Start by measuring the battery voltage before you start the engine and after it is running. Expect to see near 12.6 volts with the engine off. If it is closer to 11 volts, there is a shorted cell and the battery must be replaced.
When running, the voltage must be between 13.75 and 14.75 volts. In addition to that, GM generators are well-known for developing excessive voltage spikes that are normally dampened by the battery. As the battery ages, it loses its ability to do that, then those spikes interfere with the normal signals the sensors supply to the many various computers. If your mechanic performs a load test that shows excessive ripple, both the battery and generator should be replaced. You can also try charging the battery, if necessary, with a battery charger, then if the problem occurs again, immediately unplug the small connector on the back of the generator. If the problem clears up, replace the generator.
It is common to go through four to six generators in the life of a GM vehicle. Replacing the perfectly good battery at the same time will reduce the number of repeat failures. The old battery may still work fine in a 1986 or older car.
Friday, February 11th, 2011 AT 7:17 AM