Squealing belt issue

My issue: only when I turn the heat onto the defog/defrost (windshield) setting, my S-belt will squeal badly. It does the same when I run the A/C, eventually stopping only to start again after a while. While this is going on my battery voltage will dip to 12V while the belt is screaming, once the tantrum stops the battery voltage will return to 14. My A/C compressor was replaced 6 months ago along with the S-belt. I'm thinking it's the idler pulley, if so what is the "correct" idler pulley size (mm/in.) for a 2001 XJ 4.0L? Advanced auto parts gave me the wrong size 6 months ago (much smaller) than what I currently have on my XJ, stating according to their system it was the correct size, not according to my XJ it wasn't.
If not the idler pulley what else could be going on?

This may help you.. I keep a close eye on my 91XJ all year round and i have spent some time checking on my S-belt. I found that it works a little better when adjusted slightly looser than what someone might tell you. Because when all that heat builds up under your hood while driving that S-belt is tightening up all by itself. you could loosen a little when you get up in the morning then check and see how tight it is when you have drivin awhile. Sometime's it might take a few day's to get just right... Good Luck!

Thanx, I'll give it a try

From the symptoms you are describing, the belt is slipping on the crank pulley. Generally, that is indicative of two things. One is a loose belt. The other is an AC pump that is stiff or seizing. When you had the pump replaced, it is possible that this system is over charged. This would make it very difficult to start up. I would suggest having the system checked as well as monitoring belt tension.

I just replaced the belt on my friends 89 because it was doing the same thing they have a different tensioner and idler setup though. My 2000 did this right before it blew up and i just tightened the belt and it stoped. Loosening it and letting it get tighter as you drive wouldn't really help since it is apparently loose and slipping even after it is warm now. good luck