Excuse me for butting in to your conversation, but there's more to the story. GM has had a very poor generator design since the '87 model year. Repeat failures are very common and are caused by a battery that may test just fine but is more than about two years old. These generators develop huge voltage spikes that can damage its internal diodes and voltage regulator, and confuse the many computers and make them do weird things. The battery is the key component that smooths out and dampens those voltage spikes, but as it ages and the lead flakes off the plates, it loses its ability to do that.
If one of the six diodes in the generator fails, the generator will develop even more harmful voltage spikes and excessive "ripple" voltage. Those can cause all kinds of elusive running problems that defy diagnosis.
Start by measuring the battery's voltage with the engine off. It should be 12.6 volts if it's good and fully charged. If you find it's near 11 volts or less, it has a shorted cell and must be replaced.
Next, measure the battery's voltage again with the engine running. It must be between 13.75 and 14.75 volts. If that is okay, it is okay to have the second part of the charging system tests done. That involves measuring full-load output current with a professional load tester. If you find the most current you can get is exactly one third of the generator's rated current, it has a bad diode and must be replaced. Anytime you replace a generator on an '87 or newer GM vehicle, always replace the battery at the same time to prevent a repeat failure, unless the battery is less than about two years old.
With a failed diode, 30 amps from the common 90 amp generator is not enough to meet the demands of the electrical system under all conditions and may allow the battery to slowly run down over days or weeks. The battery should be charged at a slow rate for an hour before it is tested to insure a valid test result.
Sunday, January 31st, 2016 AT 3:52 PM