Interesting problem yesterday. It was such a nice day I thought I would pop out for a drive. Got in the car, turned on the ignition and noticed the alternator light didn't come on. Started the engine and the voltmeter stayed fixed at 12V. So obviously alternator was not working. Found the problem very quickly, the warning light/exciter wire had snapped at the crimped joint at the alternator So built a new crimped joint and soldered the wire onto the existing one and everything worked as it should.
But from a philosophical point it was a bit poor. If I hadn't fitted
the voltmeter and hadn't noticed the alternator light not working (easy
to do in bright sunlight) then I would happily have driven off just on
the battery. I assume everything would have ground to a halt after a
few miles as the battery drained. The problem is that the alternator
warning light assumes the wire is connected so it can sense when the
alternator stops producing it's 14V and drops to 0V. So if the wire
had snapped while driving I would be none the wiser.
It also proves that the alternator I have is not self-exciting and
the warning light circuit is essential. Not surprising I guess based on