Use an inexpensive digital voltmeter to measure battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If it is low, replace the generator. It's possible it is failing intermittently and it was tested while it was working.
It is real common to go through four to six of these piles in the life of the vehicle. Many professionals are finding out that to reduce the repeat failures, you must replace the perfectly good battery at the same time. They will work fine in pre-1987 vehicles, but as they age, they lose their ability to absorb the huge voltage spikes these generators produce. Those spikes can wreak havoc on sensor signals for all of the computers on the truck as well as damage the internal voltage regulator.
Monday, December 6th, 2010 AT 6:54 PM