re: 1990 200 Air Conditioning Problems...

[Philip Rose <pjrose@servtech.com>]

>Date: Fri, 21 May 1999 17:36:43 -0600
>From: "Stongle, Casey" <Casey_Stongle@kne.com>
>Subject: 1990 200 Air Conditioning Problems...
>
>I have a 1990 200 and the AC stopped working.
>
>I took it to be recharged (at a non-audi dealer) and was told that the
>compressor wasn't turning on.
>
>anyone heard of this before?

Uh, no, all the rest of us _never_ have problems of any sort with our Audi
A/C... har, har.

>Also, my control panel for the climate control reads the wrong temp for
>outside, and for econ mode, it blows warm for any temp setting but "lo".

Aha!

The CC panel is telling you an absurdly *low* outside air temp, right? Even
for Colorado? That's probably your problem--definitely so if the
temperature it's displaying is below 40F, the temperature below which  the
system  acts to prevent the A/C compressor from running--to avoid freeze-up
damage (as in wintertime).

So you need to determine which of the *two* external temp sensors is
defective. Generally it's the one located up front--just behind the
grill--a little to inside of the driver's headlight. Whichever sensor is at
fault, you need either to replace it (see post at bottom for  a source from
which  to obtain the thermistor), or, as a temporary solution, you can pull
the bad sensor off and replace it with a suitable fixed resistance
(resistor), which will signal a false--but sufficiently high--temperature
to the CC controller so that it will allow the compressor to cycle
normally. See next quoted post, where I describe using a 1.9kohm resistor
(=approx. 52 F ext temp reading) in place of the temp sensor.

Of course, the downside of *leaving* things like this is that the CC will
think it's never any hotter than 52F outside. Not a big problem for the CC,
but it gets boring to look at day after day. Once the outside air temp
actually falles below 52F, the other (working sensor) will be the one that
is displayed by the CC panel since it "believes" the lower of the two
sensor values--if they differ.

Of course _other_ things can go wrong to prevent the compressor from
cycling on. But since you mentioned the "wrong temp" being displayed, this
is my first guess. Hope it helps.

Phil Rose


*****************
>
>This summer, the ext. temp readout went bananas. Ext. temps of -50 degrees
>were typical and obviously were causing the AC compressor to shut off.
>Now, the ACC channel 1 put out a code "5", indicating an open circuit for
>the cowl temp sensor. I unplugged the cowl temp. sensor and this time
>measured the sensor resistance. It was around 300 Kohms (over 300X too
>high)!!! Clearly the problem was the sensor and not simply a poor contact.
>I substituted a 1.9 Kohm resistor; the ACC now thinks it's a moderate 52
>degrees outside and the A/C comes on like gangbusters.
>
>I'm sure there are _other_ problems with my A/C system that will surface
>before long. But even temporary victories are worth savoring, however. :)
>
*****************

> reading the ACC diagnostics: there is a separate channel for each of the
>two external sensors, so it is easy to check which one is
>defective--although their output is not given in degrees. The  Bentley
>manual has a table relating sensor diagnostic output--->temperature. See
>below.
>
>The one (sensor) up front is just behind the grill and very easy to reach.
>Access to the other sensor requires removing the large plastic cover
>behind the firewall; I believe it's over toward the passenger side. I
>believe they are not outrageously expensive--meaning only semi-outrageous
>:) There is some info in the archives about replacing with a non-OEM part,
>I recall.
>
>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 smaller.

******************************
>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 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

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