The fan should come on low with the A.C. on and the car at idle on a hot day.

The aux fan should run on high with engine temps above 115.

To check, just bridge the high pressure switch on the a.c drier. (red) The fan should come on low.

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Benzmac:
Donnie Drummonds
1991 GMC Syclone
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM