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?