checking outside air sensors

[Phil and Judy Rose <pjrose@servtech.com>]

>Date: Mon, 17 Nov 1997 11:27:26 -0500
>From: Frederic Breitwieser <frederic.breitwieser@mcione.com>
>Subject: RE: "Outside Temp" adjustable?
>
>>The outside temp display on my 89 100q also behaves erratically.  It is
>>somewhere between 10 and 15 (F) degrees off at all times.  Strangely
>>enough it comes closer to reality in cold rather than hot weather.
>
>Audi's (as well as other cars) have the outside temp sensor on the
>firewall, thus within proximity of your exhaust manifold, typically giving
>an innacurate reading.

In fact Audis use _two_ outside temp sensors, but neither IMO is "within
proximity" of the exhaust manifold. One is mounted just behind the front
grill to the left of center, the other--for political balance ;-)--is on
the right within the covered plenum section, probably 2 or 3 feet away and
completely isolated from the EM. On my '89 100, the outside temp reads
within about 2 degrees of what I'd expect--although it might take a few
moments of driving for it to stabilize. The problem is likely to be just a
defective sensor--easy to replace if you can figure out which one.

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.

HTH

Phil Rose

         **********************************************
         *  Phil & Judy Rose     E-mail:              *
         *                       pjrose@servtech.com  *
         **********************************************