Again, do you mean the starter doesn't turn long enough, as in it cuts out, or fast enough for the engine to start? "Won't quite turn over" doesn't tell me what's happening.
Since it was working okay for a few days with the new battery, it's most likely the generator has failed. Since the '87 model year, GM has had a real big problem with them. Due to their design they develop huge voltage spikes that can damage the internal diodes and voltage regulator, and interfere with computer sensor signals. It's real common to go through four to six replacement generators in the life of the vehicle, but to reduce the number of repeat failures, you must do what you already did. That's to replace the battery at the same time unless it's less than about two years old. The battery is the main component in damping and absorbing those harmful voltage spikes but as they age and the lead flakes off the plates, they lose their ability to do that.
What probably happened is a diode failed in the generator thanks to the old battery, so now it can't keep up with demand. With one bad diode of the six, you will only be able to get exactly one third of the generator's maximum rated current, and that's not 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.
Once you check the battery voltage with the engine running, (it must be between 13.75 and 14.75 volts), that will tell you it's okay to do the rest of the test but that requires a professional load tester. That will test for maximum full-load current and "ripple" voltage. Besides full-load charging current that is too low with a failed diode, ripple voltage will be very high.
Friday, May 8th, 2015 AT 9:13 PM