Outside Temp Sensor

["Fred L. DeRoos" <flderoos@mmmpcc.org>]

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 determins
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.

Fred