In hot weather, with the AC on the engine fan will come on with the aux fan to cool off the radiator and the condensor. This is normal and should not happen on the highway. IF it does look for bugs clogging up the fins of the condensor and or the radiator.

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