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Testing Voltage Regulators
for your BMW Airhead motorcycle

Copyright, 2014, R. Fleischer
article #20

Note:  Information on voltage regulators is also found at, Technical Tips section; although it may not be as up-to-date as this article, below.  

There are articles on servicing or adjusting the various models, or making them adjustable, on this Snowbum website and the above website. 
Almost any voltage regulator found in a junkyard car that your Airhead's plug fits into, will work.
There are sources for adjustable and non-adjustable ones.  Just one of these is the Borg-Warner (Pep Boys, etc.) R588, and far cheaper, probably, is the Transpo IB301A which is adjustable.
Particularly good sources for an inexpensive and quite decent adjustable voltage regulators are   and


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 use a flooded type battery.  It will also cause the incandescent lamps to burn out somewhat sooner.   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 due to the high cost.  If you want a higher voltage and also adjustable regulator, try the above sources.  NOTE, however, that if you have a METAL CAN voltage regulator, it CAN BE MODIFIED EASILY to be fully adjustable, as it CAME adjustable, but the control is sealed.
Article is on this website:  CLICK!

NOTE:  I recommend use of an adjustable (or not) voltage regulator set to about 14.3 to 14.7 volts, for use with sealed AGM and Gel type batteries.  If you want longer lamp life, set the regulator to 13.9-14.1.


Method #1:  UNplug the voltage regulator.  In the socket, not the regulator, jumper the opposing two female's, they are likely blue and blue with black stripe. DO NOT jumper the brown wire!!  How best to do that jumpering?...well, this is a good time to obtain two standard male spades, a 4 inch or so piece of insulated stranded copper core wire, and make a jumper wire to keep in your on-bike tool kit.   If the regulator fails, you CAN get home with the socket jumpered, if you keep the rpm down so the battery does not overly 'cook'.

With the socket jumpered as described, 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, telling the alternator, in effect, to 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 the fairing voltmeter may read about .3 volt lower that a battery reading voltmeter would.

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, although I have done this with a far lesser capability power supply.

You can do this with ANY of the voltage regulators on the airheads, Bosch mechanical, electronic, or Wehrle electronic, or any aftermarket regulator out of a car, etc. 

The electronic regulators were used with a different rotor, an early one and a later one with slightly different resistances.  The electronic regulators can be used on all years of airheads including the /5/6/7 and later. 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. Use of the mechanical regulator in a 1981 and later airhead should be considered a very temporary measure, as the vibrating/arcing contacts MIGHT upset the electronic ignition.

Regulators 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 the same Voltage Regulator terminals.   It is best to connect that voltmeter at the regulator terminals, not at the power source terminals, to avoid any possible voltage drops at the connections at the regulator.

Connect a load between Df and D-.   I recommend a 12 volt (nominal) 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
slowly increase the voltage, noting that the lamp will begin to glow if the regulator is capable of passing current. As the voltage is SLOWLY increased, the lamp will get brighter, and if you keep one eye on the digital voltmeter, and the other eye on the lamp, when the voltage reaches the 'regulating' set level, the lamp will suddenly turn off completely. NOTE the voltage JUST BARELY before the voltage is at the light extinguished point. Back off the voltage a bit and retry several times. You want to actually know the voltage at which the lamp turns off, but your power supply MIGHT JUMP UPWARDS when the lamp turns you will have to estimate it.  You should be able to get within 0.2 volts of correct value, or even less, with several tries.

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.

Copyright, 2014, R. Fleischer

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