There are some rubber bushings that suspend the compressor from its mount that may have collapsed. You'll be able to tell because one or more will be flattened on one side and the bolt running through it will be off-center.

Cheap part, easy to replace.

If I understand your question correctly, the a/c belt is not aligned squarely with the crank pulley.

I have dreaded dealing with the a/c belt for years, but I figured out a solution last year when I needed to remove my compressor so I could convert the system to 134a.

On my 1990 240DL there are only two bolts to deal with.

With a friend to assist me, I took a strap wrench and wrapped it around the middle part of the body of the a/c compressor and then had my friend apply pressure on the strap wrench to tighten the belt.

Then, I reached in, and tightened the two nuts that keep it in place.

I was done and it was perfectly aligned with the crank pulley.

I hope this helps.
--
If it needs to be maintained, repaired or replaced on a 1990 240, I've probably done it. '90 240DL, 279K looking forward to 300K badge (or sticker??). >>You haven't really worked on a car until you draw blood<< :-}