[audi20v] Re: overheating

Eugene Kanter eugene at bcl.bz
Thu Aug 28 09:35:12 EDT 2003

Could anyone tell me when (according to in-dash temperature control)
radiator fan starts if car left idle for a while? The first stage radiator
thermoswitch should close just above 90C I think. But temperature control
has no digits!


-----Original Message-----
From: 200q20v-admin at audifans.com [mailto:200q20v-admin at audifans.com] On
Behalf Of Brett Dikeman
Sent: Monday, August 25, 2003 11:14 PM
To: Peter Schulz
Cc: b.benz at charter.net; 200q20v
Subject: Re: [audi20v] Re: overheating

>>> From: Peter Schulz <peschulz at cisco.com>
>>> the ECU and Temp gauge get  temperature information from the
>>> Multifunction temperature sender ofrMFTS, which is on the left side
>>> of the engine, below and forward of the distributor
>>Wrong, Peter.  Neither the MPI nor the Motronic ECUs derive coolent
>>temp info from the MFTS.  Both use a dedicated sensor located on the
>>back of the head.

> At 01:51 PM 8/25/2003 -0700, you wrote:

> If thats the case why does removing the lead to the MFTS in some cases
> restore boost (part of diagnosing a bad MFTS)?

[beats head repeatedly against the wall for this is only the billionth time
we've gone over this :) ]

The MFTS only provides a we're-about-to-overheat signal to the ECU/Climate
control, a holy-crap-we're-overheating signal to autocheck, and a
temperature level to the gauge.

The MFTS sinks current from autocheck to indicate an overheat, sinks current
from the dash gauge, and sources current(possily from the gauge in the 3-pin
sensor versions?) for the ECU and CC head.

The ECU derives exact coolant temperature for purposes of starting and
timing, from the sensor bernie mentions- the sensor on the back of the
block.  If said sensor fails/shorts/is disconnected...the ECU assumes
maximum temperature and sets timing appropriately(I don't recall if boost is
also dialed back a little or not, I think it might be.)

Mike is right- the MFTS sends the overheat signal to the climate control
unit and the ECU(via pin #2), and also drives autocheck(pin #4).  If you'd
all kindly flip to page 545 in your Bentley repair manuals, you'd see pin 2
of the MFTS goes to current track 473(CC head) and 43(ECU).

Pin 4 is marked as tied to the float switch's supply side(pin #1 on the
float goes to ground point #82), but supply is marked as track #276- a track
which does not exist in my copy.  Obviously the circuit works in the car,
and the other leg is ground, so #276 must go to the auto-check system, and
it must be a source.  I don't think they would use pin #4 as supply in the
3-pin version for signalling the ECU+CC, as a low-coolant condition would
drop the voltage on pin #4 to zero.

One sidenote- pin #2(ECU/CC overheat signal) triggers earlier than pin
#4(Autocheck).  If you were to disassemble one of the old-style sensors,
you'd find a wax-expansion-driven piston setup, whereby the piston presses
against a spring-loaded canteliever-type deal, relay-style, with two sets of
contacts. Playing with the lever, you'll find one set closes sooner than the

This of course makes perfect operational sense- if the car's overheating,
the sensor first tries to save the bacon by cutting A/C and telling the ECU
to knock off the high-HP shenanigans.  Failing that, scream at the driver...

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