Alternator / Charging issues

[81SCTarga]

I have a 1981 911 SC Targa (i think my screen name may have given that away) that I finally brought home after over a year of body / paint work. I have put the car back together mostly, but now I have to deal with an issue that I have been putting off for a while.

When I turn the key, the Battery light comes on as it should. When the car is started, the light turns off. When I first start up the car, I get about 13.8 V at the cigarette lighter, showing me (as far as I know) that the alternator is doing its job charging. This will teeter between 13.5-13.8 as long as the car is NOT at normal operating temperature. Once the car has warmed to normal temperature the voltage drops to 12.0 V or less and continues to drop and the Alternator appears to not charge any longer.

I am wondering if it may have a short or something in the O2 sensor circuit (i may be way off). If I remember correctly, until the car warms up the Warm Up Regulator takes care of the Air / Fuel mixture, but after that the O2 takes over.

Other than that I don't know enough to really have any educated guess as to why this may be happening, but hopefully that's where all of you come in.

Any ideas?

Thanks in advance


Pic of the car right after paint (taken with my cell phone - in the paint booth in my dad's barn)