Have a load test done on the generator that includes a ripple test. Sounds like there is a defective diode inside the generator. That will reduce its maximum output to exactly one third of its rated capacity which is barely enough to run the fuel pump, injection / ignition systems, and all of the numerous computers on the car.
There's no way to sugar-coat it, since the 1987 model year, GM went from the second best generator to the world's worst pile ever and they have no plans on improving it. Due to its design, they develop huge voltage spikes that can destroy the internal diodes and voltage regulator, and they can interfere with the computers' sensor signals and cause all kinds of problems from confused computers. It is very common to go through four to six generators in the life of the vehicle, but to reduce that number of repeat failures, replace the perfectly good battery at the same time. As they age, they lose their ability to dampen and absorb those spikes. Problems usually start to develop when the battery is three to four years old. The old battery will work fine in a 1986 or older vehicle.
If there is a defective diode, besides the low output on the load test, it will also show very high "ripple". That is an indication that one of the three circuits, or "phases", is not working. The common 90 amp generator will only be able to produce about 30 amps which will eventually lead to a discharged battery and the need for a jump-start.
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Monday, December 26th, 2011 AT 8:39 PM