How do you know the generator is okay? The warning light means the generator is not recharging the battery properly. GM has had a huge problem with their generators since the '87 model year when they redesigned them. They develop large voltage spikes that can destroy the internal voltage regulator and diodes, and interfere with computer sensor signals. It is real common to go through four to six replacements in the life of the car. To reduce that number of repeat failures, replace the battery at the same time, which you already did. As they age they lose their ability to dampen and absorb those spikes. That usually is not a problem if the battery is less than about two years old.
There's two tests that need to be performed and you can do one of them yourself. Measure the battery voltage with the engine running. It must be between 13.75 and 14.75 volts. If it is low, the generator needs to be replaced. If it is within that range, the second test requires a professional load tester. The generator must be tested for maximum current output and "ripple" voltage. If one of the six diodes has failed, you will lose exactly two thirds of the generator's capacity. 30 amps from the common 90 amp generator is not enough to run the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over hours or days. A bad diode will make ripple voltage very high too.
Thursday, February 13th, 2014 AT 5:22 PM