'01 S-80 Timing belt moving off pulleys
 

Help! I replaced belt, tensioner, and water pump 1K miles ago and the timing belt suddenly has moved 3/4 inch off center pushing serpentine off. I shut down as belt is hanging on by 1/4 inch. WHY? Repair?

Is the crankshaft balancer pulley tight?

I believe so. The water pump gear seems too far "out" to allow belt to seat against its inner most (closest to the block) and slanted gear face though.

Double check to make sure the water pump bolts are tight & torqued. They probrobly are or you would have a leak. Post a few pics please. Can't imagine the water pump shaft bent or coming out?

Where did you get the parts from? Sounds like a quality issue..

This can only be a few things. I will assume it's sudden and was aligned well when the job was done.
A damaged or loose pulley or pulley bearing and sometimes an internal cord on a belt can let go. It happens, manufacture a million of something and you can get a bad one. Sometimes a washer can be left off behind a pulley or a spacer missed. I've done it.

You didn't mention the idler.
Ed