The clue is turning the fan off affects the symptom. That reduces the current needed to be supplied by the generator. GM has had a real big problem with their generators since they were redesigned for the 1987 model year. They develop huge voltage spikes that can destroy the internal voltage regulator, the diodes, and interfere with computer sensor signals. It is a real common cause of elusive engine running problems.
If you start with a fully-charged battery, unplug the small plug on the side/back of the generator. Drive just long enough to see if that changes the symptoms. If the running problem gets worse or clears up, have the charging system professionally load-tested. The three numbers of importance are charging voltage, full-load output current, and "ripple" voltage. I can explain each of those in detail. Ripple voltage is usually just listed as "normal" or "high", not with an actual voltage.
Be aware that it is real common to go through four to six replacement generators in the life of a GM car. If you need to replace yours, to reduce the high number of repeat failures, also replace the battery at the same time, unless it is less than about two years old. I can explain that too if it becomes necessary.
Jul 30, 2017 at 6:36 PM
