Hi after many hours of trying I need some help!!!!!!!!!!!

The aux drive belt keeps sliping off the pre-tensioner on my V40 T4, Has anyone any ideas??

The belt came off whilst I was driving and I have tried to replace with new but the problem still exists, (most puzzling).

Any help would be great cheers.

 

Is the bearings good in the tensioner and idler?

Make sure everything looks like it is inline.