Squealing S-belt problem...help!

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, by according to my XJ it wasn't.
If not the idler pulley what else could be going on?

How tight is the belt?

I had my a/c replaced, and had the same problem. Tighten the belt; although by the time mine was doing the voltage fluctuation like that it was too late; I destroyed the compressor.

Replaced it again, used a Krikit II gauge to check the serpentine tension, and now it works great.

Until you fix it, unplug the electrical connection from the a/c pump so you don't burn it up when running the defrost

Remove the belt and try spinning each pulley by hand. If any of them feel like they are grinding or stiff, you might have an issue. If they all spin freely, put the belt back on (or a new one if it's badly glazed, it probably is after squealing like that) and tighten it to factory spec. Factory spec is 180-200 lbs for a new belt (less than 15 minutes in operation) or 140-160 lbs for a used belt (more than 15 minutes operation.) Use a gauge (the Krikit II is $15 at NAPA) rather than the old "twist it" or "push on it and check deflection" methods, they really aren't accurate enough.

IslanderOffRoad said:

How tight is the belt?

I had my a/c replaced, and had the same problem. Tighten the belt; although by the time mine was doing the voltage fluctuation like that it was too late; I destroyed the compressor.

Replaced it again, used a Krikit II gauge to check the serpentine tension, and now it works great.

Until you fix it, unplug the electrical connection from the a/c pump so you don't burn it up when running the defrost

Click to expand...

I’m thinking (praying) it's not too late for my compressor. No odd noises coming from it like last yr. Since the squealing issue I've been using the heat on the floor setting with no problems (no squealing). Is this ok or should I disconnect the A/C comp completely? The 30 degree weather in Baltimore is putting a damper on diving in and checking all the items you guys recommended.

I had this same thing and after changing out every pully on the motor it ended up being the Hapmonic Balancer. It wore off the rubber and was hitting the timing cover. Just to make sure check that.