Diagnose an HVAC issue on a 2001 Audi A6

[ToManyAudis]

This is my first post here so bare with me, if I didn't follow the rules.

I have been trying to diagnose my 2001 Audi A6 2.7T. Last fall the HVAC stopped working, and as spring is started, I really need to get it working again.

Basically there are lots of pressure on the AC system on my gauge it is just at the edge of entering into the red zone. The compressor does not run when I activate AC, and the pressure does not fall as it usually would do once the AC is started.

Tried to use VCDS to diagnose the system, but I apparently does not get any error codes, and I suspect something is not right with the AC control unit.

I have placed an auto scan of the car below. It is basically clean.

Not sure now much VCDS support there is with the specific AC control module but the scan says no errors.

When I try to use the measuring blocks, there are weird responses. Typically no units on the read-outs, and they all more or less look the same. It definitely looks wrong, but then I am not sure how much support there is for this specific module.

I also have a VCDS-lite with another cable, but it errors out on the HVAC control module.

The AC-compressor have never been replaced in this car so 23 years old. The HVAC system have always worked flawlessly in this car.
I suspect either the compressor or the AC control unit, but the compressor is pricey and quite a bit of work to replace, so I would like to get some advise on the control module.

On more observation: Both fans run, when AC is on.

Scan:

Saturday,06,April,2024,16:00:59:24098
VCDS -- Windows Based VAG/VAS Emulator Running on Windows 10 x64
VCDS Version: 23.11.0.0 (x64)  HEX-V2 CB: 0.4643.4
Data version: 20231027 DS351.0
www.Ross-Tech.com


VIN: WAUED64B61N154433   License Plate: A6-APRIL2024



--------------------------------------------------------------------------------
--------------------------------------------------------------------------------


Chassis Type: 4B (4B - Audi A6/S6/RS6/allroad C5 (1998 > 2005))
Scan: 01 02 03 06 08 15 16 17 18 22 34 35 36 37 45 55 56 57 65 67
          75 76 77
 
VIN: WAUED64B61N154433
-------------------------------------------------------------------------------
Address 01: Engine        Labels: 078-907-551-AZA.lbl
   Part No: 4B0 907 551 M
   Component: 2.7l V6/5VT     G   0002
   Coding: 06712
   Shop #: WSC 87119
   VCID: 2E05F267B902A75A8DF-513C

No fault code found.
Readiness: 0000 0000

-------------------------------------------------------------------------------
Address 03: ABS Brakes        Labels: 8D0-907-389.lbl
   Part No: 8D0 907 389 E
   Component: ABS/ESP allrad      D56
   Coding: 06395
   Shop #: WSC 12345
   VCID: 3537DD0B5C549482CC5-513C

No fault code found.

-------------------------------------------------------------------------------
Address 06: Seat Mem. Pass        Labels: None
   Part No: 4B0 959 760 C
   Component: Sitzmemory R1 BF    0102
   Coding: 00003
   Shop #: WSC 00000
   VCID: 2D07F56BB404DC42B45-4B00

1 Fault Found:
00668 - Supply Voltage Terminal 30
            07-10 - Signal too Low - Intermittent

-------------------------------------------------------------------------------
Address 08: Auto HVAC        Labels: 4B0-820-043-MY1.lbl
   Part No: 4B0 820 043 AG
   Component: A6-Klimavollautomat D65
   Coding: 00160
   Shop #: WSC 02325
   VCID: 5F9B5FA38E287ED2769-25B6

No fault code found.

-------------------------------------------------------------------------------
Address 15: Airbags        Labels: 4B0-959-655-AI8.lbl
   Part No: 4B0 959 655 F
   Component: Airbag Front+Kopf  2002
   Coding: 00307
   Shop #: WSC 02325
   VCID: 3333E31352208AB2E21-5160

No fault code found.

-------------------------------------------------------------------------------
Address 16: Steering wheel        Labels: 4B0-907-487.lbl
   Part No: 4B0 907 487 D
   Component: Lenkradelektronik    D03
   Coding: 00102
   Shop #: WSC 65535
   VCID: 2D07F56BB404DC42B45-4FDA

No fault code found.

-------------------------------------------------------------------------------
Address 17: Instruments        Labels: 4B0-920-xxx-17.lbl
   Part No: 4B0 920 981 GX
   Component: C5-KOMBIINSTR. VDO D12
   Coding: 02264
   Shop #: WSC 65535
   VCID: 025D76D75D1AF33A397-513C
   WAUED64B61N154433     AUZ7Z0A1826686

1 Fault Found:
01177 - Engine Control Unit
            49-00 - No Communications

-------------------------------------------------------------------------------
Address 35: Centr. Locks        Labels:. 4B0-962-258.clb
   Part No: 4B0 962 258 J
   Component: Central Lock/Alarm  D34
   Coding: 15885
   Shop #: WSC 40707
   VCID: 3039EC1FA336B1AA9F3-4EEA

No fault code found.

-------------------------------------------------------------------------------
Address 36: Seat Mem. Drvr        Labels: None
   Part No: 4B0 959 760 B
   Component: Sitzmemory R1 F     0101
   Coding: 00001
   Shop #: WSC 00000
   VCID: 2C01F86F8F1ED54AA3B-4B00

No fault code found.

-------------------------------------------------------------------------------
Address 45: Inter. Monitor        Labels: 4B0-951-178.lbl
   Part No: 4B0 951 178 A
   Component: Innenraumueberw.    D04
   Coding: 00101
   Shop #: WSC 02325
   VCID: 26150A4791F2EF1A75F-50AE

No fault code found.

-------------------------------------------------------------------------------
Address 55: Headlight Range        Labels: 4B0-907-357-AUT.lbl
   Part No: 4B0 907 357
   Component: LEUCHTWEITEREGLER  D004
   Coding: 00006
   Shop #: WSC 02325
   VCID: 65974D4BAC742402BC5-5160

No fault code found.

-------------------------------------------------------------------------------
Address 56: Radio        Labels: 8D0-035-1xx-56.lbl
   Part No: 4B0 035 195 A
   Component:  Radio              D00
   Coding: 00114
   Shop #: WSC 02325
   VCID: 1E6522A7C9A237DA3DF-4B1E

No fault code found.

End----------(Elapsed Time: 04:37, VBatt start/end: 12.3V/12.2V)-----------

 

[Uwe]

he compressor does not run when I activate AC,

The first thing to check is Measuring Blocks, Group 001. What do you have in the first field with the A/C activated?

-Uwe-

 

[ToManyAudis]

Group 001 reads like this:
Box1: 7.0 (no units) Box2: 12.46 V Box3: 12.46 V, Box4 00:02 (Clock)

Groups 2,3,.. have no units at all.

 

[Uwe]

Box1: 7.0 (no units)

Means:
Kompressor AUS: Abschaltung über Bedienteil "OFF"

In English:
Compressor OFF: Shut-off via Control Head "OFF".

Any possibility you have a stuck button on the Climatronic head?

-Uwe-

 

[ToManyAudis]

I do not have that impression. The control panel behaves normal, so I can turn it off and on, regulate requested temperature and so on.

I notice a brief hesitation on the engine at idle when I turn AC on, like is faced a short resistance. But when I crawl under the car to look at the compressor, it is not turning with the engine.

I guess it points towards the control head then.

 

[Uwe]

I guess it points towards the control head then.

Not certain. Does this car still use a clutch-style A/C compressor or is it already a variable displacement one?

-Uwe-

 

[ToManyAudis]

I would not know the difference. This is with 95% likelihood the original compressor, so I would assume the clutch style one. Car is from 2001.
A correction on my part. I based my statement on a screen capture I had done earlier, when I was surprised by the lag of information, so I redid the test.

With car running and AC turned on, box 1 states "0". Turning AC-off, it changes to either 6 or 7. I tried a couple of times and got different results.

Sorry for the confusion.

 

[Uwe]

With car running and AC turned on, box 1 states "0". Turning AC-off, it changes to either 6 or 7. I tried a couple of times and got different results.

'0' is the HVAC module commanding the compressor ON.

Digging a bit deeper, it looks like that's an old-school compressor with a magnetic clutch. Have you checked voltage there when it's supposed to be running? Have you checked resistance of the clutch?

-Uwe-

 

[ToManyAudis]

Thanks for the update. I will dig into the compressor. It is not easy accessible when it comes to the connections. I initially suspected the control module as it responses didn't line up with what I found only, but I understand now that on this module, this is likely a normal behavior.

I will be traveling the next week, so it will take me some time to get into this. I will return with an update in about a weeks time.

Thanks for your help. I like your picture, Dr Strangelove - a great movie.

 

[ToManyAudis]

I got back to this issue and did some more trouble shooting. Turns out the control module is providing 13 V to the compressor. Detected after the relay.
The impedance on the compression is infinite.

I guess it concludes that the compressor needs to be replaced.

After crawling under the car and removing parts to get to the compressor it turns out that the connector for the AC-compressor site in the top left corner of the engine room. Along with the fan connectors. Beneficial to know for others.