Thermostat and 3pin MFTS = low water temp at motorway

[Jonas_R]

So,

When I'm driving on the motorway my temp gauge seems to drop 3-4 bars below 90 degrees.

Ofcourse I'm thinking that it is the thermostat, but then I find this answer (link to thread) by Porkkis that mentions that the 3-pin MFTS is affected by the wind and thereby showing wrong temperature.

So I have a couple of questions:

Looking at S2central.net it seems that the ECU uses the 2-pin water temp sensor and not the MFTS to read the coolant temperature? Then I guess I could use VAGCOM to check and see what the water temperature is? If it stays at 90 at motorway then it is not the thermostat but the MFTS that is the problem?

Second has anybody besides Porkkis tried to screen the MFTS sensor? Any suggestions or pictures?

If indeed it is the thermostat, then is there a guide that shows how to remove and refit it? Or can some tell me how much **** I need to remove to get to it?

/Jonas

[dobbin]

i had same problem 2 weeks ago changed mfts but did not cure , changed stat now car runs around 90 all time now, i fitted stat myself took around 30mins to fit

[UrS4boy]

Coolant Temperature Senders

There are four coolant temperature senders of interest:

1. The ECU gets its coolant temperature signal from the G62 sender at the back of the head. http://forums.quattroworld.com/s4s6/msgs/20985.phtml


2. The Automatic Climate control gets its temperature signal from a separate sender located in a "T" at the back of the head. This temperature signal can be read through Channel 51 of the automatic climate control faceplate. http://forums.quattroworld.com/s4s6/msgs/22679.phtml and

http://forums.quattroworld.com/s4s6/msgs/21881.phtml

3. The multifunction temperature switch (MFTS) that sends the temp signal to the gauge cluster. This reading can be compared to the Ch. 51 reading to help diagnose whether you have a MFTS problem or a thermostat problem. http://www.audiworld.com/tech/int5.shtml

Note: this MFTS switch is supposed to have a rubber boot ("grommet") over the connector (Item 29 below) and exposed switch portion, as shown as Item 27 in the diagram below. http://forums.quattroworld.com/s4s6/msgs/22584.phtml

4. The after-run pump climate control switch - this is one that triggers the after run pump. http://forums.quattroworld.com/s4s6/msgs/21503.phtml

[Jonas_R]

Thanks for the info seems to confirm what I thought.

However it does not seem that the information mentions if the MFTS can have false reading due to wind cooling it down?

[UrS4boy]

Thanks for the info seems to confirm what I thought.
However it does not seem that the information mentions if the MFTS can have false reading due to wind cooling it down?

I would say that is because I personally do not believe that to be true. That said, I think the MFTS should be run with the large rubber boot (grommet) (aka "condom") to keep warm and dry.

[newsh]

Yes, if the rubber boot is missing it will cool and show a low temp in cold weather on the motorway. I had a bunch of water warnings and found that the connector and terminals were all corroded because the boot had split and was full of watery muck, so I threw it away and fitted a new sensor but ended up with this issue, I cured it with a bit of rad hose split down the side and tie wrapped on.

[Jonas_R]

Yeah the rubber boot will help with protection against water. But the part of the sensor where you thighten it will still be exposed.

Anyhow I guess I will give it a go with some kind screen and see if it has any effect at all.

[UrS4boy]

AAN Coolant Sender Info

As a result of my above post, I turned that info into this single (but hyperlinked) kawf post for my home AAN-based forum.

http://forums.quattroworld.com/s4s6/msgs/96144.phtml

[UrS4boy]

Seems like the "wind" in the engine bay *might* be related to running or not-running a belly pan. Maybe not. Might be a B3/B4 versus a C4 thing. (To do with the different radiator arrangements).

[gedu]

I have this exact problem with C4 chassis. Might have to do more with missing radiator "sideflaps" than anything else on my ride, as it used to read correct temp with those air guides intact.

[UrS4boy]

I have this exact problem with C4 chassis. Might have to do more with missing radiator "sideflaps" than anything else on my ride, as it used to read correct temp with those air guides intact.

That would definitely make sense since without the air guides in place, raw, cold, air can sneak by the radiator into the engine bay. With the air guides in place, the air entering the engine bay from the front will be warmed as it is forced through the radiator. Good call.

[huksa]

This forum is indeed filled with excellent info.

I have recently noticed that the temp gauge drops while cruising on highway.

It was - 7 deg. C and I was going around 120-140km/h at night. I noticed that the water temp needle pointed exactly between 60 and 90 degrees. Oil temp stayed same at the bold bar between 60 and 130 deg. as usual (I think that's around 90?)

So I cross checked with Ch. 51 readings and here is what I found:

- Ch. 51 reads 88-90 in notmal condition (the fan kicks at 89-90 deg.). The temp gauge points at a tick below 90.
- Cruising at 120 km/h, the temperature reading (Ch. 51) drops to as low as 84 degree C and the needle goes down by 3-4 bars, settling between 60 and 90.

Now. two questions pops up in my head..

1. Is it reasonable for the water temp drops approx. 5 deg C while driving at a cold (-7 deg C) night?
2. Is it reasonable for the temp gauge needle drops by 3-4 bars for such change in water temperature?

I think the water temp always changes, but I am not sure how much change is acceptable as a normal operating condition.. Your thoughts are appreciated.

[UrS4boy]

F76 Multifunction Temperature Switch (MFTS) Info

- Ch. 51 reads 88-90 in normal condition (the fan kicks at 89-90 deg.). The temp gauge points at a tick below 90.
- Cruising at 120 km/h, the temperature reading (Ch. 51) drops to as low as 84 degree C and the needle goes down by 3-4 bars, settling between 60 and 90.

1. I don't think that any fan should come on until about 93 or 94 C, unless you turn the air conditioning on.

2. At 120 km/h in -7C weather, you are getting a significant cooling effect, so dropping to 84C on the climate control Ch.51 readout is perhaps not surprising.

3. IF you do not have the rubber boot over the F76 MFTS, all that cool air will cool the MFTS and the analogue gauge reading you get will be too low.

Click on the MFTS label in this hotlinked Engine Bay Device Map (all the other labels are live links too - for future reference):

http://members.shaw.ca/speedtek/UrS%20Engine%20Bay%20Device%20Map.html

[Michael S2]

Great info as always. Seems many have the gauge dropping at motorway speeds. Does this mean Audi made a car which shows the wrong temperature if travelling at high speed? I find this hard to believe.

Can anyone confirm there gauge stays at close to 90 all the time once warmed up? Gauge on my avant drops and if I remember correct my old coupe did the same
Maybe i had the same fault in both cars.

[Jonas_R]

No it just means that we have old cars and lots of stuff go missing over the years.

Such as the rubber gormet that urs4boy mentioned or the radiotor airguides or the belly pan etc. All these things can effect the mfts getting exposed to cool air.

But if you read out the engine temp sensor and it shows close to 90C then everything is fine. If it is as low as the dash gauge, then it may be the thermostat.