Battery Light woes

[guitarfish]

98 Grand Prix GT 3.8, my wife's car.

She's out & about one day, and the battery light comes on. She shuts the car off. The battery is dead, so I replace it. Next day, battery light comes on again. OK, bad alternator? I replace it (a warranty swap, so it's free).

The car's good for a few days, then she goes out for a drive, the battery light comes on. I have her turn around and go home. I get home - all the connections at the batt & alt look good. I start the car and run it for 15 minutes, no problem, no battery light.

When the car's off, the batt measures 12.8 volts, something like that. When it's running, it shows 14.4 volts, or therabouts.

Two weeks go by, she's driven a hundred miles or so with no issues. Now today she drives about 25 miles from the house, and you guessed it - batt light again. Comes on, goes off, comes on.

I'm not a mechanic, I'm stumped. Is the alternator flaky? Is there something else I can check? I'd like to measure the voltage at the batt. when the batt light is on, but haven't been there when it's happening (yet).

Ideas? Thanks all.

[richtazz]

Check the serp belt and see if it looks shiny (indicating the belt is slipping). IF so, the tensioner could be bad allowing the belt to go slack and slip, or the belt itself is worn out and glazed and needs replacing.

[guitarfish]

UPDATE: I met my wife and we switched cars. I had to drive 40 miles to get home. As soon as I start the car, the batt light comes on. Rev it, it goes off, then comes on 10 sec. later. It became apparent that if the engine is over 3,000 RPM, the light stays off. SO I had a tricky drive - keep the car in 3rd gear, but don't go too fast and burn the transmission. Anytime the RPMs went below 3K for 5 seconds or more, the batt light would come on.

Once home, I put a meter on the battery - 11.6 volts. Should be 12+.

SO, does this sound like a bad alt? It's 3 weeks old.
I'll check the belt. It has only 14,000 miles on it (I usually get 40K out of those things). I don't believe it was slipping when the car was sitting there in the driveway idling, however.

Again, any help is GREATLY APPRECIATED !!

-Dave

[BNaylor]

What brand alternator did you install. It is not uncommon for aftermarket parts to crap out making you doubt your sanity.

The PCM module is what sets the battery light. It is a specific voltage range it looks for. If this input from the charging system is bad then the PCM will trigger the light.

BTW - Although the PCM is part of this circuit it is very seldom the cause so I would rule it out for now.

Sounds like you are on the right path.

[guitarfish]

bnaylor - yes, it's aftermarket all right. It's by Excel. I think it's the alt. I'm going to call the supplier tomorrow. Thanks for your help.
-Dave