89 300SE
I currently have a full and working A/C system... after several years of not working. The Aux fan is not coming on even with a jumper at the dryer switch.

The fan works great with the high temp trigger at the high speed fan.

Is this telling me to change the aux fan resistor??

ALSO...Are these aux fans set up to run after the car is shut off to bring down the motor temp on hot days... like 80 deg., long trip, a/c was on etc...??
If so mine at high speed fan engagement shuts off with key, and I thought it should run for a few minutes. If you go to shortly re-start the temp guage can be sometimes up to 115ish before restarted and the fan gets going again.

Resistor is one possibility, the other is a corroded connection at the resistor terminal box, but are you sure you are jumping the wires at the right place?

When you separate the plug at the high pressure switch to check, make sure that you jumper the wires going to the fan, and not the wires going to the switch.

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'95 E300 Diesel, 264,000 Miles. [Sold it]

I did jump the right set of wires... no action. Where is the resistor box you mention? I fould the resistor under the master cyl... looks like a fun time getting in there.

That's where it is. IIRC, you need a very long philips screwdriver with a magnetic tip to remove the screws that hold the wires to the resistor. Mark

Thank you for the responses.
Does the aux fan also suppose to stay on , after the engine is turned off, if the motor is hot enough? Mine shuts off always with the key.

Yes, the aux. fan is supposed to turn off when the key is turned off. Mark