Belt squeal with A/C off, gone when ON???

There may be a big difference in the makeup of the two belts as in the amount of cording and even the rubber compounds used. The new one is probably a less ridged more stretchable construction than the OE.

I put the belts side by side and they are identical in length, or so it seems. Obviously the original is shorter thus giving a tighter tension. But it's not visible to the naked eye. This has an automatic belt tensioner and it was replaced 20K ago. It works fine. I am not sure if it was Napa. I am looking for the receipt. I know I shopped around for a high quality belt. I don't go cheap. I don't want to have a trip ruined with mechanical issues. I do most of the PM myself and replace components per Ford, that's why I changed out a perfectly good belt at 50K.

NAPA offers very high-quality belts. Better than OE if you pick the premium flavors. Yes, the bottom-of-the-barrel option may not be up to par, but that's why its only $15.

They make a severe duty belt which is green on one side. This is what I run now, after testing it out on our repo trucks. Those trucks all had a hydraulic pump driven by the Serpentine belt, and it only pumps when you use the hydraulic controls.

There is an electric clutch on the compressor. Perhaps it's the bearing on the clutch, instead of the belt.

Dusty

need to tighten up the pulley tensioner,happy-camping

Maybe you should have compared belt lengths before you put NAPA belt on last time.

It took 2 minutes to remove the belt. I checked all pulleys for wobble. All were tight, nothing obvious. Then I took the old belt I saved for emergency. It actually looked better than the one I took off. I had my notes from last time and the order I used with a diagram. Still took 75 minutes to put it on. Finally used duct tape to secure the belt to the pulleys. When I used my 1/2 drive with 3/4" PVC 24" pipe for leverage on the tensioner, I found the old spare belt required more room to get the belt on. Had to remove the air intake housing that the filter attaches to. The belt is significantly tighter than the one I just took off. I removed all the duct tape and put the air filter back on. Fired it up without the AC, reving the engine, no squeal. Turned on the AC and still no squeal. So I got off lucky. I can't imagine what the AC clutch and whatever plus labor would have cost.
I will pickup a new belt, this time from FORD. I think I got that belt from Napa. Will change it out in 10-20K as the old one still looked in great shape.

I agree with you, and in the same order you gave. Good luck.

(The belt is the cheapest and easiest to replace. The tensioner is second in both cost and difficulty. Finally, the clutch is the most expensive and the most difficult.)