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Re: quattro-digest V4 #1373
If anyone is interested, the negative temperature coefficient thermistor
used in the 5k and 200 Audi (probably 100s also) is available from Digi
Key in Thief River Falls, MN for approximately $2 each. As mentioned
before, there are two thermistors used by the AC controller. It reads
the two temperatures (resistances) and chooses the higher resistance
(lowest temperature) to display. This is Audi's way of trying to avoid
biasing the indicated temperature to a high value due to engine or road
Anyway not all 1 k negative temp thermistors are equivalent. You also
need to spec an R value that is the ratio of the resistance at 0 deg C
(could be 25 deg C, I'm getting forgetful) to the resistance at 50 deg
C. The Audi thermistor has an R of 9.1. If anyone wants the part number
and the actual R value, send me EMAIL direct so I don't miss it and I'll
send them to you.
Also, the system will run fine with only one thermistor. So if you
suspect a thermistor problem, try the system with only the plenum
thermistor and then with only the front (behind grill) thermistor. It
is unlikely that you have two bad thermistors, so one way should give
you the correct temp. I replaced mine and am now within +/- 1 deg of
actual (measured in the garage before the car is started) temp when
ambient is around 25 deg C (77 deg F).
If you want to force the AC to operate below its lowest temp, you can
find the wires going into the programmer and parallel a couple of
resistors across the thermistor leads to fool it into thinking it is
warmer than it really is outside. A single pole double throw switch
could be easily wired so you can switch it on and off. You could even
use microswitches that open and close with the activation of the
defroster flap if you wanted. As a starting point, 2.2 k should make
sure the indicated temperature is > 40 deg F.
Hope this is helpful.
Fred L. DeRoos
> Date: Tue, 18 Nov 1997 13:51:36 -0500 (EST)
> From: Audial@aol.com
> Subject: Outside temperature sensor resistance values...
> You can check the resistance of your sensors to see if they are out of
> The resistance of the sensors follows a gently sloping, declining curve from:
> ~3300 ohms @32F
> ~2200 ohms @ 46F
> ~1500 ohms @ 59F
> ~1250 ohms @ 68F
> ~1000 ohms @ 72F
> ~ 800 ohms @ 86F
> ~ 650 ohms @ 95F
> (This data is for the 1986-88 digital climate control, as per the Bentley)
> Based on this, if you were sufficiently motivated, you could change the
> outside temperature that the control head sees and then verify it using the
> fault-code channels.
> Also, the A/C compressor clutch is regulated by the thermostat on the A/C
> evaporator housing. It has a capillary tube that's inserted into the
> evaporator and controls the power to the compressor clutch if the evaporator
> temperature falls below 32F, preventing ice formation. The resistance values
> for this sensor are:
> ~ 40 ohms @ 59F
> ~38 ohms @ 68F
> ~36 ohms @72F
> ~35 ohms @86F
> ~34 ohms @95F
> You'll need a good meter to measure this one properly.
> There is also an "Ambient temperature switch" that is _independent_ of these
> sensors located on the A/C evaporator housing. It opens when the temperature
> falls below 37F, interrupting a ground signal. The A/C control head will
> then prevent the compressor from being switched on.
> Other devices that can kill the compressor are:
> A/C high pressure sensor (in plenum chamber, near left side of heat exchanger
> A/C kick-down switch on auto trans. cars - sends a ground signal to terminal
> 9 of A/C control head, turning the compressor clutch off for 12 seconds.
> Engine coolant overheat switch - sends a ground signal to terminal 20 of AC
> control head, which then sends a signal to A/C programmer to shut off the
> compressor clutch when coolant temperature rises above 247F.
> If anyone needs the full procedure for diagnosing the outside temperature
> sensors, you can email me.
> Unfortunately, the Bentley doesn't just come right out and hand us the code
> for the control head's integration of data from these sensors. Anybody want
> to take one apart and disassemble the code? You have to reconstruct what the
> engineers were thinking from the testing procedures. :-(. Now if _I_ was
> publishing the "official" manual...
> Date: Tue, 18 Nov 1997 11:12:25 +0000
> From: Sargent Schutt <firstname.lastname@example.org>
> Subject: Re: "Outside Temp" adjustable?
> I don't know that the temp sensor is adjustable, but it can go bad. Mine
> was reading all over the map. It went out for good on my way back to San
> Diego as I was leaving Las Vegas two summers ago and the A/C, though ice
> cold, would not turn on as the outside temp sensor was indicating that
> it was a cryonic -45 degrees F outside! (it was actually 110+ degrees F
> - - NOT THE IDEAL TIME FOR A/C FAILURE!!) Upon installing a new sensor
> (just remove the grill and plug in a new one) all was well again.