Re: Water temp switch in readiator for S4 turbo
This switch/sensor is located at the bottom drivers side corner of the radiator.

What is it for? Why does the FSM use two different names for it and never describe how to test it (the FSM doesn't say what temperature it should switch at, whether it should have continuity at high temp or low temp, or what effect this has on the operation of the vehicle).

The only references to this sensor that I can find in the FSM are (for the S4 Turbo model):

Cooling section:
3-14 - in the picture showing how to remove the radiator, item number 8 is labeled "Radiator Switch Connector", implying that the switch would be called the "Radiator Switch"

Wiring Diagram section:
5-22 - Here the switch is labeled "Water Temp Switch" and is shown with one contact grounded and the other as an input to the ECU (plug F-31, pin H). The connector to the switch itself is labeled F-37

5-23 - shows a close up of connector F-37, again calling the switch "Water Temp Switch" and indicating that the "connector" is actually two separate plugs, one for each switch contact.

50-114 - Front harness (page 1/2), shows double connector for "Water Temp Switch"

I just replaced my radiator and am curious what this switch does? I had never heard of it before.

 

It is the temp sensor for the 3000 rpm warm-up, which happens during the first 30 seconds of starting a cold engine. Starting the car in gear, unplugging this sensor, or unplugging/emilinated the AWS eliminates this system.

 

The temp switch is covered in the EFI section because it's part of the emission control system. It's for both the secondary air injection system and accelerated warm-up system. You'll find how to test it on page 4B-46.