Serpentine belt slack when AC is operating??

Hello all, I have a 90' Cherokee XJ Pioneer, 4.0L I6, 5spd. The AC hasn't worked in a looooooong time. I finally decided to trouble shoot and fix it. I found that it is out of refrigerant (figured as much) so it hasn't operated due to low pressure. Anyways, I found and repaired a leak on the suction line service valve. Long story short, the AC now operates and cools wonderfully. Back to the issue, when I turn on the AC I noticed a sound coming from the engine. I found that the serpentine belt from roughly the alternator pulley up to the AC pulley is bouncing/vibrating pretty bad causing that "Loose belt Sound, not squealing, more like flapping ". It has never done this before until I added to load of the AC compressor to the equation. With out the AC compressor running it runs like a top and very quiet. I checked the tension on the serpentine belt and it seems to be just right. The belt itself is in good condition. All the pulleys are in good shape and spinning true and rigid. The belt appears to be loose in that area from the alternator up to the AC when the AC compressor is operating. ANY SUGGESTIONS WOULD BE GREATLY APPRECIATED! Thanks again my friends!

Sounds like a bearing in the compressor is shot, but I would think its the same bearing thats loaded when the clutch is disengaged also. Very Interesting situation. Hopefully a compressor whiz has an answer for you.

The clutch on the AC is operating properly as far as I can tell, I have repaired several automotive compressors, AC systems over the years. I have never seen this particular problem. Thanks for the reply!

Your belt "seems" tight enough.

Tighten it, squeal or not, see what happens. If it's too tight (short term) and it starts making a different sound like grinding or binding, you can loosen it. You'd probably do well to get the Krikit II gauge and know for sure.

I will tighten it some, that is a good starting point for sure. On the 90' XJ , I think I can adjust the tension from the power steering , alternator, or water pump , does that sound about right? I cant run out there right now and look. I will have to first thing in the morning. Thanks y'all.

I'm no AC expert but went through the same thing a month ago - the compressor was the problem. I did not spend any time dissecting the compressor internally for the failure, but all other aspects of the system operated properly. The system held the proper charge, the clutch operated correctly, the belt and all pulleys functioned normally.
Not sure how long the compressor would have lasted, I didn't use the AC for a week and it held together, but replaced it, and everything is back to normal.

edit: I put a new belt on initially because I had a spare, wasn't the problem.

At Ronbo, thanks for the reply. Was your serpentine belt loose and "flapping/vibrating and noisy" only when you ran the AC? So you replaced the compressor huh?

Right, only with the AC 'on'. With the AC off, everything idled/ran normally. Belt tension was fine with the engine off.

Yeah, gonna have to say compressor. If you cqn actually see yje compressor pulley oscilating when the clutch is engaged, thats definatly it. Take care of the cooling fan while you're poking around.

Looks like its about $185 + shipping for a new compressor. You might want to buy the delete kit from napa if your sick of your a/c, but I'd fix it!!!

When you say oscillating, do you mean wobbling? What do you mean "take care of the cooling fan"? Thanks again.....

wobbling, yeah. Take care you dont catch your parts in the cooling fan while you watch the compressor clutch engage, (fingers, ears, face). Just saying "S" for safety.