View Full Version : Aux Fan Running - Thermal Switch Question

09-09-2008, 11:51 PM
I'm new here, I've got a Laser Red '01 1.8TQ and I've got few questions.

My Auxiliary fan is always running whether the A/C is on or off. It is not super loud, I believe it is running in low speed. I've done some searching, and it looks like the first thing to check is the Thermal Switch at the bottom of the radiator, driver's side. When I unplug it the fan stops running - great - looks like a bad switch. But....

Everything I've read says it should be a 3 pin switch (usually white or gray) and it should screw in. Mine is 4 pin (d-shaped) and green and it clips in with a black plastic clip. It looks exactly like a coolant temp sensor. Am I missing something? Can somebody shed some light on this for me?

I've got pics if that would help. Thanks

09-10-2008, 10:34 AM
A little hard to photograph as it's in a tight space. Here are photos looking at the thermal switch from the top and bottom.

Notice it is green, 4 wire, and held in with a clip. Has anyone seen this before on a B5?



09-10-2008, 05:06 PM
that IS the coolant temp sensor.

The Thermo switch should be on the left (Driver's) side of the radiator about 5" from the bottom on the endtank.

If you have a multimeter handy it should open the contacts at temps below 160*F and close the contacts(turning on the fan) at temps 180*F+.

09-10-2008, 06:25 PM
These pictures are of the thermal switch location. We are looking at the bottom of the radiator, driver's side.


Intake tubing running along bottom of radiator in the first pic, and the power steering pump pulley in the second pic.

09-10-2008, 07:08 PM
I recognize the location, the thermoswitch should thread directly into the radiator endtank.

The hole above the lower coolant hose.

Switch/sensor looks like this.

09-10-2008, 08:25 PM
Hmm. There are no other electrical connectors on the radiator there, I've looked. You can see in the second picture. it shows the area above the lower coolant hose. There is nothing there.

Unplugging the connector from the green switch does make the electric fan stop. Plugging it in makes it start again. Unplugging this wire does not affect the Temp reading on the dash or the coolant temp reading in Vag com. Unplugging it does not set a code or turn on the CEL.

This isn't the coolant temp sensor. That is on the back of the block - I've already checked to make sure it was the updated green one ;)

09-10-2008, 09:05 PM
Some cars have two sensors. I may be wrong on this one, but the 2.7T A6 I did turbos on had two as well.

It may be a facelift thing, I'm not 100% sure to be honest. The secondary sensor may be an auxiliary input to the ECU for fan control.

09-19-2008, 09:39 PM
Figured it out. Looks like the PO's mechanic took out the (probably good) thermal switch in the lower rad hose and swapped in a Coolant Temp Sensor in an attempt to prevent all those "pesky temp sensor issues". The size and electrical connector happen to be the same on my A4. The sensor was allowing enough current through to kick on the relay for the Aux Fan low speed.

Audi dealership parts guy took my VIN and gave me this part number for my Aux Fan thermal switch:

8D0 959 481B

He also said that it was "uncommon" on the A4's. Must be just one model year, or half a model year? Who knows. Bought the part at ECS. Wasn't listed for the A4, but if you click S4 it comes right up.


You learn something new every day.