1998 Cherokee 4L. Replaced serpentine belt that was frayed and came off. I tightened the belt up all the way and it is still loose. When I start the motor it squeaks. Then if I turn on the AC it gets really bad and the belt actually stops spinning. SO what do I do? Try a new belt 1st, then what? AC compressor? Need some help please. My daughters truck and IM tired if her driving mine

Well...I suspect an incorrect belt or incorrect routing if the belt is correct. Also, make sure that the tensioner pulley center bolt is loose enough to prevent a false sense of tightness on the tensioning adjuster bolt. If that center bolt isn't loose enough the tensioner could be binding giving you a sense of reaching an extreme limit when there's actually more adjustment that can be made.

I can see on the tensioner bracket mount that it has traveled as far as it can. I will try another belt. Now part 2. When the belt is off, all of the pulleys spin freely. Squeaking when motor is on-can't really locate. Then when the AC is on, it gets very loud and the belt even stops spinning at times and going very slow-but still very loud. AC off-goes back to rhythmic light belt squeak. That would not be caused by a loose belt would it? Compressor/compressor clutch maybe?

A mechanic's stethoscope is your friend.

Thank you-on it. I listened with a long screwdriver. Tension pulley seemed loud, but I could not hear anything with the AC on. Plus the belt was not spinning or spinning slow.

X2. Wrong belt or routed wrong. The belt fraying and coming off is a classic symptom of the harmonic balancer (the pulley on the front of the engine crank) starting to come apart. Have a look and see if the rubber that separates the inner and outer portions is falling apart. When it starts to fail, the outer section of the pulley can get wobbly, or start walking inward or outward which tends to tear up the belt. It can also walk far enough inward to rub on the timing cover.

X3 on the wrong belt or routing of it. double and triple check that.
Possibly a belt that is for a model without A/C ? IDK... just tossing that out there.

I had this problem on a cherokee. I never could figure out what the root problem was, but I got frustrated and took the adjustment bracket out and made the slot for adjusting a little longer. Didn't need much. IIRC, I used a drill bit and just drilled a hole at the end of the slot and it made it bigger by the width of the drill bit.

They make 2 different sized idler pulleys all effective for almost all 4.0. One model , sadly cant remember which.. both from advance auto....one is a good bit larger than the other one. The bigger pulley takes up more slack than the smaller ones you have 2 one on the tensioner and one below the ac compressor. If you have 2 small ones you might not be able to take up the slack even with the correct belt

Interesting there Blue.....
I've always been in the practice of when I take an old part off (for replacement)....I always compare it to the new one to make sure dimensions, holes, etc all line up the same as the original part before installing the new one.
Saves a boatload of headaches.