Horrible screeching noise after replacing alternator

My jeep was not starting, I had the battery tested, then the alternator, it was bad, replaced alternator and has battery charged at store, car now turns on fine sounds good no noises, the second I turn on the AC there is a horrible screeching noise, I have video, but it says I cannot post it.

Screeching is usually due to a loose belt. It could also be a pulley that won’t turn.


Did you replace the belt?


Did you reinstall the belt correctly?

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Might be a problem with belt that drives the alternator , either too loose, too tight, or worn out. If the A/C is driven by a different belt, could still be the alternator belt’s that making thenoise. the reason is when you turn the A/C on, more electric power is needed to run fans, and that puts more mechanical load on the alternator pulley. Suggest to post your Jeep’s model year and engine ID for more ideas. If you’re unsure where the sound is coming from you might try using a short length of garden hose as a sort of stethoscope. I had a belt screech problem on start-up on my Corolla a few years ago, fixed straight-away by a simple re-tensioning of the alternator belt.

OK , I give up . Just how many people have a short piece of garden hose laying around .

Hee hee, you should be replacing your washing Machine hoses from time to time. I save the old ones and cut them to the length I want. Work good in the utility sink and draining the water heater.

But gee, it ain’t hard to figure out which belt is making noise by opening the hood. Never been a fan of sticking stuff like hoses or screwdrivers close to spinning stuff though.

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Is that a pun? … lol …

Sometimes spraying a little water on the belt is diagnostic. If it makes the sound stop, belt or belt pathway is the problem.

Its only one belt, ill try to upload a separate video and sound cause it doesnt let me upload an mp4, again it only starts screeching when the AC is on when its off the car purs and runs fine, i took it on a test drive and it drove fine. it doesnt let me add the modified file either lol, ill try to make a youtube video and post a link.

again it only starts screeching when the AC is on

It sounds to me like the AC clutcth is bad.

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Audio/vdo files rarely helpful, not clear enough for diagnostic purposes, but no harm done if you want. Most folks seem to provide u-tube links for that.

ok i uploaded to youtube here are 2 videos, the auto store seems to think its the AC bi(by) pass

with noise

without noise

sorry i think i uploaded the same video twice

The belt is rather loose, check if the tensioner moves freely.

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ill check here is without noise

So the belt appeared not loose to me, I probably didnt check everywhere I was supposed to check, but it seemed to be on fine. let me know if I missed a place i could check, and I could not move the tensioner at all by hand. I also tried to tighten the nut more on the tensioner but that just started to move the tensioner up and the belt starts to loosen.

Did you use a new belt? Yeah tensioner may not be doing it’s job after being disturbed. Worse comes to worse your ac compressor may need attention, but it was ok before? Pulley on the alt same size? Right size belt? I’d try exercising the tensioner a little to see if it will tighten the belt up.

are you able to elaborate on exercisizing the tensioner? im kinda new to cars, so it would help alot, ty, also if there is a video you can link that would be greatly appreciated as well.
oh, and the belt is the same one as before, its not an old belt the auto store guy also stated the belt was in good condition, and correct this noise was not happening before the alternator was replaced.

It looks like the tensioner isn’t removing the slack from the belt.

Replace the tensioner and the belt.


Just asking, if that was the case wouldn’t it make the screeching noise regardless if the AC was on or not? if the belt was too loose?