Measure the battery voltage with a digital voltmeter, engine off. It should be 12.6 volts if it's fully-charged. Measure it again with the engine running. It must be between 13.75 and 14.75 volts. If it is low, double-check the voltage on the large output terminal on the back of the generator. Whatever you find at the battery, you should find at the generator.
GM has had a really huge problem with their generators since the '87 model year. It's not uncommon to go through four to six in the life of the car. To reduce the number of repeat failures, replace the battery at the same time unless it is less than about two years old.
Saturday, November 23rd, 2013 AT 8:36 PM