The 3B 20vt engine used the Multi-function Temperature Switch (MFTS) (aka "electronic thermal switch" in the 3B diagram below), located under the intake manifold on the auxillary coolant manifold, to send a coolant temperature signal to the 3B Engine Control Unit (ECU)to help control boost and fuel. (See link to SJM Autotechnik below)(Replacement MFTS PN is 034919369C, the plastic electrical connector, if it breaks, is 893971974)



However, with the development of the AAN, the 1992-97 Audi S4/S6 ECU does not use the MFTS as a boost and fuel control data source; instead, it uses the signal from the ECU coolant temperature sensor mounted at the back of the cylinder head:



Here it is one the back of Tom S's S4 engine:

Unlabeled photo courtesy of Tom S.

If the AAN G62 coolant temperature sender is broken, the AAN ECU may "think" its cold out all the time, leading to higher duty cycles on the injectors and, therefore, rich runnng conditions (which, by the way, can burn out the catalytic convertors) and poor fuel economy. Part No. is 035919369M (KATE calls it a thermal switch - No. 9 Illustration 919-65 (A10Q, 1993))(Scroll down and look for Item 9 in the diagram) (Note: the 035919369M G62 coolant sender is a Bosch 0280130037)

This is what the study guides say about the G62:

VAG SSP 111 (3B) :



VAG SSP 143 (AAN):



The G62 is a temperature-influenced variable resistance. The ECU drives out a voltage (5? 12?) to the G62 via ECU pin T45/55 via a Grey with Brown stripe (GY/BR) wire. The return current is returned to ECU ground pin 30 (presumably via a brown wire).

If the G62 fails, it *might* throw a check engine light (CEL). Blinking the code out (or using a VAG tool) would reveal:

2312 (Blink Fault Code), 00522 VAG1551/2 Fault Code Number
Coolant sensor (G62)
Open or short circuit, Defective sensor or wiring, high resistance between sensor G62 and ECU J220
Symptom: Cold starting difficulties at low temperatures, Poor idle and acceleration during warm up
(Reference SJM Autotechnik)

From Bentley, the G62 (Engine Coolant Temperature (ECT)) sensor test procedure is as follows:

Allow the engine to cool to room temp (68F/20C)
Disconnect ECT sensor from harness connector
Switch multimeter to resistance range
Connect multimeter to terminals 1 and 2 of the ECT
Resistance must be 1500 to 3000 ohms

IF NO = replace ECT
IF YES = check wiring between sensor and ECM (ECU)

Reference: Bentley pg. J28-22

Here is a link to G62(035919369M) pricing at Autohaus AZ

The MFTS (F76 Electro thermoswitch) on the AAN is located somewhat as shown below (only more horizontal):



Here is another view (with PNs) (Note: MFTS = Item 16, the after-run pump temp switch is Item 15)


Just in case you needed to know.
Link to More MFTS Info at SJM Autotechnik