I have a 1991 Chrysler New Yorker who's serpentine belt comes off on its own too often. This has happened 3 times in the last year. What could cause this? I have replaced tensioner, idler pully, alternator and new belts. The last belt that was put on only stayed on for 3 months. The belt is not shreaded and was put back on by the mechanic that installed the tensioner and he can see no reason for this failure. Belt runs straight and smooth once installed but I can no longer have confidence in driving the vehicle too far. Anyone have similar problems with belts? Any help would be appreciated.

 

I would check all accessories for play or looseness, if that's good then take a straight edge and put it from the crank pulley to all the other pulleys they should all be exactly straight across from each other. if just one is off, it could cause the belt to jump or move over enough to come off.