Technical 1.2 Drive belt jumps a rib

[garyjpaterson]

So just noticed the drive belt has jumped a rib (away from the engine) on the crank pulley, and seems to have been like that for a while given the light rust on the exposed rib.

I was able to get it back on properly, but sure enough on startup it pops back out again.

The belt definitely needs replacing, as it is cracking. Could that cause it, or is something else likely at play, tensioner maybe? Nothing looked obviously out of alignment or anything.

 

[RalphM]

I would change the belt, count the ribs and the groove's some have a extra groove in the pully,
if so make sure it's running in the right ones.

 

[garyjpaterson]

So I figured out what the problem is - someone has fitted a non-A/C crankshaft pulley to my A/C car. The offset seems different, looking at pictures.
Should be an easy fix to swap the pulley out, just need to make sure its lined up same as before for the sensor?

I am quite perturbed as to why it's been changed at all though. They have a rubber damper section, but do they usually need replacing before 90k miles?