'91 F150 Serpentine belt

you don't 'remove' the tensioner pulley, you SWIVEL it on its bracket, agains spring pressure

Re-install the pulley and Use a long breaker bar in that square hole on the tensioner to swivel it away. The belt should then slip over it easily

belt back on! I

the pulleys. The

just as depicted on

shrink ???

It seems there can only be two possible problems.

1. Alternator not mounted properly.
2. Belt not routed properly. It may be on the wrong side of something or hooked on a bolt or other protrusion.

Recheck everything. As mentioned in other responses, it is not necessary to remove the tension pulley. It should be held against spring tension as far as necessary to get the belt on it.

Thanks for the replies. It was my own stupidity, it was routed wrong. That's what you get working on something until midnight!