The first thing to do is replace the battery. Second thing is to have the charging system professionally tested. You can start the testing yourself with an inexpensive digital voltmeter. With the engine running, measure the battery voltage. It must be between 13.75 and 14.75 volts. If it is low, replace the generator again. If it is okay, have your mechanic finish the testing. He will measure full-load output current and "ripple" voltage. If the charging voltage is a little high, clean and tighten the battery cable connections.
Since GM redesigned their generators for the 1987 model year, they have had a real big problem with them developing harmful voltage spikes that can damage the internal diodes and voltage regulator, and interfere with computer sensor signals. The battery dampens and absorbs those voltage spikes, but as it ages and the lead flakes off the plates, it loses its ability to do that. It is real common to go through four to six replacement generators in the life of the vehicle. To reduce the number of repeat failures, replace the battery at the same time that you replace the generator, unless it is less than about two years old.
If the voltage spikes destroy one of the generator's internal diodes, the most output current you will be able to get is exactly one third of its rated current. 30 amps from the common 90-amp generator is not enough to meet the demands of the electrical system under all conditions. The battery will have to make up the difference as it slowly runs down over days or weeks. That low voltage can cause running problems by itself, but a failed diode also cause ripple voltage to be very high, and the resulting pulsing current magnetically induces voltage spikes into adjacent wires in the harness, and if those wires are for sensors, the Engine Computer will interpret those voltage spikes as coming from those sensors. It will try to run the engine based on those "noisy" signals. This is where unplugging the generator stops the production of those voltage spikes, stops the interference with sensors signals, and the engine runs better as long as the battery voltage does not drop too much.
If testing shows the new generator is only producing one third of what it should, and ripple voltage is high, it has already been damaged. Replace it and the battery.
Saturday, August 13th, 2016 AT 11:07 PM