If you have a charging problem, you may want to find out if the VR (Voltage Regulator) is the problem.  A faulty VR can cause various problems, the most common full failures are no charging or too high a voltage during charging. NOTE that the stock factory VR internal setting is such that the battery terminal voltage is typically 13.8 volts during cruising (the higher voltage Authorities…Police…regulators are about 14.2).  The 13.8v is a reasonable compromise for water use on flooded fillable batteries, but really too low for best battery life, particularly on AGM/VRLA batteries.

There are TWO very easy ways to test a voltage regulator that do NOT involve the technical methods in the rest of this article. These two methods are:

1. Substituting a known good voltage regulator (any 1970+ BMW airhead regulator, or any three terminal automotive regulator that fits the airhead plug).  If whatever the problem was now disappears, then the VR was faulty.

2. If there is no charging, or minimal charging, UNplug the voltage regulator plug.  Short the two OPPOSING terminals inside the female cable plug (DO NOT short the brown wire to anything) to each other. If that makes the alternator go to full output (that is, whatever the alternator is capable of at that particular RPM), and you had been unable to get over 13.7 or whatever previously, then the regulator is faulty. Don’t test at high rpm for longer than it takes to see maximum safe voltage at the battery; that is 14.9 volts.

THE NERDY TECHNICAL METHODS:

This is not for everyone as you will need an adjustable (voltage adjustable) power supply. A regulated power source of high stability is NOT needed, but nicer. The power source should be a fairly ‘clean’ DC…that is, not much A.C.’ripple’ riding on the D.C. Most ‘bench’ power sources with any sort of built-in capacitor-type filter will do. The power source must supply enough current to operate the lamp you select (read further…).

You can do this with ANY of the 3 connection 12 volt voltage regulators on the Airheads….Bosch mechanical or electronic; or, Wehrle electronic; or, any aftermarket regulator out of a car, etc. Results in testing the mechanical regulator are for reference purposes.   Specific information on the  mechanical and electronic voltage regulators, for such as modification, adjustment, etc., are likely on this Airheads website, and in more detail, on the Author’s website:  https://bmwmotorcycletech.info/index.html

NOTE: The electronic regulators were used with one of two available lower resistance rotors…approximately 3.4 or 2.8 ohms, as opposed to the original /5 type rotor, that had approximately 7 ohms and used a mechanical regulator (that was used through the seventies too).  ANY mechanical regulator can be replaced by the electronic type.   Rotors interchange; however, the lower resistance ones have slightly better performance, particularly with the electronic voltage regulators.   The electronic regulators are capable of reliably passing the increased current demand of the later lower resistance rotors, and also will have no problem with the earlier higher resistance rotors. Thus the electronic regulators can be used on all years of airhead back to and including the /5. The mechanical regulator MIGHT not last as as normal, if used with a 3.4 or 2.8 ohm rotor, but, I have no proof of this.  MANY early rotors have been replaced or rewound, so one can NOT go by model and year to know what rotor is in a particular Airhead.   Bosch made two types of electronic regulators.  The earlier one, is of very high quality and is in a metal can, although shorter in size compared to the earliest original Bosch mechanical regulator.   The much later Bosch regulators, from roughly the end of the seventies or starting in the early eighties, was plastic cased, like the Wehrle.  Use of the early mechanical regulator, in a 1981 and later airhead that has electronic ignition, should be considered an emergency measure, as the vibrating/arcing contacts MIGHT upset the electronic ignition.

The regulators all have THREE male spade connections. One is marked D+, one is marked Df, and one is marked D-.    D+ refers to the incoming power to the regulator (from the GEN lamp …..and small diodes on the diode board which supply the rotor current once the rpm is high enough).  Df refers to the terminal of the regulator that goes to the Df BRUSH of the rotor.  Thus, Df is the OUTPUT of the Voltage Regulator.  D- is the ground, or battery negative, side of the voltage regulator.

Connect the variable voltage power source positive (+) (usually RED) output to D+; and the negative output (-) (usually BLACK) to D-.  Have the voltage down to near zero, and the power OFF.

Connect an accurate digital voltmeter to the same terminals, NOT at the power source ends of the connecting wires.   This method avoids any slight errors from voltage drops in your connecting wires.

Connect a ‘load’ between Df and D-. I recommend a 12 volt (nominal) LAMP, such as an old car headlight (save one of these ‘burned out’ lamps; they usually have one section still working).  You can also use an incandescent (NOT LED) brake lamp or a turn signal lamp…if you use those, it is nice to have the proper socket with leads.   I suggest you do NOT use a resistor. Yes, you can  use a resistor, but, because you are looking for a triggering point, I recommend a LAMP [and NOT a resistor and you would need another voltmeter!!!]. .  Don’t use a load that draws over about 5 amperes at 14 volts. For a resistor, that means not under 2.8 ohms.  A resistor of about 5 to 10 ohms and rated at perhaps 25 watts is OK.   Frankly, a lamp is BETTER, as you can watch the lamp for its extinguishing point out of one eye, as you watch the variable voltage on the digital meter!!   Headlamps are rated at 55 or 60 watts typically, and that is about a 4 ampere drain; and brake and turn signal lamps pull about 21 watts.  You can use a smaller lamp, but I recommend one of these three types, as the current drain is better for your purposes here.  That is, don’t use a running lamp or instrument illumination lamp, etc.   You really want to have the voltage regulator pass a more typical real-world current, so use the lamps I suggest….especially the old headlamp.

Turn on the variable voltage power source, and very slowly increase the voltage, noting that the lamp will begin to glow. As the voltage is 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 ‘regulating’ set level, the lamp will suddenly turn off completely.   What you want to do is to know the voltage JUST BARELY BEFORE the light extinguishes. You are looking for the closest to extinguishing the lamp voltage, but NOT the voltage AFTER the lamp extinguishes. Back off the voltage and retry several times. You want to actually know the voltage at which the lamp turns off, but your unregulated (even some regulated ones!) power supply will JUMP upwards in voltage when the lamp turns off, and there is some voltage drop in the leads…so you will have to estimate as close as you can, which IS good enough.

You should be able to get within 0.2 volts of correct value, or even less.
Other articles of mine will describe how to adjust the VR, and to what voltage, if your VR does not have a readily available adjustment.

The following website contains well over 100 articles for mostly BMW Airhead motorcycles, with a large section on electricals:
https://bmwmotorcycletech.info/index.html

Snowbum #1843
Rev. of 02/16/2021