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Testing Alternator Voltage Regulators
for your BMW Airhead motorcycle

Copyright 2020, R. Fleischer
article #20

There are articles on servicing/adjusting the various Airhead and /2 alternator voltage regulator models, including making them adjustable, on this Snowbum website.  Almost any voltage regulator found in a junkyard car that your Airhead's plug fits into, will ""function"" in your Airhead motorcycle.  There are sources for new adjustable and non-adjustable VR's.  Just one of these is the Borg-Warner (Pep Boys, etc.) R588, and far cheaper, probably, is the Transpo IB301A which is adjustable.   At last check, Amazon had those for ~$16.    Good sources for inexpensive and quite decent adjustable voltage regulators are  and   Last time I checked, these were about $24, and looked like the same Transpo, but re-labeled    ?


BMW offers a higher output voltage regulator that some have installed.  It was standard on the Authorities (police) bikes.   Like any voltage regulator that is set higher than the original factory model, it may cause you to check the water level more often if you do considerable highway miles and if you use a flooded type battery.  It will also cause the incandescent lamps to burn out slightly sooner (very small effect though).   This regulator ends in part number -737.  It is rated for 14.5 volts at 20C; 14.3 volts at 30C.  I would not recommend its purchase from BMW due to high cost.  If you want a higher voltage capable adjustable regulator, try the mentioned sources, above.  If your Airhead already has a Bosche or Wherle metal can electronic voltage regulator, it can possibly be easily modified to be fully adjustable, as most came adjustable, but the control was sealed by a paint dot. How to do it article is on this website:

The mechanical regulators can be serviced and adjusted for voltage.   See article 21. Articles on the voltage regulators as a group, are 21, 22, 23A, and 23B.

I recommend voltage regulators be set to 14.3 to 14.7 volts, for use with flooded lead-acid as well as sealed AGM and Gel type batteries.  Battery life is improved at 14.3 volts or slightly higher.


Method #1: Press the plug connector at the little tab that releases the plug from the voltage regulator, and now you can UNplug the regulator.   In the socket plug, jumper the opposing two female's, they are likely blue and blue with black stripe. DO NOT jumper to the brown wire.  How best to do jumpering?  This is a good time to obtain two standard male spade connectors, a 4 inch or so piece of insulated stranded copper core wire, and use these to make a jumper wire to keep in your on-bike tool kit.   If the regulator ever fails in the open condition, you can get home with the socket jumpered, if you keep the rpm down so the battery does not overly 'cook'. In an emergency, make a jumper with almost any piece of wire, bent paper clip, etc.

With the socket jumpered as described (NO connection to the BROWN wire), and monitoring the battery, or at least the bike's voltmeter (if it has one), start the engine and slowly raise the RPM.  The GEN lamp should go out (if it does NOT, then stop, as you have other problems than just the regulator), and the voltage should begin to rise fast as rpm is raised, especially if the battery is near fully charged.  Don't have the engine at high rpm very long, as the voltage might rise to over 15 volts, cooking the battery.  This test BYPASSES the regulator (after all, it IS disconnected!), and the alternator should put out whatever maximum it can.  If the alternator produces lots of electricity, and the reading comes up to at least 14 volts at the battery with the regulator bypassed (but will not come up to at least 13.6 with the regulator in place), then the regulator is likely faulty. NOTE that a stock fairing voltmeter may read about .3 volt, sometimes as much as .5 volt, lower than a voltmeter would if attached AT the battery.

Method #2:  This assumes that you have good reason to remove and bench test the regulator.   This is a generic method, definitely not for everyone, as you need an adjustable power supply. A regulated power source of high stability is NOT needed but nice. However, the voltage should be a fairly 'clean' DC ...that is, not too much A.C. riding on the D.C.    Most 'bench' power sources with any sort of built-in (or you adding) capacitor filter will do. The power source must supply enough current to operate the lamp you select.  Probably a source capable of at least 2 amperes is adequate, as you could likely draw 5 amperes without damage, at least for a few seconds at a time, and that IS all that is required.  If your power source is rated higher, fine, no problem.

You can do this with ANY of the voltage regulators on the Airheads, mechanical or electronic, and this includes all the Bosch and Wehrle regulators ...and includes aftermarket regulators and those from a car.

The electronic regulators were used with a different rotor, an early one and a later one, with somewhat different resistances.  The electronic regulators can be used on all years of airheads including any from the /5 to the end of production of Airheads in 1995.  The mechanical regulator MIGHT NOT last too long if used with a 1981 and later rotor or a rebuilt rotor with low resistance (the original rotor for the /5 era was 7 ohms).  I have no proof of this, it is purely conjecture on my part, and I have never tested the idea. Use of a mechanical regulator in a 1981 and later Airheads should be considered a temporary measure, as the vibrating/arcing contacts, which produce a bit of electrical noise in the motorcycle's electrical system, might upset the electronic ignition.  This would be more likely with a poor old tired battery.

Regulators on BMW Airheads all have THREE male spade connections. One is marked D+, one is marked Df, and one is marked D-.

Connect the variable power source (power off!) positive (+) output to D+; and the negative (-) output to D-. Connect an accurate digital voltmeter to those same Voltage Regulator terminals.   It is best to connect the voltmeter at the regulator terminals, not at the power source terminals, to avoid any possible voltage drops to the connections at the regulator.

Connect a load between Df and D-.   I recommend a 12 or 14 volt (nominal values) LAMP ...such as an old car headlight with one section still working, or a taillight bulb in a convenient socket, or whatever you have.  A tail brake lamp is OK.  Don't use a load that draws over about 5 amperes at 14 volts, which is 70 watts.  Because you are going to be 'looking for' a triggering point, and test circuit readings will likely instantly change at that triggering point, I recommend a LAMP [and NOT a resistor and another voltmeter].

Turn on the power source, and VERY slowly increase the voltage, noting that the lamp will begin to glow if the regulator is capable of passing current.  As the voltage is VERY SLOWLY increased, the lamp will get brighter.  Keep one eye on the digital voltmeter, and the other eye on the lamp.   When the voltage reaches the VR set point, the lamp will suddenly turn off completely.

NOTE the voltage JUST BARELY the tiniest amount before the voltage is at the light extinguished point. Back off the voltage or turn the power off..... and retry several times from a much lower voltage. You want to actually know the voltage at which the lamp turns off, but your power supply MIGHT JUMP UPWARDS when the lamp turns off, as the load has gone to zero you will have to estimate the point, as close as you can, JUST BEFORE the lamp extinguishes.  You should be able to get within 0.2 volts of correct value, or even less, with several tries and a smoothly adjustable power unit.

02/03/2003:  Add top of page information on adjustable and other regulators.
04/22/2003:  Add .htm title
05/30/2004:  Add article number to correspond with T. Index; add voltage/temperature information on -737 high output regulator.
11/22/2009:  Slight updating for clarity and URL's for sources.
04/13/2010:  Minor emphasis and wording changes.
04/26/2010:  Fix hyperlinks for Transpo.
10/15/2012:  Add QR code, add language button, update Google Ad-Sense code (language button removed in 2013).
09/14/2013:  Add hyperlinks for the adjustment article.
04/02/2016:  Update with latest metacodes.  Fix layout, fonts, colors, etc.
12/09/2016:  Clean up some HTML excesses.
05/03/2018:  Final html, colors, fonts, cleanup.
05/08/2020:  Minor cleanup and minor clarifications.

Copyright 2020, R. Fleischer

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Last check/edit: Saturday, December 19, 2020