The symptom suggests a defective diode inside the generator. In the case of my minivan, I've been driving it like that for over a year. To determine if that is the problem, a load test must be performed with a tester that can measure "ripple". With one bad diode, the ripple will be very high and the most output current you'll be able to get will be very near one third of the unit's rated capacity. A 90 amp generator will only produce 30 amps which is barely enough to run the fuel pump, injectors, ignition system, lights, and radio with little left over to recharge the battery.

If you measure battery voltage while the engine is running, you'll find it's a little on the high side. Normal is between 13.75 and 14.75 volts. The voltage regulator sees the dip in voltage during the period when the defective diode should be passing some output current. It responds to that dip by increasing generator output voltage to try to get the current back up, then it doesn't cut back quick enough when the good diodes are conducting. That's why you might find from 15.0 to 15.5 volts at the battery even though the output current is low.

Answered by caradiodoc (expert)
17,308 answers provided