Only two or three things can cause this. Loose belt, faulty compressor control unit, or faulty sensor on the rear of the compressor.

If the belt is the least bit loose, start with the tensioner.

------------------
Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM