Hi friends, greetings from Argentina! I¨ve been a happy owner of a 1948 model U with sidecar for a couple of months now. Since I acquired it, I have been improving many details typical of a motorcycle that has its years, and for many of the last it has been standing still. I decided to remove the dynamo for examination and cleaning, as it was not able (even increasing engine revs) to get the GEN light on the dash to turn off, so I assumed the generator was not charging. The first thing I notice, is that it was missing the metal and bakelite washers between the conmutador and the back cover. Also the insulating papers of the field terminal coil and those of the switch terminal. Also the wires of the shunt field coil were badly connected. Once this problems have been solved (place the corresponding washers, the insulating papers and connect the wires according to what the manual explains), try to test if the dynamo rotated as motor when connecting current from battery, but no.....its does not rotate. -it is clear that according to videos I have seen, connect the positive of the battery to the SWITCH and RELAY terminals, and the negative to the housing-. The surprise was that I accidentally connected the positive battery cable to the RELAY terminal, and the negative to the third brush holder and now it WORKS AS A MOTOR!...what could be happening here??? Have I connected or isolated something wrong? (If I can, I will try to upload a video. That is more graphic than all words).
Thank you in advance for your comments!
I have not been able to add the video in this post, so I leave the link below to see it if anyone is interested. Thanks!

 

The voltage regulator might be your issue