any mercedes built before 1993 may also have an auxilary fan problem. The problem is they decide to run whenever they want; even when the car has just started and the tempurature is well within normal operation range.

This problem is indicitive of a defective climate control, believe it or not, 3 different things control the auxilary fans:
1: AC high pressure switch though a resistor, located on the reciever drier
2: High Engine temp control switch, located on the top of the cylinder head
3: Climate control circuit board, located inside climate control unit in console.

I had this problem with my car and it annoyed the crap out of me. After viewing the wiring diagrams i ruled out the two sensors and narrowed in on the climate control, since these units seem to be so unreliable. What i ended up doing is figured out which relay controlled the auxilary fans on "high" operation. Pin 85 is responsible for turning the aux fans on and off on high operation. DO NOT simply cut the wires to this pin because this would render the fans useless when the high temp sensor closes. Gain axcess to the climate control plugs through the console and unplug (i believe it is the left) plug, take a voltage meter and test which pin inside the plug connects to pin 85 of the aux fan relay. once you have found the pin/wire remove the plastic cover to the climate control plug and carefully remove it from the plug, carefully put the cover back on. (I say carefully because all the wires are loose once you open the plug). wrap the single pin in electrical tape and plug back into climate control.

this is all that has to be done to repair this problem. In all honesty I haven't a clue why the climate control needs say in the auxilary fan operation, when the other two switches control it already. I hope this saves someone from simply replacing parts to find they don't fix the problem!

I've attached the wiring diagram for the aux fans, for the v-8 engines, the other engines also run on the same diagram:

the pink color indicates climate control input to the relay, the green is engine high temp input to the same relay pin.
blue indicates refridgerant high pressure circuit via resistor (low speed operation)
red and yellow indicate high voltage wire to run fan motors.

the fans will run independent of the climate control input.



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