Mercedes designed the a/c compressor with a cut-out system to protect the multi-rib belt in case of compressor failure! The relay & related parts have costed Mercedes owners $$ & frustration. Sounds like your compressor relay doesn't THINK the the a/c compressor is going the correct speed, most likely a BAD A/C COMPRESSOR!! I understand that some "aftermarket" rebuilders of electronic componets (a/c relay) rebuild the relay & leave out the speed signal imputs.
MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS M.B. TECH