Dandy. The charging system problem I'm concerned with requires a professional load tester so you'll have to visit a shop for that. Due to their design, starting with the '87 models, these GM generators develop huge voltage spikes that can damage the internal diodes and voltage regulator, and interfere with computer sensor signals. The battery is the key component in damping and absorbing those spikes, but as they age and the lead flakes off the plates, they lose their ability to do that. It is common to go through four to six replacement generators in the life of the vehicle, but to reduce the number of repeat failures, replace the battery whenever the generator has to be replaced, unless it is less than about two years old.
When one of the six diodes fails, you'll lose exactly two thirds of the generator's capacity. 30 amps from the common 90 amp generator isn't enough to run the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over days or weeks.
Those voltage spikes also radiate into other wires magnetically and cause weird signals to show up as sensor signals. The Engine Computer can interpret those as legitimate signals and try to run on them. That's where those elusive running problems come from that defy diagnosis. One trick to identifying them is to disconnect the small plug on the side / rear of the generator to disable it. If the running problem clears up, the charging system should be tested.
You can start the testing on any charging system by simply measuring battery voltage with the engine running. It must be between 13.75 and 14.75 volts. If that is okay, it just means the rest of the tests will have validity. The first thing the load tester will do is draw the maximum full-load current to see how much can be developed. That is either going to be very near the generator's rated current or one third of that. If it's one third, one of the diodes has failed and the generator must be replaced. It's not practical economically on other brands to replace the diodes because rebuilt generators aren't that expensive, but on GM's generators it is nearly impossible to do so without destroying other parts when trying to get them apart. These generators weren't designed to be repaired.
AC generators have three-phase output. When one phase is missing due to a failed diode, "ripple" voltage will be very high. That is also measured by most professional load testers. Some show it as a voltage, and others use a bar graph to show relative voltage. That can't be measured accurately with a digital voltmeter. You'd have to use the AC voltage scales and those are only accurate at 60 Hz as in house wiring.
Monday, April 29th, 2019 AT 8:04 PM