UrS4boy

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, plastic connector, if it breaks, is 893971974)

<img src="http://pictureposter.audiworld.com/15624/3b_enginedevices.jpg">

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:

<img src="http://pictureposter.audiworld.com/15624/enginecoolantg62.jpg">

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 035 919 369 M (KATE calls it a thermal switch - No. 9 Illustration 919-65 (A10Q, 1993))

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).

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

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

<img src="http://pictureposter.audiworld.com/15624/aanenginesensors_non-turboside.jpg">

Just in case you needed to know.<ul><li><a href="http://www.sjmautotechnik.com/trouble_shooting/cooling.html#gauge">Link More MFTS Info at SJM Autotechnik</a></li></ul>

alpinab7

It seems to test ok per bentley but it seems very likely that there may be an intermittent problem with it. High emissions and very hard starting after sitting for a few days in cool weather. I have to crank for about 15 seconds and then again for about 10 before it turns over and stumbles to an idle. For $30 or so it's worth a shot. BTW, new parts: MAF, FP, 1.8t coil upgrade, plugs, fuel filter, o2, and lots of other stuff so i am ruling that out.

UrS4boy

$30 plus a few minutes time is cheap insurance and piece of mind.

alpinab7

I have no cat so not TOO dangerous

maciu

"If the AAN coolant switch is broken, the AAN ECU may "think" its cold out all the time"

Not always. If it outputs slightly higher resistance than it should then yes, I agree.

It's not a switch it's a sensor and in case
of failure (decision is up to ECU) intake air
temp is taken for period of three minutes
when air temp is below zero and after
3 mins is constant of 80C substituted
also this constant is substituted when intake
air temp is &gt; 0C.

It all depends on what are the
conditions inside ECU for it to assume
that this sensor is broken. Maybe only
wiring problems such extremely strange
resistances. Maybe not.

Anyway this sensor is very cheap
and very easy to replace so no worries

UrS4boy

Good to know. Easier to just replace the sensor.

Bisarre

I don't agree... The analogue signal is just for the instrument in the dash.
There's two digital signals as well. One shorts to ground if the temperature is over 119 degrees centigrade. This tells the AC control unit to shut off the compressor. The other is for the auto check system on the S4 and S6. This one is also a short to ground if the temperature exceeds 119 degrees centigrade. Tells the auto check system that the temperature is to high. But also for the ECU on the 200 models. And will cause the ECU to lower boost. NOT on the S4/S6 models, where boost is controlled by the G62 sensor on the back of the cylinder head.

4x5x5

Has anyone Ohmed one of these recently? Saw the thread and having experienced very poor mileage thought why not. I could only get momentary readings on the existing unit. I'd get an instantaneous reading and then my DMM would go back to full open/infinity readings SO I bought a new one and it was almost exactly the same. Not sure why resistance measurement will not read out.