Bad alternator? Autozone messing with my confidence...

[stom]

New to the forums, thank you for the opportunity!

1991
Oldsmobile
silhouette
V6 3.1 Liter
130,000 miles

Ok here we go. My friend's minivan won't turn over. There's been a few electrical problems in the last few days. The windows wouldn't go up or down, the fan blower wouldn't work, and the engine stalled. This happened about two days ago, but we were able to drive it home. Now today, the engine wont turn over.

So I brought out my multimeter, and connected it to the battery. It read 10.95 volts - a bit low. So I jumped the van, and it started. Again, I grabbed my multimeter and connected it to the battery. Because I kept the jumper cables on for a few minutes, it read about 12 volts, but slowly trickled downward. I then connected it to the alternator, and there was no difference in the voltage. Again, it trickled downward - even if I revved the engine. That tells me that the alternator isn't putting out voltage, and needs to be replaced - at least that's been the case for every time I've ever worked on a car. But then I went to autozone.

I take the alternator out, and take it to one of the guys behind the counter. He puts it on their machine, and it checks out OK! I found that quite curious, so I had them check it again. They checked it several times, and each time it checked out OK. The guy then told me it could be the battery. However, that DOESN'T satisfy my question as to why the alternator wasn't putting out voltage. He couldn't answer that, and instead told me how he's been a mechanic longer than I've been alive.

Well, either one or two things is happening here. 1), their testing machine is completely worthless, or 2) what they're telling me is right. Now, mind you I've checked all the cables and connections for rust, corrosion, etc, and they're all OK. Am I doing something wrong here, or has anyone else had a similar experience at autozone? Is it possible that the battery could be so bad that it's not allowing the alternator to output voltage?

[brcidd]

An internal short in a battery will eat you alive in voltage-- yes it is possible-- to test- take the battery out of your car and put it in the van-- when shorts get bad enough- they won't even jump start--- also put a battery charger on it- see if it takes or hold a charge--it probably won't......

[merc81]

Yea, that's why mister ACE mech works at Autozone right? He wants to be there so he can help guys like you out? Gimme a break.

Look, if the alternator was working, it would put out at least 90 amps. At 14 volts, that's 1260 watts. Are you thinking the battery could short out, and consume 1260 watts and you wouldn't notice it? I don't think so. You would notice heat in the wiring, boiling in the battery and belt squealing at the alternator if it was working.

The battery may indeed have a short, but that alternator isn't feeding into it either. Easy enough to change batteries with a good one and then test the alternator on the car.
John

[rockwood84]

130,000 miles is about the life of the factory alternator brushes. don't know if you can still get brushes for a '91 g.m. alternator or not. i know that '98 up they don't sell brushes you have to buy rebuilt or new. if you can get a set of brushes and new voltage regulator for the alternator just redo it . as for autozone testing equipment they told me my ford starter was working and the bendix wasn't even kicking in to start the car. the starter would turn over but wasn't kicking to hit flywheel.