(climate) Updated May 22, 2002
Climate Control System

Fresh air flap, or "it's foggy in here"                      (12/1/99)
Temperature sensors
Temperature regulating flap
Heater blower and heater core
Error codes
Fixing the A/C Clutch Suppression Diode
    (see also the Radio repair group)
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Fresh air flap, or "it's foggy in here"

Defroster/Econ = Humidifier ????
A broken recirculate flap spring is a common problem.
The recirc flap is accessed from inside the car.  It's up behind the glovebox, which will need to be removed.
The spring is attached to a plastic plug in the flap which hard to reach.
The easy way to attach the new spring is to:
1. Remove the plug from the flap by twisting it a half turn.
2. Attach the spring with the plug in your hand.
3. Then re-insert the plug in the flap.
Attaching the other end of the spring was not a problem.
In some cases it's possible to re-use the old spring.  If the end loop is all that broke off, which I think is typical, you can just bend the next loop up and reattach with it.
Dave Conner
David Conner  Columbus, OH
'87 5KS
 '89 100E
  ' 86 4KCSQ
Audi issued some technical service bulletins dealing with this repair; they are here:
Page 1
Page 2
Page 3

Temperature sensors

Date: Tue, 18 Nov 1997 16:46:58 -0600
From: flderoos-at-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-at-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 smaller.

Phil Rose      Rochester, NY
'89 100
'91 200q       pjrose-at-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.


Temperature regulating flap

The climate controller head regulates temperature by cycling the air conditioning compressor on and off (unless set to "off" or "econ"); by turning the flow of hot engine coolant to the heat exchanger core on and off via a valve located behind the engine head on the firewall; by closing the recirculate flap behind the glove box; and by controlling airflow using a temperature regulating flap. The flap is located under the hood, beneath the black plastic plenum on the firewall.

The climate control head controls an electric motor with a feedback circuit that tells the control head the flap position. I've found that if the electrical connection to this motor degrades, the climate control head cannot properly control the flap positions. You'll notice this when you get too much heat, or no heat at all. Run the climate control diagnostics (below) for details.

This motor has an audi part number of 443 820 511 A (Bosch 0 132 801 003) and runs around $70.

The fix involves either replacing the temperature regulating flap motor, or using electrical contact enhancer to improve the connection at the harness under the plenum.

Start by opening the hood, and removing the windshield washer arms (pull the protective caps, remove the 10mm nuts, mark the position of each wiper arm on the windshield, then bend each wiper at the joint and pull them off. Remove the couple screws holding the wiper assembly in place, and set it aside. (This gives you enough room to work, and only takes a minute to remove.)

This is a good oportunity to pull out the shop vac and remove all the dead leaves, small animal nests, and years of dust and debris. You'll need to make up a small diameter extension to get down into the bowels of the system; I used a paper towel roll cardboard.

Lube the moving parts/joints for the flaps. The motor is located on the driver's side of the blower box, above the glove box area. Follow the wiring back along the engine compartment wall; unsnap the connector, and use a good contact enhancer/cleaner; then reconnect. Now, start up the car, and run the climate control through the various settings and temperatures to make sure everything moves smoothly.


Heater blower fan, heater box, heater core 

updated 5/22/2002

Heater Box Removal and Repair

The Bentley has -on page 87.18, what appears to be detailed instructions on removing the heater box. What stopped my was when I came to the line "Remove all retainers between the body and heater." What does that mean? Why did I
take out the center console. What to do now?

1) Look at the picture on 87.22. From the interior, remove the two footwell air outlets (two screws each). After disconnecting any vacuum lines that might be clamped to them, remove the two large elbows on either side of the
vacuum servos- they pull down, and a little out. No screws involved. Remove the corrogated rubber duct connection that is up closer to the firewall. I did not need to remove any other ducting.

2) Look at the picture on 87.28. Disconnect the three vacuum lines -yellow, blue and green- from the vacuum servos. Disconnect the red, black and white lines at the joints located right next to the vacuum servos. (No need to
remove the two vacuum servos)

3) Back on top in the engine compartment, disconnect the red, black and white vacuum lines located on the passenger side of the heater box . Disconnected top and bottom, a section of these three lines will come out with the heaterbox. Notice the metal A/C line running left to right in front of the airbox. Cut any cable clamps from it and move flexible wires and lines out of the way. There is very little clearance between the A/C line and the heater box as you later pull, push and rotate the box out.

4) Use a narrow (like 1") thin bladed, flexible putty knife to break the seal between the box and the firewall where you have access ( primarily at the top of the box, and the drivers side. Next, carefully use a thick bladed, ridgid 1" putty kniffe to begin to pry the box away from the firewall. Follow up with a big flat bladed screwdriver. At this point, I had
about an inch of motion on the upper drivers side of the box. Push the bellows A/C duct connector on the passenger side into the heaterbox, I taped it back so it would not interfer. Keep working it. I spent a lot of time laying on top of the engine, arms reaching around both sides of the box, gradually pulling. Slowly, the adhesives would release. Finally, the box was loose all around.

(Next you need a helper. Word to the wise: if your helper has a strong emotional attachment to the car and has not been following your progress, warn him or her about what they are about to see. Its not a pretty picture. Your helper will look at the mess in the interior and not realize that you labled all the wires and switches with masking tape, tucked the screws and bolts into labled baggies, and of course have the Bentley, Chris' pages, and all your buddies on the list to help figure out how to put it back together.

My wife is convinced I have managed to turn our car into junk that would have to be towed away.)

5) Have the helper push from the interior, more or less on the vacuum servos. The need to be pushing up and forward- at about a 45 degree angle. Meanwhile, assume a position laying on top of the engine, hands grasping either side of the airbox. pull back and up, favoring the passenger side.

When you have clearance, remove the black bellows connection to the A/C unit, and push the airbox back down a bit. Then start pulling up favoring the drivers side, until the box clears the metal A/C line. Now a big push from below while you lift and rotate the top of the airbox back a little.

Disassembly of the heater box is not tricky, but I had to go buy something like a long 14" blade Phillips to get to one of the screws holding it all together. Take lots of pictures and make sure you see what is going on with the various servos etc. Assembly is tricky because you have to get the various flaps in the right place. I had one side of the box on the
workbench, inserted all the flaps and parts, and then glued some fishingline to the top end  of the flaps, and threaded the line through the appropriate holes in the upper half of the box, so that I could slide the top half onto the bottom and get all the flaps into the right place. The last two sentences will make no sense to you at all until you have the box apart.

Tom F.
Last week I replaced the heater core in my '91 200 TQW. Tom Forhan was kind enough to forward a detailed description of the process. From all the messages and help from listers, the bugaboo is obviously getting the box loose from its seal. I researched Bentley and found a picture, with no description, of an Audi extraction tool - a jack screw on a beam across the shock towers - obviously not something any of us would have access to. Taking a clue from the picture I wrapped coat hangers around each end of the box, and over a four foot 2x4 positioned across the shock towers, and lifted up one end. The box was out 2 minutes later.
Contrary to Bentley, you do not need to remove the center console; just take off the console side panels, drop the two foot well heat ducts, and disconnect the little plastic vacuum manifold.
The rest was easy. You pop off all the box center clips, remove the three screws, and note the flap positions. The trick there is to place pieces of masking tape on the upper part of the box, and draw the flap locations on the tape, along with a picture of the lever arm setup.

Re-assembly can be done with your fingers inserted in the box as it comes together. You must seal the installation end of the new heater core with silicon sealant, and let it cure over night.
George Sidman


Error codes

updated 2/3/2000
Bentley states that some faults are indicated w/o activating the fault
memory system.  The LED next to the outside temperature button will flash
for ~1 min after a fault is detected or the ignition is switched on when
there is a fault.  Note that the manual devotes 100 pages to
the digital climate control system.  It includes detailed troubleshooting
flow charts as well as individual component tests.  If you've got a real
problem it's probably best to obtain the manual.  I'll include a discus-
sion of the fault check system here.

The fault system can be activated when the ignition switch is in the ON
position, the engine need not be running.  The fault system is activated
by pressing the "OFF" button while holding down the "OUTSIDE TEMP"
button.  After activating the fault system the "OUTSIDE TEMP" button can
be released.  When initially activated the numeric LED displays will in-
dicate a code of `01.  This value indicates the "channel" that is select-
ed for monitoring.  Notice that the Climate Control system will continue
to operate in the mode it was in before the fault system was activated,
and it is possible to terminate the fault system display by pressing any
button on the head other than the "WARMER" and "COOLER" buttons.

The fault channel selection will increment when the "WARMER" button is
depressed ... there are 22 fault channels.  The fault channel value is
accessed by pressing the blue "COOLER" button.  Once the channel has
been accessed it is possible to check other channels by pressing the
"WARMER" button.  The channel definition is:

Chan    Function
`01 System Faults (see fault code table below)
`02 Measurement value of inside temp sensor (roof mounted) (refer to chart in Bentley)
`03 Measurement value of inside temp sensor (dash mounted) (refer to chart in Bentley)
`04 Ext Temp Sens - Plenum
`05 Ext Temp Sens - Radiator
`06 Measurement value of coolant temp sensor G62, phase in during production
`07 Graphic Control Head Output for A/C programmer
`08 Actual value of feedback potentiometer for temp regulating flap servo motor
`09 Specified value of feedback potentiometer for temp regulating flap servo motor
`10 Specified Blower Motor Voltage
`11 Vehicle Voltage <=== Useful for monitoring battery too!
`12 Total of electrical system voltage interuptions, values between 5 and 9.5 volts (less than 5 read as open low-pressure switch)
`13 Program number (not significant for troubleshooting)
`14 Switch position of high pressure switch (0-5 means high pressure switch closed)
`15 Specified voltage on fresh air blower in volts (depends on program)
`16 Pulse counter (not significant for troubleshooting)
`17 Graphic display of compressor shutoff conditions
`18 Graphic display of electrical outputs
`19 Number of times compressor shut off via high pressure switch F118 (red housing).
`20 Number of times compressor shut off via high pressure switch F118 (red housing) since last ignition switch cycling.  After 8, compressor shuts down until system restarted.
`21 Program number (no significance to troubleshooting)
`22 Speed signal

System Fault Codes for channel `01
 00 - No problems!
`01 - Interior temp sensor open
`02 - Interior temp sensor shorted
`03 - Exterior temp sensor in plenum open
`04 - Exterior temp sensor in plenum shorted
`05 - Exterior temp sensor in front of radiator open
`06 - Exterior temp sensor in front of radiator shorted
`07 - Feedback potentiometer on motor for positioning heater flaps  Open Circuit/interuption
'08 - Feedback potentiometer on motor for positioning heater flaps  Short circuit
'09 - Coolant temp sensor G62 open circuit
'10 - Coolant temp sensor G62 short circuit
'11 - Inside temp sensor (roof) open circuit
'12 - Inside temp sensor (roof) short circuit
`13 - Battery voltage is/was less than 9.5v during current running period
`14 - High pressure sensor - Excess pressure in system, compressor off
`15 - Adjustment motor for temperature regulating flap with feedback potentiometer is set incorrectly
'16 - High pressure sensor - short circuit
Definitions for graphic control head output - channel `07
(I'll eventually add the diagram showing the various flaps and levers; for now, the text version will have to do)

Channel 7

When Channel 7 is read, each segment that is lit (or not) has a meaning, as follows:
1 lit or flashes:  temperature regulating flap moves towards "heating"
2 lit: Recirculation fresh air flap is in recirculation mode
3 and 5 lit: "air from instrument panel outlets"
4 lit:  Heater valve is closed
5 lit (but not 3):  "air from instrument panel outlets and to regulating flap (footwell/defrost)".
6 lit:  "air from footwell outlets"
7 lit or flashes:  temperature regulating flap moves towards "cooling"
8 lit:  compressor on.
9 lit:  Radiator cooling fan runs on first speed.


Channel 17

When Channel 17 is read, each segment that is lit (or not) has a meaning, as follows:
1 lit:  compressor on.
2 lit:  compressor off.  A/C refrigerant high pressure switch (red housing) open.
3 lit:  compressor off.  Outside temperature too low, or mode = ECON or OFF (check outside temp sensors)
4 lit:  compressor off.  Electrical system voltage less than 9.5 volts.
5 lit:  compressor off.  A/C refrigerant low-pressure switch open.
6 lit:  compressor off.  Kick down function activated. (switches on after 12 seconds).
7 lit:  compressor off.  Coolant temp warning switch, or multifunction temp switch closed.


Testing air control doors - This is from the earlier system, and may or may not be accurate for our cars.
If the diagram is correct, the same line that selects outside air or recirculated air
controls the flow of coolant to the heater core.  The table below shows
actuator positions for several conditions.

OFF              |    RECIRC    |    CLOSED      | WINDSHIELD |    CLOSED
ECON   64F |    FRESH(?)  |  FULLY OPEN| FOOTWELLS  |     OPEN(?)
ECON   84F |    FRESH     |    CLOSED       | FOOTWELLS  |     OPEN
BI-LEV 64F  |    FRESH(?) |  HALF OPEN  | FOOTWELLS  |     OPEN(?)
BI-LEV 84F  |    FRESH     |  HALF OPEN  | FOOTWELLS  |     OPEN
AUTO   84F  |    FRESH     |    CLOSED       | FOOTWELLS  |     OPEN
DEF               |    FRESH     |    CLOSED       | WINDSHIELD |     OPEN
Notice that I question a couple of the entries in the table in regards
to the heater valve being opened when the temperature setting is low.
I'm noting that here, because the heater valve servo and fresh air/recir-
culate servos are connected together.  One important thing to note is
that the recirculate/fresh air door has only two valid positions ... if
yours appears to be somewhere in between it is an obvious fault.  BTW,
the heater valve servo is open when the actuator arm protrudes from the
vacuum motor and the valve is closed when vacuum is applied to the motor.

Date: Wed, 27 May 1998 22:18:21 -0700
From: "Avi Meron" <avi-at-cosmoslink.net>
Subject: RE: Need A/C Comp. for 87 5KQ

Compressor rebuild Co.
7850 North Willis
Van Nyes, CA
818 908 9272

Fixing the A/C Clutch Suppression Diode    (see also the Radio repair group)

Subject:  A/C clutch suspression failure.

The A/C clutch relay contains a supression diode to suppress the inductive
voltage spike generated by the clutch coil when the relay contacts open.
this diode is subject to infrequent (?) failure.

 1.  If the compressor runs all of the time rather than cycling
with A/C demand, the diode is shorted.
2.  If compressor disengagement causes radio noise and other electrical
disturbances, the diode is open.

The Fix:
No need to replace the (outragiously priced) relay, just the diode
within the relay with any Radio Shack 2 amp, 50 volt or better diode, (not

Bernie Benz

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All rights to this info in this format are reserved; feel free to use it, but don't just copy it verbatim! Not responsible for screwups in typing etc. YMMV, etc. I haven't  tried many of these procedures, and some are adapted from procedures for other Audi models.

Please send me any comments, updated procedures, etc.

Chris Miller