It comes on for two reasons. One is if the pressure of the refrigerant is at a high level. The fans will come on low.
IF the engine is running hot, the fans will come on high.
IF your fans are not coming on the low speed with the A/C on in the summer, you have a problem. (We see the 15a fuse under the fuse box cover blow, it should be updated to a 2Oa.)
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Benzmac:
Donnie Drummonds
1992 500E (very soon I hope

1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM