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Re: Outside Thermometer, Radioshack Fix??
In a message dated 10/8/99 6:43:15 PM Eastern Daylight Time,
> Hi all,
> My 1988 5kcstq (Actually Paul Meyers, I'm just holding it til he can pick
> up) shows 20 degrees F. outside whilst it is really 95 F.
Here's what I've got off my web site
hth, chris miller, windham nh
Date: Tue, 18 Nov 1997 16:46:58 -0600
Subject: 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 heat.
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
You can check the resistance of your sensors to see if they are out of
spec... The resistance of the sensors follows a gently sloping, declining
~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
-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
Best Wishes,Alex'86 5KCSTQ
Date: Sat, 18 Apr 1998 08:03:30 -0600
From: "Fred L. DeRoos" <email@example.com>
Subject: Outside Temp Sensor
Hi Vince, I don't know which car you have, but for the 200 quattro there are
two sensors used to display outside temp. One is located just behind the
grill in the front of the car and the other is located in the
plenum chamber below the windshield. The AC unit reads the two sensors and
displays the temp of the coolest one. This is Audi's attempt to get around
an erroneous temp from engine heat.
Anyway, the sensors are negative temp coefficient thermistors. As the temp
goes down, the resistance goes up. They should each read 1000 ohms at 25 deg
C. Audi wants approximately $100 each for
them, but if you are able to do some soldering, you can replace them for less
than $5. Digikey in Thief River Falls, MN sells the thermistors as part
number KC016N-ND for $1.95 each. I think with
shipping and handling, it comes to about $5.
If you want to look around for this thermistor, you need to specify one that
is 1000 ohms at 25C with an R value of 9.10. The R value determines the
slope of the resistance change with temperature. The old
sensor can be dissasembled and the new thermistor soldered in place of the
old one. Use a low wattage pencil iron as you can damage the thermistor with
too much heat. Also, it will be necessary to slightly
enlarge the plastic holder with a pencil so that the new thermistor will fit.
This will be clear when you look at the old and new thermistors.
By the way, a quick test without measuring the resistance of the old
thermistors is to disconnect one of them and see if the temp reading is now
correct (with driving to make sure you aren't seeing engine heat).
If that doesn't work, reconnect it and disconnect the other one. One of them
is probably slightly high in resistance. It is unlikely that both are bad.
Good luck. Let me know if you need any more information.
Here is part of some information I posted to the qlist last year about the
external temperature sensors:
To check the status of each sensor, you can read channels 4 (sensor in
plenum) and channel 5 (sensor on cowl) on your climate control fault
diagnostics readout. Compare the output to the chart in Bentley
(D8-240) to get the temperature. Each reading should correspond (via the
table) to a temp within about 5 degrees F of actual T outside.
BTW, the diagnostic readout values _decrease_ in a nearly linear fashion as
the temperature _increases_. Temperature of 50 degrees F gives a readout of
127. The diagnostic number changes by about -1.4
units for every 1 degree F increase. To a useful approximation (good to
within about 3 degrees between 32 and 100F) you could relate T to the
readout, D by the following equation:
T (deg F) = (198-D)/1.4
But beware that the accuracy is way off when much below 32 F.
BTW, in case I'm wrong about which channel reads which of the sensors, you
can easily touch (warm up) the front cowl-mounted sensor and then quickly
check to see which channel's value has become much
Phil Rose Rochester, NY
'91 200q firstname.lastname@example.org
You got to love a $4.00 fix on a 20V. Especially when I was ready to toss
around $150 into an A/C recharge. I can confirm the KC016N-ND part number as
the correct one for the coupe 20V as well as
the 200. Getting the sensor out of the plastic sleeve was a bit of a pain, I
ended up sort of peeling it like a banana (cut all 4 corners and pried
apart). After I stretched the plastic tube the sensor fit in, I pushed
the new sensor in and taped the sleeve shut then pulled the rubber boot back
over it....good as new.