I thought I would share this with others in the hope that they didn't end up spending alot more money than needed to cure what seemed to be an alternator problem.
I recently bought a 1993 Chev Lumina Sedan with the 3.1 Liter V6. I got it real cheap because it had been in a minor accident with only cosmetic damage to the front bumper and some plastic parts. The owner also said it needed a new alternator and possibly a new battery, then went on to describe how you could boost it to get it going but the engine would die after about 10 minutes and the battery would be too low to restart it. He then demonstrated and the car died at about 8 mninutes after removing the cables.
So, I bought the car and towed it home.
The battery was a pain in the butt to get to, but I managed to get one clamp of my booster cable on the positive terminal and one on the engine block and got the car started again, so that I could test the output of the alternator. Result only showed a repid decrease in the battery voltage, so I figured the previous owner was right and that the alternator was the culprit.
Next day I finally disassembled all the junk in the way of getting at the battery and found that even after a 12 hour charge the previous night, it had dropped in voltage from about 12.79 volts earlier that morning when I took the charger off, down to 12.02 volts. I suspected that even if I could start the car at that voltage, the battery might not have enough power left to energize the magnetic field in the alternator in order to generate power. (The battery was a "Sure Start"
650 Amp, 465 Cold Cranking Amp's.) So, I replaced the battery with one I knew was good out of my old car, (Crappy Tire, 800 Amp, 650 Cold Cranking Amps), and checked the voltage before starting the engine. Pre-startup voltage was 12.62 volts. With engine running was 14.52 volts. After running for about 5 minutes, then shutting off the engine, voltage was 12.85 . Looks like the alternator works.I have to swap plates etc with my old car before I can take it for a serious test drive, but I'm reasonably certain it will continue to charge properly. If not, I'll repost to let people know.
D.BC