Strange Problem with Alternator Light - Any Ideas?

[ber911]

While driving my 76 Carrera 3.0 tonight, the alternator warning light was erratically going on and off - on for say 10 minutes and then going off for a few minutes and then coming back on again e.t.c. This was whilst driving at a steady speed on the freeway and in traffic.
The light generally stays on for a while after I start the car.
Checked the battery voltage:
12.5V with the car off
14.2V at idle (approx 1000 rpm)
14.2V at higher revs.

Any ideas as to what is going wrong?

Rob

[Rot 911]

Check for a loose fan belt.

[woland]

Maybe the alternator brushes near the end, unless you changed it recently

[JeremyD]

Or a loose ground wire (brown one) mine did this - found out the brown ground wire had loosened with the connection.

[ber911]

Maybe I am still missing something obvious?
I have had the alternator checked out by a reputable auto electrician who replaced the excitor diodes and bearings, I ran an additional earth wire to the alternator and checked the fan belt tension when renstalling. The alterrnator light glows brightly all the time now irrespective of revs but the battery voltage when running is 14.2V.

Rob

[jklaspok]

I second the worn brushes. I had a similiar issue and purchased a voltage regulator from our host. Problem solved.

[304065]

Short in the "blue wire." Trace it from the bulb holder in the oil pressure gauge all the way back to the alternator. Maybe it is lying on the cylinders and the insulation burned through. Search my name and "D+/61"

[Silveresrty911S]

Could be the diode in line with the alt light, but prbably not if you were at steady speeds and was going on and off. I haven't installed one I have, but my light will show up at low RPM's and go dimm as the revs go up.