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Re: Outside Thermometer, Radioshack Fix??

In a message dated 10/8/99 6:43:15 PM Eastern Daylight Time, 
Gary.Lewis@West.Boeing.com writes:

> 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
Temperature sensors 

Date: Tue, 18 Nov 1997 16:46:58 -0600 
From: flderoos@mmm.com 
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 
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 housing). 
-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. 
Best Wishes,Alex'86 5KCSTQ 
Date: Sat, 18 Apr 1998 08:03:30 -0600 
From: "Fred L. DeRoos" <flderoos@mmmpcc.org> 
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 
'89 100 
'91 200q       pjrose@servtech.com 
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.