An awful squealing noise comes from my '99 xj when I start it up. It seems to be related to the A/C. any thoughts? I am nowhere near a mechanic, so have no idea.

 

Only when the a/c or defroster is on could indicate a problem with the compressor or it's clutch, or the belt is loose.

 

Place a mechanics stethoscope (or long screwdriver) on the body of the suspect component

There are bearings in most of the rotating pulleys.

I believe there is a replaceable bearing for the A/C

 

"Seems related to the AC"
As in: it is not squealing when you start without the AC on?

And if so, what happens when you then switch the AC on?

I think you will have the answer to your question there..

 

Even a cheap mechanics stethoscope from Amazon is worth its weight in gold,, it takes away 90% of the guessing

 

The first thing to check is belt. make sure it is not loose, make sure it is not deteriorated with cracks, nor wet with oil splashed on it.

Loose, deteriorated, or oily belts may slip under normal loads at times, and thus make noise.

Easy to inspect for this. Cheap to fix.

https://www.amazon.com/Gates-91107-Belt-Tension-Tester/dp/B000MUTAGS/ref=asc_df_B000MUTAGS?tag=bngsmtphsnus-20&linkCode=df0&hvadid=80127027892111&hvnetw=s&hvq mt=e&hvbmt=be&hvdev=m&hvlocint=&hvlocphy=&hvtargid =pla-4583726553823217&psc=1

Belt tension gage is handy.

However note that these belts must be extremely tight, over the longest span of belt between pulleys, pressure with your thumb should yield very little belt deflection, with moderate force for the average healthy adult, a bit less than 1/4 inch deflection max. for the pressure I apply. but you kind of need to calibrate your thumb first on a known correct adjusted belt so you get a feel for it.
The jeep has a Serpentine Belt, not a Vee Belt. you can probably find youtube videos on how to check Serpentine belt tension without a gage. Vee Belts are set much looser than Serpentine Belts, you oft get twice or more deflection with a vee belt. So understand the difference.

Look for cracks on both sides i,e. inner and outer sides of belt. if cracked, replace.

New belts do stretch at first and oft need to be readjusted after so many miles.

I own the gage linked above, but you can also get a rough estimate using your thumb if your thumb is calibrated. It is easy, but takes some experience . A gage is best when adjusting however, the thumb can be used in a pinch. Great pun, no?


If not belt, then it is something else. lets hope it is simply a belt issue

Besure to oil the belt tension adjuster block and the long thread for it before turning it. Made from powdered metal,****then sintered under heat and pressure, these adjusted blocks are known to split if too much force is applied, and its internal thread strips easy. Mine failed, and that is a show stopper, you NEED that little adjuster else you cant drive! So oil it well before turning! also make sure all clamp nuts on the movable bracket are loosened before turning the adjuster. Never force turn the adjuster, it will split or strip if you force it, Delicate! It is much more likely to fail if not lubed when turned.

Mine was replaced after it split , now with aftermarket part machined from cold rolled steel bar stock, this one will not fail.

Powder metal,parts have their place, but here it was a very poor design.

Have a spare on hand.

**** As found on my 1990, other models may differ.

 

Belt needs to be as tight as it needs to be. No belt needs to be super tight.
For years I followed the 'advise' for the XJ that those need to be "really tight". I didnt overdo it but started wearing out pulleys/components left and right.

Tight enough it does its job and doesnt squeal when warmed up is what you should try and achieve imo and should be enough. If things driven by the belt are failing or starting to make a noise, look into those individually.

 

Just get the gauge and stop guessing. 140-lbs for a used belt, 180 for brand new as I recall.