutch switch at the handlebars, or ground the terminals, or the starter motor can not be powered in gear.  Do not pull in the clutch, as the idea is to use the starter and have it be heavily loaded, through the transmission, wheel, tire, tarmac.  Turn the ignition ON.

Press the start button, holding it for APPROXIMATELY TWO full seconds. The starter solenoid will make a noise and the starter will try to rotate the engine but can not hardly rotate the engine due to being in gear and the brakes on.  The voltage should not drop under 9 volts (preferably 10+); the engine of course will not start.  Immediately move the positive lead of the voltmeter to the starter terminal; since this would require another hand, either have a buddy there helping you; or, re-do the procedure, again with 2 seconds ON.  The voltage should be not over 3/4 volt less than the first reading.    If less, there is probably a cable/connection problem. Note that this sort of 'locked starter' test is quite severe, drawing a very large amount of current from the battery.

This test can also be done with two digital voltmeters at the same time, thus only one test period is needed, and the results are more accurate.

This battery load test is BEST done with a formal load tester.  In that method, you do not engage the starter motor.   You can set the amount of actual ampere load (usually 3 times the ampere-hour rating of the battery, but I usually use 1/2 the CCA rating of the battery).    The Harbor Freight two-meter load tester has meter scales for temperature, etc., and the test is done automatically.  Use the voltmeter scale amounts for that test, disregard the voltages I mentioned.  NOTE that if the Harbor Freight two-meter Load Tester is used, and combined with a digital meter at the starter motor, you can get further information; and, even more information if the Load Tester is connected to the starter itself!   If the two-meter Harbor Freight unit (or, your digital voltmeter connected to the battery) shows anything much more than a small decrease in voltage during the LOAD checking, the battery may be failing.  I use a load of 1/2 the CCA specification of the battery to help avoid very specific voltage charts for these tests.
The Harbor Freight TWO-meter load tester is adjustable by a large knob on the front, to set the amperes of load, at the instant the wires are connected.    This tester works well on car batteries too.

Starter motor system and problems:

The basic starting system is the same on ALL Airheads: a starter motor, a starter motor solenoid (which is a very large switch), a small starter relay, the starter switch on the handlebars, a battery ...and the necessary electrical wiring.  There are some variations in peripherals, including a special circuit inside the starter relay of the /5 series; and, some variations in the 5 speed models circuitry which includes the clutch and neutral switches.   At its core, the Airhead starting system is very similar electrically and mechanically with what has been in use on cars and trucks since its invention over a hundred years ago.

The starter motor is a powerful electric motor.   To obtain the type of mechanical power needed from the starter motor, upwards of 1/2 to 1+ horsepower may be needed if the engine is quite cold, has thick oil in it, or the engine has high compression ratio.  Thus, the starter motor requires a quite high electrical current.  Over 100 amperes is not unusual.  This INCLUDES NOT ONLY THE STOCK BOSCH OR VALEO STARTERS BUT ALSO THE AFTERMARKET STARTERS.

There needs to be a means of switching on and off the very large amounts of electricity to the starting motor.   The heavy duty starter solenoid switch, physically located on the Airhead starter, is fed by a large diameter copper-conductors-cored cable, directly from the battery.  The starter solenoid itself is activated by a rather small amount of current delivered by the small starter relay.  The 12 volt power travels from the starter relay via a thin BLACK wire that goes to the SPADE terminal on the starter solenoid unit. The starter relay is, therefore, a small relay that sends electricity to another much larger relay, called the solenoid (or, solenoid relay).   When you press the START button on the handlebars, you are simply energizing the small starter relay.

With the ignition key ON, and any KILL switch in the ON or RUN position,  pressing the handlebar START switch button supplies electricity (completes the circuit) to the coil of the starter relay located under the fuel tank.  The starter relay contacts close, sending a modest amount of current to one of the solenoid switch coils via a relatively thin black wire, that ends in a female spade connector that has been pushed in place over the starter solenoid unit's male spade.

The solenoid switch unit is located on the starter motor in the Airheads. The current from the small starter relay causes a substantial magnetic field in the solenoid switch PULL-IN winding coil.   This magnetic field magnetizes a substantial-sized steel slug.  One end of the steel slug has insulated contacts or a insulated contacting strip with flat copper on it, which is pushed towards and strongly touches (slams into) large copper electrical contacts. The force causes secure contact closure.   Now electrically connected, the large battery cable at the solenoid can pass a very large flow of electricity into the starter motor.  At the same time, the Pull-In winding inside the solenoid switch (there are TWO interconnected coils) disconnects and the connection that remains is to a secondary STAY-IN coil that draws less current, giving  more current to the starter motor that keeps the solenoid piece pulled-in and the massive switch in contacting position; and is primarily done this way to prevent pull-in coil overheating.   The starter solenoid is nothing more than an electrically operated high power electrical switch, easily capable of passing well over 100 amperes.

As the starter begins to rotate, a mechanical device called, generically, a 'Bendix' or 'Bendix Drive', begins spinning. It is located on the rear of the starter motor, on the starter motor shaft, somewhat loosely.   This device, via spinning forces and guide grooves/splines, and with massive help from the plunger being linked to the Bendix drive, moves rearward with quite considerable force, and as it moves rearwards it engages its teeth into the outer gear-like teeth on the engine flywheel (called Clutch Carrier on 1981+ models) teeth.  It is those teeth that you may see if looking through the engine timing hole.    This all, together, causes the starter motor shaft to, in effect,  rotate the flywheel, and thus the engine rotates.  The teeth of the Bendix drive, and on the flywheel gear, are shape-designed to enable proper meshing at high levels of force.

It is entirely possible to design a Bendix unit that does not use a solenoid via mechanical coupling, but inertial forces,  to move the Bendix into the ring gear on the flywheel, and this has been popularly done on many vehicles; usually the Bendix unit is reverse rotated by the engine flywheel when the engine starts, and so the Bendix comes back into the original, starter motor not energized, position.  A different method is used by BMW on the Classic K-bikes, and the starter motor has no Bendix unit at all, but is constantly connected via a one-way clutch (sometimes called the Sprag Clutch) to the engine. BMW is not the only motorcycle manufacturer doing something like that, witness the Honda 'Wings. 

The actual operation of the solenoid, electrically, is a bit more complex, and I ran across nice videos that have a schematic and discussion, and here are the links.  Be sure to watch the entire video, first is #2 of a series of 3 to analyze a starter that won't stay engaged:

Here is #3:

You can probably access all of the videos from any one of them, not sure, but it is what I had to do to get #3.  You can probably just scan down the right side list for Moto Phoenix articles. What is particularly good about #3 is that it clearly shows how to take the Bosch starter completely apart.

Later model Airheads may have a Valeo starter motor which has permanent magnets instead of field coils as in nearly all Bosch starter motors.   The Valeo uses a planetary reduction gear set inside its nose, which allows the starter to develop higher rpm and hence higher power.

There is a factory bulletin on the starter relay on 1985 and later bikes (most specifically, 1985-1988), information will be found in my Bosch-Valeo article:

There is also a link in that article to another article on how to do general maintenance on the Valeo starter.


There are at least THREE types of Bosch starters used on the Airheads as original equipment.  I say "at least" three, because there are some very rare instances of other models used on SOME VERY FEW foreign-shipped bikes.

The number of teeth on the Airheads flywheel and starter is always either 8 teeth on the starter with 93 tooth flywheels; OR, 9 teeth on the starter with 111 tooth flywheels.

Bosch starters up through 1974 were 8 tooth, rated 0.5 Kw and 290 A.  The /6 bikes for 1975 and 1976 used  8 tooth, rated 0.6 Kw and 320 A.  The 8 tooth starters are used ONLY with the 93 tooth flywheels; and are, for practical purposes, interchangeable.  BMW phased-in the Bosch starter at different times depending on the motorcycle model, so it is possible for some 8 tooth starters to be on somewhat later bikes ...So it is a good idea to count both the number of teeth on your starter Bendix gear and the number of teeth on your flywheel.

For SOME 1976, and all 1977 and later, the starter has to be 9 teeth, for use with the 111 tooth flywheels  (more properly called the clutch carrier from 1981).  The starter is, rated 0.7 Kw and 320 A.

Valeo permanent magnet starters that incorporated a planetary gear set were installed from the later 1980's, and they are all 9 tooth.  Early Valeo starters were quite troublesome with glued-magnets coming loose and locking-up the starter.   See for fixes; or, new ones, without the problems.

ALL Starters can be repaired.

Starters do not always fit perfectly:
The fitting problem is slight, but can cause broken starter nose-cone's, so do check.

If the starter motor bearings wear considerably, the starter may tend to bog down, and might even start to rub its armature against the pole pieces; if let go too far, the armature is trashed.  Add in worn brushes, grunge in general, worn armature commutator, and poor solenoid contacts, & these things may seriously reduce starter power output.  Early Valeo starters on the Airheads had a habit of the glue letting go on the field magnets, jamming the starter internals.   Valeo replacement starters from such as are modified and don't seem to have these problems.   If you decide to install a Valeo (or other aftermarket starter), in place of an originally installed Bosch, be sure the Valeo, ETC., fits exactly & properly.  See: There are photos, etc., showing the areas to be checked, and if needed, lightly filed. Always be very careful that the starter is properly fitting, and fully, the mounting cradle, and evenly tightened to it.

It is a very good idea to disconnect the battery before removing (or replacing) the aluminum cover over the starter area. The reason for this is the potential (bad pun!) shorting of the cover against the battery terminal on the starter solenoid.  That connection needs proper orientation (to avoid shorts to the case/cover); and has to be TIGHT; ....things to check on before you later re-install the cover.    You need only remove all the wires at just the negative terminal of the battery to ensure the power is disconnected.  If you have ONLY the ONE large wire at the negative terminal, you could disconnect there or at the speedometer cable bolt.   Yes, I do think it is acceptable to modify the battery cable lug, so it will just barely push onto the hollow speedometer cable bolt.  That way, no need to totally remove the hollow and somewhat fragile speedometer cable bolt.   Be SURE the TWO flat washers are on that bolt.  DO NOT OVERTIGHTEN!

If the starter does not work correctly, you need to determine if the problem is in the starter motor, and/or its solenoid; or, someplace else, such as connections, cables, or relay.   Just what your bike's symptoms are will determine your proper approach to your problem.  If the starter does not work at all, the tests may be a bit different.

It is vital that the battery be in good condition.   You can monitor its terminal voltage, and see what the terminal voltage does during cranking attempts AS THE ENGINE ROTATES.  If the starter is very bad, perhaps drawing HUGE amounts of current, that can drag the battery to under 11 volts.  This is a simple test (and is NOT the same as the 'locked' test, well above in this article.   A formal load test on the battery is done in a well-equipped shop; although Harbor Freight has two versions of decent testers available quite cheaply, and they ARE accurate enough.     Monitoring the battery voltage while you try cranking the engine (assuming the starter functions at all) might tell you quite a bit.   If the battery wires are poor, corroded, etc., the voltage at the battery during cranking may be much higher than at the starter.   Monitor the voltages at both places.  Is the battery OK during cranking attempts?   A battery CAN have good terminal voltage at rest, and fall dramatically during a cranking attempt (usually a failed inter-cell connection or a lot of sulfation causes this).   You can also easily test for a bad + cable or bad - cable.

If there is no starter function, no loud starter motor solenoid noise, no (or yes) tiny under tank click noise from the starter relay; then the starter relay, or its sometimes troublesome later model socket connections, are suspects.  A meter or test lamp will easily reveal the problem.  Monitor the battery, and also trace the power into and out of that under fuel tank small box starter relay.  The wires for the input and output power are RED at the relay.  ALL the red wires must ALWAYS be very close to battery voltage, whether or not the ignition key or any other switch is on or off.  The only exception is if the relay is UNplugged.  Test with lights on to provide a modest load, in case of 'just poor' connections in the socket or inside the relay.  It can be inside the relay for all years.

If that small starter relay does make a small light click when the ignition key is turned on (listen in a quiet area!) and the starter button pressed,  but there is no major click/clunk from the solenoid, then check the solenoid, be sure it is getting power to the SMALL black wire terminal (the solenoid's male spade terminal) DURING cranking attempts.  That black wire is the power coming from, you hope, the starter relay.

No cranking?  All seems OK?
Remove (pull off) the black female small wire connection at the solenoid.  Run a temporary jumper wire (any small gauge wire or a screwdriver, etc., is OK) very temporarily from that male spade terminal on the solenoid, to the battery terminal next to it.  If that causes the starter to operate, where it would not before, then your problem is NOT in the starter area; but more likely the thin black wire or the starter relay; or push button ...also possible, the starter relay contacts may be moving but not making electrical connection.  If the starter fails to operate with the starter solenoid connected temporarily, the starter or solenoid is at fault.  A test lamp or voltmeter will tell some or all of the story, at the starter relay (they are openable for cleaning contacts).

Certain years and models of BMW Airheads (after the /5) have various types of starter relay interconnections with the neutral switch at the transmission and the clutch lever switch at the handlebars.  These can introduce complications in your testing.  You may need to find out if power is going through the starter relay.  The easiest way to begin this process is to leave the black small push-on wire DISconnected at the starter.  Key ON, Transmission in Neutral, pull in Clutch lever, emergency shut off switch centered (ON position).  Push starter button:  is there now power at the unfastened wire? If so, the primary problem is NOT in the starter relay.   I like to use an old headlight lamp for this test, saving the ones with one burned-out section, as using this lamp draws about the same current as the solenoid coil does, and is a better test on the starter relay and its contacts.  I connect one terminal of the lamp to the chassis as a ground connection, and the other lamp terminal to the black wire.

Release the clutch lever; does it still have voltage?    If so, the problem not in clutch lever switch.

Transmission to any gear but not in neutral.   Clutch lever pulled back.  Starts?  if so, problem not in transmission switch nor clutch switch.

The /5, ONLY, had a peculiarity; and, the circuitry arrangement needs understanding.   The battery feeds power to the ignition switch, as in all the Airheads, but there was NO handlebars-mounted emergency cutoff (KILL) switch.  From the ignition switch in the /5, the battery power fed a special version metal can starter relay that has had various names, including Starter Lockout Relay.  It is located furthest forward on the left of the backbone under the fuel tank.  This is not just a simple relay.  It contains a simple transistor circuit; which, when stock (that is, UNmodified), is unfortunately overly-sensitive to cold temperature (possibly failing at even modestly cool temperatures) and it is also sensitive to even modestly decreased battery charge.    When the 'problem' occurs, the relay may chatter, or sound like a cricket, and may even power the starter solenoid that way too; making a much louder cricket or rapid clickety-clackety noise.  To an old time mechanic, it sounds just exactly like what happens to cars with a quite low battery.  The transistor circuit purpose was to eliminate possible starter relay operation (as in YOU trying to start an already running engine!) as soon as the alternator is spinning at engine idle or, actually, somewhat above idle.   Bottom line is that there are low battery and low temperature problems with that starter relay circuit, and a modification is HIGHLY recommended by me, and there is an article on this website specifically dealing with that problem, and the relatively easy fix:

Other than the added safety anti-start function, the /5 starter relay coil is wired such that the grounding side of its coil connects to the starter pushbutton switch.  It is grounded when the pushbutton is pressed.   If the starter relay on the /5 fails to operate, it can be corroded inside or might even have a small amount of something on the contacts...both problems are often fixable as it is in a metal can that can be carefully UNcrimped and things cleaned-up inside.  If the transistor fails the relay will not usually work at all.   The modification, see link just above, preventing problems at lower temperatures and/or slightly weak battery, is highly recommended by Snowbum.    There are NO complications in the /5, as to clutch switches, neutral switches in the starter circuitry, etc.

***The slash 5 (/5) ONLY can also have another problem; two possible causes, and the reasons these even exist is due to the above discussed special /5 relay.   If the alternator rotor electrically opens, or especially if the voltage regulator opens, it is actually possible for either of these faults to prevent the starter relay from working.  Said differently: The /5 can have a rare problem of 'no start' ....the starter relay does not work at all, yet all of the starter circuits seem to test OK.  The problem can be an open alternator voltage regulator, or an open rotor.  These are not being discussed further in this article ....except that you won't get any or proper charging if either of those two things happen.

After the /5, some things started becoming more & more complicated in the starter circuits.  One simplification, as there was NO special transistorized anti-start relay, which was deleted in favor of a more standard type.  Naturally, BMW had to complicate even that, much later starter relays contain one or two diodes.

In the early /6  the neutral indicating lamp, which is green (and the GEN lamp which is red) both went through a fuse, before connection to the ignition switch.  The starter relay coil in the 1974 /6 models was wired to the same fuse as the INDICATOR lights if those lights are not working, chances are the fuse is blown!  That particular fusing did NOT continue after that very early /6.

Beginning with EARLY 1975  /6 models, BMW incorporated an emergency kill switch ....added between the ignition switch and the starter relay coil.  Sometimes these kill switches become corroded internally and act strangely intermittent.  They have been known, particularly in 1981 and later models, to, if quite rarely, affect "just" the ignition operation.

It was in the LATE 1975  /6 models, and then carried forward into 1977 and beyond (in some ways), that the beginning of complications came about that drive some mechanics or would-be mechanics sometimes crazy.  As usual, failure to understand the circuitry was the primary problem.   BMW added three things to complicate it all.   The starter button, when pressed, no longer just grounded the starter relay negative side of its coil to the chassis.  While all the 5 speed transmissions from the earliest /6 had a neutral switch, the early models were NOT wired into the starter section, like the later ones were.  Instead the path from that starter relay coil could take TWO directions:

1.   If the neutral switch, located on the underside of the 5 speed transmission, was in the ON position (which means the transmission WAS in neutral), then the circuit was such that the starter relay coil was grounded.... but through a DIODE.  If the diode was working properly, the starter relay was then operable via the starter pushbutton. The location of the diode varied, see 3. just below.

2.   If the bars clutch lever mounted switch was closed by pulling in the clutch, the starter relay could be operated from the starter button, no matter if the transmission was in neutral OR NOT.

The diode was to prevent the neutral lamp from indicating every time the clutch lever switch was activated.  These diodes have been known to fail by SHORTING.  If so, the neutral lamp will light up every time the clutch lever is used.
If the diode failed OPEN, the neutral switch will not allow starter operation UNLESS the clutch was pulled in.

There are complications in the circuit!   THE LOCATION OF THIS DIODE VARIES!   For 1974-1980, except the R45/R65, it is UNDERNEATH the connection board in the headlight shell, where it can't be seen, and is hard to get to for replacement.  For 1979-1980 R65, it is part of the wiring harness, near the Voltage Regulator.   For all the others from 1981, it is INSIDE the starter relay!  THUS, the starter relay from 1981 is a SPECIAL relay; although an aftermarket one can be substituted, and the diode added externally (or internally, perhaps).

Regarding that diode:
The diode located under the connection board in the headlight shell is the one that usually fails, but it is a relatively rare failure.  More rare is a failure of the diode when located elsewhere's.  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 or 600 volts; and 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 (gray or silver stripe) is in the original direction.  I have seen these diodes installed backwards. For the diode when mounted on the underside of headlight bucket wiring board, the banded-end (gray or silver stripe) of the diode is connected to terminal LKK.

In 1978, BMW changed things again.  This lasted into 1980.  These models had an under-fuel-tank brake master cylinder.  BMW incorporated a low brake fluid level switch, which, when closed, turned on a brake failure RED light.  The switch was wired to that lamp via a diode (yes, another diode) to the starter relay coil.  This modification did NOT interfere with the starter operation, even if the diode failed in the open condition.  If, however, that diode shorted (very unusual), and at the same time one had a failed (closed) switch, or the fluid level was low (the switch will close), the starter could fire up mysteriously, all by itself, and NOT release! ...unless the ignition or kill switch was used.   This is an rare thing to have happen.

ANOTHER diode! ....BMW added a diode the headlight relay.  The location of that relay varies with year and model.  The wiring and operation was such that during the time the starter motor was powered, the headlight relay coil was NOT energized, which turned off the headlight.  Depending on model, whether USA or European, this varied somewhat, and in most models the instruments and rear running lamp WERE left ON during cranking, accomplished by the diode inside the headlight relay.  What complicates things here, is, at least for USA models, the diode in the headlight relay was so connected that if it SHORTED, then in some conditions of ignition switch setting, etc., it was possible for the starter relay to fire up, and NOT be releasable, until the battery was disconnected.    Yes, the same sort of thing like the 1978-1980 symptom of the shorted starter relay diode.  Thus nearly similar faults could be had via a shorted headlight relay diode or shorted starter relay diode!

Theoretically this problem could only happen in the PARK position of the ignition switch.  The fuse was also now incorporated in the headlight relay section, and also fed the parking light.    Normally, if you used the starter button, even if the fluid level was fine, the brake failure light would illuminate at every start ...this tested that lamp at each startup.  It wasn't necessary to have the fluid be low, then, to turn on that lamp. At this point you are likely very confused!

With the 1981 models, no longer was there an under-tank master cylinder and the low fluid switch and associated lamp and associated diode was eliminated.  This lasted through the 1984 models.

There were several more variations and changes until the end of production.  These may further spin your head.   In 1985 and later, BMW put the starter button into the + power feed to the starter relay coil, and the negative side of that starter relay coil went to the same diode and the same neutral switch and clutch switch as before.   By moving the position of the starter button connection, BMW could incorporate the diode into the relay housing.   BMW also added another diode in the headlight relay area; this time in series with its coil.  The starter worked the same, however.    The GS and ST, and 1981-1985 R65 and R45 had slightly different headlight relay contact arrangements; which in other models cut other lights during starting besides the headlight lamp itself; ...again, not a starter area to be concerned with.   [for the nerdy:  the power for the instruments and tail light comes from the ignition switch on these].  Earliest R65 models were different in this regards. Why? ...only the elves know.

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 and 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.  Be sure you understand how to use relays, and understand about relays with diodes inside them.  There are other relays from the nearest autoparts stores that may be of use, depending on what you are trying to do.  These relays are SPST, DPST, & DPDT types.  External diodes can be used, if appropriate and desirable.

11/29/2006:  Initial first draft & release.
11/29/2006:  Correct typo editing error in h.
11/16/2007:  Re-number the items, combine certain sections.   Vastly improve clarity of descriptions.
10/03/2009:  Clarify a few things and clean up some messiness.
11/22/2009:  Go over entire article, clarify certain things.
10/14/2012:  Add QR code, add language button, update Google Ad-Sense code.
03/30/2013:  Clarify and add more information to #19.
09/13/2014:  Clean up article considerably.  NO technical changes of any special note.
04/04/2015:  Improve description of the solenoid two coils operation, and copy the section to include it in the boschvaleostarter article.
05/30/2015:  Add section on the enricheners, and clean up and revise article, fix links, etc.
07/21/2015:  Fix section(s) on starter models/teeth/years/numbers.
08/12/2015:  Add video link, some minor article cleanup, additional link and comment on 08/15/2015
08/29/2015:  Add section on load testing and cable testing
03/31/2016:  Update metacodes. Improve formatting, colors, layout, and slightly clarify a few details.
11/19/2016:  Minor fixes to HTML and Scripts.
11/26/2016:  Checkover.  Minor layout and other fixes.  
11/10/2017:  Add information on mixing up starter and horn relays
12/28/2017:  Reduce excessive html, fonts, colors.
04/29/2018:  Reduce HTML fonts and colors even more.  Clean up layout.  Very minor improvements in explanations.

copyright 2023, R. Fleischer

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Last edit of this page: Saturday, August 19, 2023