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Testing Voltage Regulators on BMW Airhead Motorcycles

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 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.


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

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