Chances are the new generator is bad already. A load test will confirm that. There's no way to sugar-coat it. GM went from the second best AC generator to the world's worst pile ever beginning with the '87 model year. It is real common to go through four to six of them in the life of the vehicle, but what many professionals are finding out is to reduce the number of repeat failures, replace the perfectly good battery at the same time. These generators, due to their design, produce huge voltage spikes just like an ignition coil is supposed to do. Those spikes destroy the internal voltage regulator and diodes, and the spikes can "induce" voltage spikes into other wires. If those other wires are for computer sensors, the computers will get confused and do strange things. As the battery ages, it loses its ability to dampen and absorb those spikes. The old battery will work fine in an '86 or older car.
During the load test, if one of the six diodes is bad, you will only be able to get one third of the rated capacity from the generator. That's around 30 amps for a common 90 - 100 amp unit. 30 amps isn't enough to meet all the demands from the electric fuel pump, heater fan, head lights and tail lights, and ignition system and numerous computers. In addition, with a bad diode, one of the three output phases will be missing resulting in very high "ripple". Most professional load testers display ripple as a bar graph, not as an actual number. The goal is to have very low ripple. With high ripple, suspect a damaged diode. The fix is to replace the generator again and replace the battery.
Friday, March 23rd, 2012 AT 6:27 PM