What has been changed on the new/replacement tensioner?
This is a timely discussion since I just returned from the dealer with a new tensioner for my wife's 190e.
It was running fine except yesterday her A/C stopped. Checked it this morning and the belt was very loose. Attempted to tighten the adjustment rod after loosening the lock nut but the belt wasn't getting any tighter. Removed the tensioner and it looks physically fine, but I will certainly compare it to the new one.
I noted that the old one appears to have thread marks through the rear portion where the lock bolt goes through. The lock bolt came off with alot of material from this area. There in no threads or marks on the new one. I am assuming the old one is probably shot.