Here's what I just typed a few minutes ago for a different car:
Measure the battery voltage with the engine off and with it running. You should find 12.6 volts with the engine off. Closer to 12.0 volts means the battery is good but discharged. Around 11 volts means it has a shorted cell and must be replaced.
With the engine running you must have between 13.75 and 14.75 volts. If it is low the generator won't fully charge the battery. If it is acceptable or even a little high, have the generator professionally load-tested. If "ripple" is high and the most output current you can get is one third of the generator's rated output, suspect a defective internal diode. That can leave you with insufficient current to run all the electrical stuff on the car and it will slowly run the battery down.
To continue, GM has had a real lot of generator failures, often as many as four to six replacements in the life of the car, since the '87 model year. The biggest thing that reduces the number of repeat failures is to replace the perfectly good battery at the same time unless it is less than about two years old. As they age they lose their ability to dampen and absorb the huge voltage spikes these generators produce. Those spikes destroy the internal diodes and voltage regulator.
The first thing for you to look at is if the battery warning light turns on when you turn the ignition switch to "run". That is the turn-on signal for the voltage regulator inside the generator. If it comes on, then goes off when the engine is running, measure the battery voltage and consider that load-test.
Wednesday, October 3rd, 2012 AT 12:31 AM