I've searched and found lots of good stuff on fixing my squeaky belt. However, I can't tell if I have a pulley squeaking, or if I need a new tensioner. Is there a definitive way to tell exactly which part I should replace?

Thanks,
Scott

If you think you have a pulley squeaking, spray the offending pulley with wd40 while the truck is OFF and be carefull to get only that pulley.Then fire it up.....squeak gone? it was that pulley.If it starts to squeak again later, replace.

Remove the belt and turn the idler and tensioner pullys by hand. If they make any noises, wobble, or bind, replace them.

How many miles are on the current tensioner? You might want to replace anyway, they are known to fail at the worst possible time. Mine had 80K miles and was working fine, but I didn't want to take the change of it leaving me stranded. If in doubt of the tensioner, replace it.

Also replacing the tentiones pulley and the belt before they break leaves you7 with spares just in case...

the tensioner pulley and the idler pulley are the same part (at least on the 91-97 4.0L), Autozone has them on the shelf for like $15 ea, far cheaper then other stores I have found.

I popped the dust cover off my pully and shot in some grease. They were bone dry. The best test is to change the belt. I could see more white bushing on one side of the tensioner than the other. I went to remove the belt and the arm went BOING! Stuck at home so I made a new arm bushing for the tensioner. My design isn't any worse than Fords. 6K later and it is still holding up.