re: Outside temperature indicator

[Phil Rose <pjrose@servtech.com>]

>Date: Tue, 12 Jan 1999 10:31:22 EST
>From: LGREGF@aol.com
>Subject: Outside temperature indicator
>
>The outside temperature gauge on my 1987 5000Q has gone bonkers.  Currently it
>reads about 15 degrees F below the actual temperature.  Since the car spends a
>lot of time on the road to Vermont, I miss knowing more accurately when the
>outside temp is +/- freezing. Seems to also affect the internal thermostat as
>well. Since I still haven't solved my ABS problem, this has the potential to
>put the wrong type of excitement in my driving experience.  Re-booting by
>battery disconnect has had no effect.  Any ideas appreciated?  Thanks.
>
>Regards,
>Greg F.
>'87 5KQ

The outside temp is measured by two thermistor probes: one mounted up
front--just behind the driver's side grill (cowl?) and another under the
plastic plenum cover near the hvac blower assembly. Below I've copied a
qlister's posting from my archives. I'll add a couple of comments:

(1) A way to tell which thermistor is the defective one is to output
channel 4 and channel 5 of the CC diagnostic display. These channels are
the temp sensors' output--but resistance values, instead of degrees F.
Since the CC outside temp display is programmed to use the _lower_ of the
two when/if there are different readings, and since you're getting an
outside temp reading that's too low, thus the lower temp is the *in*correct
one. In other words, the bad sensor is the one that shows the _higher_
resistance value. As I recall, the grill-mounted sensor=channel 5 and the
sensor in the plenum=channel 4. The Bentley manual--you have one,
right?--will give you a table of temp vs. resistance values.

(2) Also, a stop-gap "cure" that I'm presently using on my '89 100 is to
insert a resistor in place of the defective sensor. I chose one with a
value of about 1.9 kohm, which acts like the thermistor at 52 deg F. If you
do this, the display will show the outside temp according to the _other_
(working) sensor during the winter--assuming it's colder than 52 F outside.
In warmer weather, you'll be seeing an incorrect temperature (52 F) all the
time, but at least your AC will be able to function. Using your bad sensor,
you may not get air conditioning to work later in the year, because the
system may think it's too cold outside to allow the AC compressor to run.

Oh, one more thing: channel 01 of the CC diagnostics will display either a
4 or 5  to indicate if one of the two temp sensors has an open circuit. It
doesn't seem to me that you have an open circuit--just a defective sensor
output.

HTH

Phil Rose


************

>>Date: Thu, 29 May 1997 16:36:48 -0600
>>From: "Fred L. DeRoos" <flderoos@mmmpcc.org> Subject: Temperature Sensors
>>
>>>Date: Wed, 28 May 1997 17:40:12 -0400 (EDT) From: "Stephen J. Siblock"
>>><steve.siblock@sheridanc.on.ca> Subject: temperature sensors (on HVAC)
>>
>>>Is there a way to correct the temperature reading on the HVAC system?
>>>Mine is about 7 degrees C "too cold".
>>
>>Steve,
>>
>>I'm not sure which car you have, but the 200s use a pair of negative
>>temperature coefficient thermistors to sense the outside temperature. One
>>of them is located in the plenum below the window (under the hood on the
>>passenger side) and the other is located in front of the AC condensor
>>approximately in the middle of the car. Both thermistors are identical
>>and are 1000 ohms at 25 deg C. They have an R value of 9.10 and can be
>>purchased from Digikey Corp. in Thief River Falls, MN for approximately
>>$2 each. The part number is KC016N-ND. The reason you are reading low, is
>>that the AC head unit reads both sensors, then displays the one that
>>senses the lower of the two temperatures. This is done to prevent the
>>display of an artificially high temperature due to engine or sun heating.
>>The temp may still be a little high, but at least it would be the lower
>>of the two. The problem is when one of these sensors goes bad, it
>>typically increases in resistance. That means that your temp display will
>>show low.
>>
>>The display will operate with only one sensor, so you can do one of two
>>things. Best choice is to check the resistance of each sensor at
>>approximately 25 C and see which one is reading high, then replace it.
>>Second approach is to unplug each sensor separately and see which one
>>makes the temp read correctly (assuming you haven't been parked in the
>>bright sun or just run the car and the engine is hot. The front one by
>>the condensor was bad on my 1990 200 TQ. When I unplugged it, the temp
>>read ok, but often needed a few minutes of driving to blow air over it
>>and reach a stable temp indication.
>>
>>To replace the thermistor, you will need to remove the bad assembly, then
>>remove the plastic tip from the holder. The original thermistor is
>>soldered to the two wires and fits into a plastic tube. The new one is
>>just slightly larger, so you will need to insert a screwdriver into the
>>plastic tube and enlarge it approximately 0.02 0.04 inches. Audi would be
>>happy to sell you a replacement, however, they list for approximately
>>$100.
>>
>>EMAIL me if you have any questions. It is an easy repair.
>>
>>Fred


Phil Rose				Rochester, NY
'91 200q				mailto:pjrose@servtech.com
'89 100