OK, got one of those that has me and the guy that does my electrical work stumped. 91 Cheby PU. When you first start it often times the battery light comes on. It will stay on a while then go out. Sometime off and on several times. Lights dim and brighten with the light so there is a voltage drop, not just an idiot light. There is a voltage gage on the panel too. He checked the alternator and couldn't find anything wrong. Went ahead and put in a regulator just in case. Worked fine for a while and then started again.
Just now it did it again when I could pull it to the Lab and check it out. Voltage at the battery is reading 12v then it will jump to 13.8 or something close. Then it would drop back to 12.something. So that confirmed there is a problem, not just a bad gage in the dash.
I checked the voltage at the Alternator output straight to the battery ground. 12.0 volts. Checked the voltage using the Alternator case as the ground same results. I was suspecting a wiring problem. But if I get the same voltage grounded to the case as the battery ground, that should eliminate a wire problem and confirm it is the alternator, correct?
OR, could a dirty ground from the motor to the battery still cause this? I was thinking that I bypassed that by using the case as the ground but the more I think about it I am not so sure.
Either way I will clean the ground(s) tomorrow from the motor to the battery and if I find any body to motor grounds. I do think that this eliminated the wire from the alternator to the fuse panel. That was another wire I was suspicious of. Had that go bad once in a VW and it was the devil to find that problem!