Installed new fan control module from M-B... with A/C engaged and coolant temp > 95 deg C, aux fans ran for about 1 sec and then stopped.


Vehicle is a '97 E420, with a production date of 11.97 (production ran late due to late EPA approval of E430).

Fan control module has six groups of pins (A-F); each group contains one or more pins. All groups are used except for group C.

With key on, module is receiving constant 12V at pins A1 & A3; pin A5 is a dedicated ground. Group D (two pins) is the dedicated power supply to aux fan.

Group E consists of 7 pins, all of which are used; function is unknown. Group B has 2 pins; group F, one pin.


Aux fan runs when module is bypassed and power/ground are supplied directly.

With engine on and aux fan activation criteria met (confirmed using HHT), voltage at group D is 0. However, module can be heard and felt to click on a regular basis; approx 1 sec after the click, the voltage at group D momentarily jumps to approx. 8.6 volts and then quickly returns to 0.

Internal inspection of the old module revealed no gross defects.


Any idea as to whats happening here? What should be checked next? Is a signal feed to the module causing the fault or is the new module defective (although wrapped in padded paper and received in a sealed box, the upper plastic lip of the module was cracked; a corresponding gouge was noted on the interior of the box)? TIA.