Re: [MBZ] Aux Fan Pooped Out

[Peter Frederick]

Check the fuse first, then relay.  Both in the fuse box on a W123, I 
think.  Also check the big rectangular ceramic resistor (up front 
somewhere, not exactly sure on that model) for corroded wiring.  This 
will allow it to run at full speed at high engine temps, but fail at AC 
speed.  Check by grounding the wire that goes to the switch on the 
thermostat housing -- must cause the fan to run at high speed.

The fan itself very rarely fails, although they can.
Peter