[A81Sturmer]

My car: US '88 951 220 hp stock, 178K miles
Instrument panel voltage gauge sometimes reads 13-13.5. Other times it reads on 12v. When it reads 12 v, the cooling fans don't come on unless I turn on the AC switch. Also, the battery isn't charging when it reads 12 v. When it reads 13-13.5, all is fine. When I turn on the AC, the volts drop .5V (whether reading 13-13.5 or 12v), temp gauge jumps up equivalent amount. Car has an aftermarket radio. It has factory alarm with may or may not work. Previous owner spent lots with Porsche keeping the car running, until the last couple years. I'm a recent buyer. The engine seems to run great. Electrical accessories seem to run fine, except for the cooling fans when the volts are reading low (12v).

Anyone have experience with this or very similar problem? I'm just starting the troubleshooting. TIA.