Ford PU Serpentine Belt

Sight along all of the pulleys to see if the belt is peeking out on one of them. In particular, look where the belt goes around the spring-loaded tensioner pulley. If it isn't inline with all of the other pulleys, the pivot bushing is worn and the tensioner must be replaced. 1/8" off-center is enough to cause problems, but usually it will cause a squeal too as the belt slides sideways across the pulley as it goes around it.

Caradiodoc

Sight down along all of the pulleys to see if the belt is peeking out on one of them. In particular, look at the spring-loaded tensioner pulley. If the belt is not inline there with all of the other pulleys, the pivot bushing is worn and the tensioner must be replaced. 1/8" off-center is enough to cause a problem, but it will usually also cause a squeal as the belt slides sideways across the pulley as it goes around it.

Caradiodoc