Slipping belt is possible, but it sounds like there's a bad diode in the alternator. With one of the six of them defective, you will lose two thirds of the rated output. Many of these alternators were rated at 55 amps. With a bad diode, the most you will get is around 20 amps. Ten amps can be used up running the electric fuel pump, ignition system, and fuel injector. Two headlights take another ten. Any more electrical demands will start to drain the battery.
Some parts stores only test alternators for output or lack of it, not how much. The system should be load-tested on the car. If you can't get at least 50 amps, do the "full-field" load test. This will bypass the voltage regulator in case that's the problem. I can copy and paste the instructions if you want to try that yourself.
Monday, March 30th, 2009 AT 11:38 PM