1980 911 SC. I had the diodes in my alternator replaced. Reinstalled alternator. Drove for 6 miles and alternator light was not lit. Stopped to fill tank. restarted engine. Alternator and OXS lights came on and stayed on. Alternator light does not vary in intensity with change in RPM. I reset OXS mileage counter, but light stays on. I removed alternator and had it tested. Works fine. There is power at exciter lead to alternator. What causes this???

 

I'm surprised that Loren hasn't chimed in here, this is his specialty field. I can only guess that if your alternator is OK, and the wiring to it is all correct, then you have to look at your external voltage regulator (used until 1982 models). Have you measured voltage at the battery, both at idle and at 2500 rpm?
Pete

 

Did you disconnect your existing external voltage regulator? The new (Updated) alternators have the integral voltage regulator. I don't believe you can still buy the old style alternators found on your 1980 car.

On my car (Also a 1980) I found this out a couple years ago when I was tracing an extra wire connector I found in my engine bay. My alternator had been replaced by the prior owner.

 

The problem is fixed.

I am still using the externally-regulated alternator. It has not been switched for the internally-regulated unit.

The problem appears to have been the usuall, for me, oddball. When the technician worked on the diodes to fix the last problem (a broken diode stud) he left a bare wire in a position that allowed it to contact something it shouldn't have contacted. That has been corrected and both the alterantor and OXS lights are out now.

Of course, the engine has developed a new oil leak, which appears to be coming from the point where the external pipe enters the case on the left side.