98 battery llight keeps coming on

We checked the alternator and it was fine. THe battery water was a little low, so we added to that. The tension pully on the alternator was broke so that was replaced. What is happening is whenever I accelerate or have the air on, the batter gauge drops and the light comes on. If I let off of the gas it goes back to reading 14. I was going to get the battery checked today, but any suggestions?

Sounds like your serpentine belt may be stretching or slipping upon load/acceleration. If you have a voltmeter, check the voltage output of your alternator at the battery with the engine idling and also at higher rpms'. Any reading under 12.5v will indicate a bad alternator if the belt is making good contact with the pulley.
If that all checks out and your belt is in good condition and not slipping, your auto-tensioner spring may be the culprit. Can you move the tensioner by hand or with little mechanical leverage?

I can ask my friend to check that out. I appreciate any and all help!

I was just at autozone and they said that everything has a good reading (battery alternator) but the belt looks glossy and cracked. It's probably as old as the car, so I guess I'll get that changed to see if it helps. If there are any other suggestions, I'd love to hear them.

Hold on, how did autozone test the alternator?

We pulled the alternator and took it in. Was this wrong?

First off, just to check, take it to somewhere else to test it just to be sure.

Second, you might have a bad engine ground.

Third, your battery needs to be charged or changed.

My dad's 98 chevy blazer is having this exact same problem. The bad part is that the charging system when out about two weeks ago and he diagnosed it to the alternator. He replaced the alternator and everything was fine for about a week and then the battery light started comming on when his REV's were between 2200 and 2700 RPM. Before that and after the battery light is off an the system looks like it's charging.

Any help is appriciated.
Thanks
Rich

Load test the battery.

It could have a internal short circuit which is overloading the alternator.

I agree with BlazerLT, a complete testing of the charging circuit is in order. Removing the alternator was a little overkill to start with. A load test would be the first thing on the battery with the engine off. Then a load test with the engine running. Second, a voltage test of the battery engine off and then running. What you are describing could be several things already touched upon to also include corrosion on the battery terminals (positive) which is extremely common in GM diesels, but that is another story.

One more thing, you mentioned that the battery water was low. Was it in all 6 cells are perhaps just 1. If there is a shorted cell you will a lower level of acid.

yea, this is a bad battery for sure.