My drive belt came off last night. So I replaced it cause it needed it. But every time I put the belt on and go turn on the car it pops off. Is this due to a faulty tensioner? My tensioner moves when I move it so I am not sure if it was that.
My car has 140000 miles.
My car has an automatic transmission.
The first thing to check is whether all of the pulleys are in the same plane. Imagine a flat surface applied up against the pulley edges. Do all pulleys touch the flat surface? If a pulley, for instance, on the harmonic balancer, is out of plane relative to the other pulleys, the belt could come off. The tensioner should be pretty hard to move (pivot) by hand without a tool applied to the end acting as a lever.
This is hard to evaluate without seeing but once the tensioner is in position against the drive belt, the belt should be very, very tight. You should not feel any slack at all when applying force to the belt in any open unsupported area of the belt (between pulleys).
Also, check to be sure the belt is routed exactly as intended. If the belt is not routed through the pulleys and accessories correctly, it will come off. Finally, all pulleys that are rolling on bearings should turn absolutely smoothly with no noise or resistance.
If you need assistance with this, consider YourMechanic, as one of our mobile technicians can come to your location to ensure a proper installment of the new drive belt.