what could cause the belt to jump off the pulley? on 1995 GMC 2500 Pickup

My belt keeps jumping off the pulley. I thought it was becuase of the air conditioner pulley so I got a new one. I also got a new belt. I dont know what else to do.

Get Your Car Fixed at a RepairPal Certified Shop.
We certify shops based on expertise, quality work, and verified customer reviews
1 answer
usually its your idler or tensioner pulley that causes this problem