Got a S5 TII.

There is a sensor with two wires that is at the base of the back of the thermostat housing. It looks like a sending unit for the gauge but I could be wrong.

 

coolant temp sensor by the sounds of it

 

It's probably the ecu waterthermosensor. Is it green?

 

There are usually two sensors on the back of the S5 water pump housing. One is the ECU's thermosensor and the other is the auxiliary electric fan's temp switch. If your car doesn't have the fan, it won't have the temp switch, just the thermosensor.

 

NZ, now that the question is answered. Would bolting on an S5 electric fan be a much better idea than buying an Efan for an s4? Also, does the temp sensor for the fan work with the ECU, or is it actually a switch that completes the fan circuit at a specific temperature?

thank you.

 

Both S4's and S5's used the same 10" electric fan, which is totally inadequate for use as the only cooling fan. It's an auxiliary fan only.

The e-fan works independantly of the ECU, and is triggered by either the A/C or the 205degF temp switch switching a relay. The S5 circuit is very simple, but the S4's uses a more complex control box because it also has a warning light. See the Cooling System chapters of the S4 and S5 FSM's for the circuit diagrams.

 

I don't have a plug on my harness for this sensor...will I need it for the engine to run?

 

S4's have the switch on top of the thermostat housing. It is not needed for the engine to run.

 

...but it's a S5...and its on the back, but not ON the housing...under the housing

 

Download the free factory service manual and look.

 

Or acurrately describe the colors of the two wires.

It should be one wire for the fan sensor and two for a water thermo sensor.

 

As I said...