I have a 1991 500 SL and my serpentine belt started squealing. I found the belt to be loose and tried to tighten it. I could not adjust the tensioner enough to tighten the belt.

I ordered a new tensioner and adjuster rod and replaced the old items. After the replacement, the new units worked and I tightened the belt (belt tight at the first mark on new tensioner). After cranking engine & checking the belt, it was tight and all worked well.

I then drove the car at normal operating temp for about a mile and stopped to recheck the belt tension and found it was loose again. I re-tightened the tensioner/belt and noticed the tensioner was then at the 3rd mark - odd I thought for a new tensioner.

I then drove the car again at normal operating temp for about a mile and stopped to recheck the belt tension, found it was loose once again, and noted the tensioner had moved past the final 4th mark.

The belt is installed correctly and all pulleys turn freely and no binding.

Thoughts on a new bad tensioner or some other problem I'm not aware of?