After I got the bike running, I noticed that the volt meter showed low voltage (under 12 volts) when I was riding the bike with the headlight on. Something in the charging system was not working correctly. I read in Bob Fleischer’s material that a failed diode in the diode board would have this symptom and dirty electrical connections and/or alternator brushes that had gotten worn down could also cause low charging current to the battery. He also talked about BMW using rubber diode board mounts at this time. These mounts can break and allow the diode board to fall enough to short against the front engine cover and also contribute to weak or intermittent grounding.
So, I decided to dig into the diode board and test the diodes, replace the rubber diode board mounts and clean up all the electrical contacts. While I am in there, I also cleaned up all the alternator electrical contacts and replaced the alternator brushes. And, I cleaned up the starter relay and voltage regulator contacts. Last, I installed a new Westco AGM battery so I have a known good battery and date of installation.
Here are the write-ups on this work:
I found that all the diodes were good on the diode board. It maybe worth replacing it due to it’s age and as insurance from an unexpected failure, but I’ll hold off on that until I do the full rebuild. Replacing the rubber mounts is not a fun or easy job. I got very frustrated more than once and had to take a couple coffee breaks to get my attitude adjusted. 🙂 But I did get them installed, so with perseverance and patience, it can be done.
After I replaced the alternator brushes and cleaned up all the electrical contacts, I now get 14.5 volts between the battery (+) terminal cable from the diode board and ground with a momentary rise to 15 volts when the voltage relay does it job. I suspect the brushes were the main reason for the low charging current to the battery.
A couple of pictures follow.
Check Diode Board, Replace Rubber Mounts.