On a motorcycle newsgroup to which I contribute, there was the following posting. Not entirely our thing, but I know we do love a problem to gnaw at.
This is what he said
"Dear All, I have a Lucas 12 volt generator with electronic regulator on my bike that has stopped working. That is, no charge showing on the ammeter and full discharge when the lights are turned on. However, just before I removed it from the bike to peer inside it, something made me decide to test it in situ.
So I removed the generator drive components and did a motoring test whilst applying rotational load to the armature shaft. All was well with no jerkiness, hesitation or stopping. I then plugged it back in and ran it up with an air drill and it works just fine with the correct charge showing on the ammeter and a multimeter across the battery showing 12.3 volts.
Put the drive components back on and start the engine and no charge. Spin it with the air drill and all correct again. So why does it work with the air drill and not with the engine running?"
See what I mean? Answers eagerly awaited ;o))