# A/C Troubleshooting?



## lite1979 (Sep 9, 2005)

I got my A/C charged recently to prepare for my trip to MD for H20i. Obviously since I'm writing this, my A/C is not working. 

The first time I went to get it charged, they found a leak at the thermal expansion valve. I replaced the valve and O-rings and went back.

Vacuum test came out OK, but when filling, they noticed a leak at the pressure switch. I replaced the pressure switch and went back.

Now the system passes the vacuum test, holds pressure and won't engage the A/C compressor clutch when I turn on the A/C from within the car.

Putting 12V to the compressor makes it engage, and I can get the system to put out cold air like this, but I want it to work like it should. Here are my current symptoms:

Pressing the Econ button to turn A/C on does not make the low-speed fans run (fans are new as of last Friday - got them for $108 shipped). It also does not engage the clutch on the compressor.

So I have a new pressure switch, new fans, and my system holds pressure and was recently checked and charged. What's next?

6 Faults Found:
00787 - Temperature Sensor in Fresh Air intake duct (G89)
30-10 - Open or Short to Plus - Intermittent
00779 - Outside Air Temp Sensor (G17)
30-00 - Open or Short to Plus
00603 - Footwell/defroster Flap Positioning Motor (V85)
41-10 - Blocked or No Voltage - Intermittent
00604 - Potentiometer Positioning Motor for Air Flow Flap (G113)
30-10 - Open or Short to Plus - Intermittent
00792 - A/C Pressure Switch (F129)
31-00 - Open or Short to Ground
01206 - Signal for Duration of Ignition Off Time
27-10 - Implausible Signal - Intermittent

I immediately double-checked the pressure switch connection, thinking that would solve it, but it's nice and clean. I also pulled the Fan Control Module to inspect for cracks in the solder or carbon build-up on the electrical contacts of the relays, and it was clean as a whistle. I'm stumped. Any ideas? I know I'll eventually have to trace all the wiring to check for continuity to track this down, but if one of you guys has gone through this before, I'd appreciate your input.

The F129 code really confuses me, since I have a G65. I don't think my system has an F129 sensor (that would be the four-wire sensor found on models before 2001, IIRC). Thanks in advance!


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## asiwa (Dec 18, 2014)

Could potentially be a bad compressor clutch, just not a bad compressor. I have a 2001 as well and last year the magnetic clutch took a dive in the dead of summer. Heard it very loudly at first, but after that it was not noticeable - just no cold air blowing through the vents. These cars are at that age where those components are beginning to fail, and a bad clutch won't read out in VCDS. 

By the way, have you tried clearing those codes out recently? Some of them could be left over from the time before you replaced the components. Clear them all out and run the car a while and then rescan and see what shows up.


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## lite1979 (Sep 9, 2005)

I stated above that when you put power to the compressor, the clutch engages and I get cold air...

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## Aaron16V1.8l (Aug 12, 2007)

Did the ac work fine before you replaced the expansion valve and high pressure switch? If so, maybe one of those components is faulty, or the system is over or undercharged. Do you have a manifold gauge set to verify what the system charge is? If it is overcharged, maybe the high pressure switch is keeping the compressor from running - you bypass that when you jump the compressor. It could help with diagnoses to hook up a manifold gauge set, then run the compressor and see what the high and low side are doing. I am no ac expert, but I have troubleshooted the systems in my Cabriolet and Scirocco which were admittedly less complex. Different combinations of readings on the high and low side could point to charge issues or blockages. 

Is it possible you put the wrong high pressure switch in? Hope you get it figured out -


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## lite1979 (Sep 9, 2005)

The pressure switch and expansion valve were both replaced to fix leaks, and both were replaced successfully. 

The A/C did not work before I replaced them.

Unfortunately, the pressure sensor I replaced is a G65, not an F129, so I cannot jumper the wires to make the compressor engage on a G65...

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## Audi_Mechanic (Mar 24, 2012)

There's a fuse panel on top of the battery that commonly causes problems with the cooling fans & compressor clutch. It has 3 green fuses (i think 30amp) which often get burned contacts. Remove those fuses and check to see if they are burnt or if the female electrical pins they plug into are burnt. There is a 3-pin connector that feeds into the side of those fuses, disconnect it and check both sides for burnt pins.


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## lite1979 (Sep 9, 2005)

Thanks. I've checked the fuses on top of the battery as well as the fuse tray twice now, but no sign of burning/melting, and I have seen a burnt tray before on a New Beetle I worked on a while back.

I can't wait to get this figured out for no better reason than to have the fans come on low speed when I'm running warm. I just put a new Coolant Temperature Sensor in the car, and my temp gauge is finally reading the right temp (it was always reading cool before), but I can see the temp creep up to one line past normal when in stop-and-go traffic. The high-speed fans then kick in and bring the temp back down to 90*C.


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## Audi_Mechanic (Mar 24, 2012)

> The F129 code really confuses me, since I have a G65. I don't think my system has an F129 sensor (that would be the four-wire sensor found on models before 2001, IIRC). Thanks in advance!


I looked up your model's wire diagram (without VIN) and it verifies that you have G65 3-pin sensor (asterik on diagram states "Automatic Climate Control with High pressure sensor, from 12.12.1999 or VIN 8N_ZY1_055070). Trouble code definitions are typically generic so don't put too much value in the scan tool's description over what's actually on your car. Erase your trouble codes, operate the system for a little while and then see what codes come up/return.

Code 00792 - A/C Pressure Switch [(F129) should be G65], 31-00 - Open or Short to Ground; looks to be static (not labeled intermittent like other codes) so if it returns, check its wiring. 

- Pin 3 should be ignition on power (black/blue stripe), it's circuit is from a welded connection (wire harness crimp) that also makes its way to other components - climate control head and fan control module. It is fused from 'S5' which is 7.5A and probably a mini fuse in the fuse panel on left side of dash (check fuse). 
- Pin 1 should be ground (brown/black stripe). 
- Pin 2 (white) is the signal wire that goes to the fan control module at connector/pin T14b/2 (14 pin connector, pin 2) and ECM somewhere in the 61 pin connector (how the wire diagram is labeled). From what the diagram makes it look like, G65 sends its pressure signal via the white wire to ECM and ecm sends signal to climate control via communication bus, probably. So if that code is present, then the ECM isn't getting the pressure signal or the fan control module isn't getting the pressure signal and thus not sending to climate control, disabling the system. 
- Check for short or open circuits between the 3 wires at G65 and check if there's a problem in the white wire between the sensor at pin 2 and fan module at T14b/2. Specific ECM pin isn't labeled in climate wire diagram or engine wire diagram so I don't know which to check. The white wire to ECM makes it way to an intermediate connector in the E-box that passes wiring from inside the cabin to engine bay, at T10x/4 - "Connector 10 pin, orange, in E-box plenum chamber". So maybe the diagram is mis-labeled and the white wire directly goes to climate control module and not the ECM.

Code 00779 - Outside Air Temp Sensor (G17) 30-00 - Open or Short to Plus; isn't labeled as intermittent so it's worth checking its wires. I forgot to mention above but check wiring to both sensors for obvious exterior damage from collision damage/repairs or rodent damage. Recently I worked on a '13-15 MY A4 that had rodent damage to ambient air temp sensor wiring. It resulted in a few warning lights and caused the cooling fans to constantly run at high speed. Scan tool stated that the outside temp sensor is used to evaluate plausibilty of engine coolant temp sensor and intake air temp sensor signals, so it may/may not have a say in how the system operates on your TT. 

- Check internal resistance of ambient temp sensor (located behind front center grill, below one of the horns), should be in Kilo Ohm. Temp range in Celcisus (Fahrenheit) = to Kilo ohm: -30C (-22F) = 18.1Kohm (1810 ohms); 640C (140F) = 0.25Kohm (250 ohms). So if you're getting more than 1810 ohms or less than 250 when checking the sensor, it's no bueno. 
- Pin 2 is ground (brown/white stripe), is shared at welded connection with sensors: G92, G112, G113, G114, G192, G107. This circuit looks like a strong candidate to check since it feeds ground to other sensors with trouble codes. Ground circuit for all those sensors starts at climate control head in the A connector pin 12 (brown/white stripe); I think the back of climate head will indicate which connectors are labeled A,B,C,D. The junction point between inside car and engine bay is at connector/pin: T17/8 - "Connector 17 pin, behind instrument panel".
- Pin 1 of G17 is the signal return (brown/yellow stripe) circuit to climate head at connector A pin 8. Check for short/open circuits. Intermediate connector/pin for inside/outside is T12a/5 - "Connector 12 pin, behind instrument panel, right"


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## lite1979 (Sep 9, 2005)

Thank you for the detailed response. I apparently am missing my ambient air temperature sensor. Since it sounds like a thermistor, can I get a generic one instead of shelling out the dough for a new one? I have two wires exposed to the air from what I can see right now. I'll get around to testing continuity to and from the pressure sensor as soon as I can.


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## lite1979 (Sep 9, 2005)

Bump: this appears to have been caused by a short to ground. If you look up lite1979 and fuse 16 you'll find the thread.


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