I replaced the serpentine belt a few months ago. The new belt kept jumping over one rib on the ac pump pulley. I thought maybe it was the belt so I put the old one back on and that one is doing it also.
I checked all the pulleys and none are loose. The belt is tight the tensioner is fairly new. Any ideas on why the belt keeps jumping?

 

Sounds like a pulley alignment issue to me. I would call around to different shops to see if any of them have a laser pulley alignment tool; it’s just a small laser mounted on a base with grooves that sit in the grooves of the pulley, then the laser is aimed at the other pulleys and should hit the same spot on all of them.

Also, does it do it when the AC clutch engages or even when it never engages (AC, defrost, blower all off)?

 

I noticed the alternator pulley was off. But that was a few months ago. I moved the alternator forward. I don't know when the belt jumps. If its when the ac pump in engaged or not.

 

I PM'ed you back.

Align the pulleys and replace your belt again.