Coolant temperature weirdness

Bernie Benz b.benz at
Fri Mar 5 20:03:14 EST 2004

> From: Peter Schulz <pcschulz at>
> Bernie:
> What we seem to be experiencing is NOT the usual failure mode of the
> MFTS..., which is what you describe ( no reading, then up to the 1/3 mark,
> then drop off again or a consistently low reading.
> My Indigo wagon, once the car warms up, the temp tends to stay at the 1/3
> mark, whether on the highway or stop and go...of course it will edge upward
> during stop and go, and the fan will come on.
> My grey wagon has the same behavior described...low readings on the
> highway, "normal" stop and go.
> since this is not the "classic" MFTS failure symptom of the erratic temp
> display, I suspected the Tstat as sticking and not closing fully.
> I am going to try to verify this using a borrowed infrared temp gun,
> measuring the temp of the water manifold when its at the "low" point, and
> again at the "normal" operating point.   If the temps DO vary, then the
> MFTS is working properly, and there is a problem elsewhere.
Peter,  Maybe no problem anywhere. (DFI if IAB!)  Remember that the
thermostat is analog, not a digital open/closed snap action device.  There
is a range of temperature between just opening and fully open, and a
slightly different range between just closing and fully closed.
I have experienced two Audi thermostat failures.  One failing open, a
mechanical, broken cartridge support bridge.  The other failing closed, a
bad thermal cartridge, taking 1K+ miles to completely fail.  If you buy a
new thermo, put both the new and the old one in the same pan of water on the
stove and observe the differences in temp. and full stroke as you heat and
cool them.
> Question for the group should be..are you running the 3 or 4 pin MFTS...
Contrary to others' experiences, I have found, on 4 Audis over the years,
that the passive 3 wire MFTS to be the more reliable of the two.   I went
through many of each type, both new and used, before finding reliability
over time, always with the 3 wire version.

> -Peter
> At 04:01 PM 3/5/2004, you wrote:
>> So, what's your complaint, just that the temp gage reads low?  That's not
>> too bad, if it reads consistently low.  The MFTS usually fails in an
>> intermittent fashion, gage reading back and forth between zero and some
>> actual temp.
>> Bernie

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