w124 87 300d
 

My aux cooling fan runs constantly. I am sure I have read here before about this so my apologies for opening another thread. One of the sensors is obviously not functioning. And btw, it runs constantly whether the a/c is on or not.
Thanks.

Should be a sensor on the drivers side under hood near the headlight on the frame rail. Two wires. Fan should turn off when unplugged.

I can take a pic when I get home this afternoon.

Sounds like the temperature switch has failed closed. Trace the upper radiator hose back to the head, shortly after the rubber stops and the metal starts, there is a sensor with two or three wires. Pull a wire or two and the fan should shut off.

Switch would be failed open. Pulling those two wires makes the fan come on. Might have to pull the relay or pull the plug for the fans. Right next to the ac dryer behind the driver headlight.

On my '87 wagon, I have to short across the temperature sensor I described in order to make the aux fan run. The switch would have to fail closed.



Looks like on later cars, your method should work. Any break in the circuit between the sensor and the PBU would cause the aux fan to run.