Sounds more like a bad diode. Have the alternator load tested with a tester that shows "ripple". If the ripple is high and the maximum output is close to one third of its rated capacity, (30 amps from a 90 amp unit), one phase is dead. That's almost always from a defective diode.
Nippendenso alternators also develop worn brushes that will cause intermittent operation. To identify that you have to measure the two voltages on the two small wires on the back of the alternator while the engine is running and while the problem is occurring. One must have full battery voltage. The other one must have less but not 0 volts. If you find 0 volts on only one wire, the brushes are open. I recently replaced mine for nine bucks.
Tuesday, January 10th, 2012 AT 12:22 AM