If you still have the original battery in the car, it is very likely the generator has failed. GM has had a lot of trouble with them since they redesigned them for the '87 model year. Due to their design, they develop a lot of voltage spikes that can destroy the internal diodes and voltage regulator, and interfere with computer sensor signals. The battery is responsible for damping and absorbing those voltage spikes but it loses its ability to do that as it ages.
Start by measuring the battery voltage, first with the engine off, then with it running. With the engine off, it must measure 12.6 volts. That indicates it's good and fully-charged. If it's around 12.2 volts, it's okay but discharged. Next, with the engine running, the battery voltage must be between 13.75 and 14.75 volts. If it is low, suspect the generator. If it needs to be replaced, replace the battery at the same time unless it is less than about two years old.
That's the first half of the tests. Even if the voltages are okay, you need a professional load tester to measure the generator's full output capacity. If one of the six diodes is defective, the most current you will be able to get is exactly one third of the design rating of the generator. 30 amps from the common 90 amp unit is not enough to run the electrical system under all conditions. The battery will have to make up the difference until it slowly runs down.
Saturday, December 20th, 2014 AT 12:51 PM