Quote found on the net
http://bmwmotorcycletech.info/startingprobs.htm
20. In 1978, BMW changed things again. This lasted into 1980. BMW added 2 diodes...and a switch! 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, and via a 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 (HIGHLY unusual), and at the same time one had a failed (closed) switch, or the level was low, the starter could fire up, mysteriously, all by itself, and NOT release!...unless the ignition or kill switch was used.
21. BMW added a diode in 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; by use of the diode inside the headlight relay. Some had various combinations. What is complicating the mess here, is that, 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 the same SORT (nearly) fault 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.
22. With the 1981 models, no longer was there an under-tank master cylinder, so the operation is slightly simplified. The wiring is exactly the same, but withOUT the master cylinder switch and, of course, its diode was eliminated. This lasted through the 1984 models.
23. There were several more variations and changes until the end of production. 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.
Can you pick which variation you have and if you have a diode that may have shorted?