[audi20v] Re: overheating

Brett Dikeman brett at cloud9.net
Tue Aug 26 12:26:46 EDT 2003

>>> 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

> 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 other.

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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