Replace the battery. Since the '87 model year, GM went from the second best generator design to the world's worst pile ever. They develop huge voltage spikes that take out the internal diodes and voltage regulator, and those spikes interfere with computer sensor signals, and the computers, which are very sensitive to system voltage. As the battery ages, it can still crank the engine but they lose their ability to dampen and absorb those spikes. It's real common to go through four to six generators in the life of the vehicle, but many professionals are finding out that to reduce the number of repeat failures, just replace the perfectly good battery at the same time.

I don't know how you figured out that unplugging the generator would provide a clue, but that IS a common test. On the off chance the battery doesn't correct the problem, look for any wiring harness that are not routed in their original orientation. Also be sure there's no undesired resistance in the fat wire between the back of the generator and the battery or fuse box. That added resistance makes it harder for the battery to dampen those spikes.

In technical terms, as the lead flakes off the plates in the battery over time, it's "internal resistance" goes up. That's a term usually only heard by electronic designers and technicians. In practical terms, the old battery can still provide the current needed by the starter, but for a much shorter period of time compared to a new battery.

Answered by caradiodoc (expert)
17,308 answers provided

check for AC voltage at battery with alternator connected. i've heard of up to 30v AC which means bad alternator.

Answered by fixitmr
24 questions asked

Battery tests good and has 12.6 volts, car starts runs for about a second and dies. With alternator to ecu harness disconnected it appears to run fine. Seems when alternator output hits 14.8 the car shuts off, does the ECU shutdown the vehicle when at or above 14.8 volts? Is there any other scenario besides a bad regulator in the rebut alternator where you would see over voltage? (i have two calibrated ECU's for the vehicle, its same with both)

Response from montanaguy
1 question asked

14.8 is just above the industry-accepted limit of 14.75 volts but it is not excessive. Did you try a new battery?

Answered by caradiodoc (expert)
17,308 answers provided