Is it the battery? No... Is it the alternator? No... Then what the heck is it?!

I had the same problem with my 2005 Pontiac Bonneville…and spent a lot of money trying to fix that problem…and it ended up being my ECM, that was going bad…because the car had above 130,000 miles on it, that’s when their ECM begins to act up

You should check the O2 sensor

How does that fit in with this thread ?

And after 13 years, I doubt it’s an issue anymore.

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I’ve been trying to find an answer to my problem and I think I found it here. This is a recent problem. When I’m driving and have the a/c on, the battery gauge needle drops from 14 closer to the red area. When I turn off the a/c, it goes back to 14. Also when I accelerate while having the a/c on, no air blows out until I let off on the gas a bit. After reading what you said, I really think it’s the alternator. Could I have your thoughts on this?

I don’t think the alternator is at fault. More likely the serpentime belt is slipping because it is old and the tensioner is weak. The ac compressor may also be failing and causing too much drag.

I already had it in the garage. I told the mechanic to change the A/C belt cause it was slipping. I had hoped that would take care of it but it didn’t. I’ll take it back and have him change the serpentine belt and check the ac compressor.

Have the shop check the voltage drop across the wire that ties between the output of the alternator and the battery with a good load on the system. There should be less than .3 volts across that lead if things are good.

The AC doesn’t draw much electrical power, just enough to engage the clutch. But when you turn the AC, you have to turn on the fan or the power to the clutch will be blocked. So when you turn on the AC, what is the blower fan set to? Is it the highest setting? Have you turned the fan to high with just air only (AC off) to see if the voltage drops? That would be an important test.

When the engine is under heavy load like accelerating or going up a steep hill, most modern cars cut the AC compressor off to save gas and help keep the engine from overheating.

It would have been better if you had started a new thread instead of tagging on to an older thread. We don’t have the particulars on your vehicle, i.e year, make, model and odometer. If your vehicle make has different engines available, then engine and transmission information would be helpful also, but not so much in your case as electrical systems don’t change due to engine/transmission. But it is necessary for engine or transmission issues.

I do think it woud be worth having your alternator tested, some parts places will test them for free but you have to remove them and take them in. Be sure to call first. I would be surprised if the alternator was bad, but there is a small case to make for an alternator that has an open diode in the rectifier section to do this. If that is the case, find out if your mechanic can check for ripple voltage on the alternator output.

I only had about five paragraphs to go and they I saw it was from 2007.