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Volkswagen Jetta, Golf, GTI: 1999-2005, R32: 2004 (A4) 2001 VR6 A/C Troubleshooting

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Last Post: Jun 8, 2007 11:19 PM Last Post By: spannerman

Posts: 3
Registered: 05/30/07
2001 VR6 A/C Troubleshooting
Posted: May 30, 2007 8:18 AM
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So my A/C stopped working last fall and I'm finally getting around to troubleshooting. After some research I think I've figured out how the A/C control system works:

I press the button in the cabin, telling the Fan Control Module to energize. (The button lights up)

The fan control module checks the signal from the pressure sensor (which is a duty-cycle transmitter, not switch.) If the signal it's sending means between 2 and 32 bar, it's OK to energize. It also checks to see if the ambient temp switch is closed. If it's closed, it's OK to energize.

If pressure is OK and ambient temp is OK, the radiator fans spin full blast and the A/C compressor clutch engages.


On with my problem...

When I hit the switch in the cabin, the radiator fans turn on but the comp clutch does not seem to be engaging engaging (no engine "bogging", no pressure change in the A/C line). If I pull the pressure sensor harness off, the fans stop spinning. I dont' have an oscilloscope so that's the only way I have of testing the pressure-sensor. So it would seem that the pressure sensor is good. This also means that the fan relay in the FCM is good.

The ambient temp switch doesn't seem to affect the FCM's operation. I'm referencing the one under the rain tray by the driver's side of the windshield. The rad fans still spin regardless of whether the sensor is connected, disconnected, or the wires at the connector jumpered. There's continuity between the 2 posts on the the temp sensor itself.

So I pulled the 4-wire connector off of the FCM and jumpered it with 2 paperclips. 2 out of the 3 possible ways you can jumper it, the fans spin (as would be expected in a normally working system). So obviously the wiring is correct and there is 12V at both relay inputs.

However, no matter how the 4-wire harness is jumpered, the compressor clutch does not seem to energize (again, no lag on the engine and no pressure change in the A/C line).

While I haven't come up with a concrete diagnosis that the A/C clutch relay half of the FCM is good, it doesn't seem to matter since I can't paper-clip it "on". So it seems like I have a bad compressor and/or compressor clutch.

Oh, there's about 105 psi at the nipple on the A/C line by the radiator. (Measured with one of those Autozone A/C refill cans with a dial gauge.)

The kicker is that (despite a few years working as an advanced control systems engineer lol) I can't seem to find 12V at any of the connectors with a multimeter. Not even the pressure sensor, even though I determined that it is powered properly when I unplugged it and the fans turned off. Once I figure out what I'm doing wrong there, I can just verify 12V at the 2-wire connector on the compressor to conclude that the compressor has gone bad.

Am I thinking about everything correctly? Any other thoughts or input?

Posts: 3
Registered: 05/31/07
Re: 2001 VR6 A/C Troubleshooting
Posted: May 31, 2007 3:15 PM   in response to: BikerFry in response to: BikerFry
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I am also fighting a similar problem. I was able to verify my clutch working by hot wiring the clutch itself.
When working properly the FCM grounds the clutch and sends it 12 volts on a second lead. Finding which was ground was done by turning the car and AC on and sensing which of the two leads on the compressor connection was connected to ground. Once that was establish I added a 12 volt tapp to the clutch's other wire and voiulla cold air.

I hope this may be of some help


Posts: 1,456
Registered: 12/09/03
Re: 2001 VR6 A/C Troubleshooting
Posted: Jun 1, 2007 11:01 AM   in response to: BikerFry in response to: BikerFry
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I end up replacing a bunch of those pressure switches. Without an oscope or a very good meter, you'll have a hard time diagnosing it.

Without digging too deep into the diagrams, I think you may have a problem with the g65 pressure switch. The info I have says it tells the fan module to turn on the clutch, and raise the fans to second speed when the pressure rises. If the fans come on low speed with the ac on, it looks like a seperate device tells it to do so, and the fans will come on faster when g65 tells it to run high speed.

The 4 pin connector has 2 main power supplys and fan power and ground, so no matter what you do to it, the compressor won't come on, it is controled from the 14 pin. Jumping connectors at the 4pin will only tell you the fuses are good, fans are good, and fan ground is good. You could jump one of the b+ wires to the clutch wire and see if it engages.

Due to how the fan module works, you having no function, and the failure rate of the pressure switches, I'd replace it first. Its much cheaper than a compressor or module, and you don't have to discharge the system, it should have a schrader under it.

Posts: 3
Registered: 05/30/07
Re: 2001 VR6 A/C Troubleshooting
Posted: Jun 1, 2007 2:53 PM   in response to: budpalumbo in response to: budpalumbo
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Found this online. This, combined with the above post explaining that neither of the 2 relays in the FCM powers the A/C clutch, ought to give me a solid diagnosis pretty quickly. Thanks!

"This summer has been a hot one, and I'm sure you have seen your share of air conditioning problems. Unfortunately, A/C systems have grown steadily more complex and difficult to diagnose.

The problem involved a 2000 Jetta GLS 2.8L manual A/C system with no compressor clutch operation. On this vehicle, the cooling fan and A/C compressor operation are controlled by the J293 fan control module, which is usually located in the driver's front corner of the engine compartment on the lower frame rail. The J293 module has both a 14-pin connector identified as the T14 connector in Volkswagen wiring diagrams and a four-pin connector identified as the T4a connector.

To diagnose this system, starting on the T14 connector:

1) Start and idle the vehicle. Select "A/C on" at maximum cooling and "blower on" at high speed.

2) Check for 12 volts at the T14 connector pin No. 8 (T14/8). This 12-volt signal comes from the A/C switch and requires both cooling fans on at low speed and compressor activation.

3) Check for 12 volts at pin T14/9. This voltage is a switched ignition source and will have 12 volts when the ignition is in the "on" position.

4) Check for 12 volts at pin T14/4. This is a constant battery source and should read 12 volts at all times from fuse S16.

5) Check for a good ground at pin T14/6.

6) Check for 12 volts at all times at the four-pin connector, T4a pins T4a/1 and T4a/3 from fuses S164 and S180, respectively.

7) Turn off the ignition. Remove the T14 connector and check for continuity between pins T14/14 and T14/5 on harness side to ensure proper operation of the F38 ambient temperature switch. Continuity must be present if the ambient air temperature is above 32 degrees Fahrenheit. Reconnect the T14 connector and restart the vehicle.

8) Check pin T14/2 using a duty cycle meter. If the refrigerant charge in the system is normal, about 30 percent to 35 percent duty should be indicated at pin T14/2 without the compressor engaged. The duty cycle signal is supplied by the G65 pressure sensor in response to system pressure changes. A duty cycle above 90 percent or below 20 percent will command the compressor off.

9) Check for an 11-volt reference voltage at pin T14/3. The reference voltage originates in the J293 fan control module and can be grounded by |the power control module (PCM) under certain circumstances (typically wide open throttle or vehicle overheat conditions) to turn the A/C compressor off.

If zero volts are present, the PCM is commanding "compressor off" or the wiring harness is shorted to ground. Raise vehicle idle speed above 2500 rpm and observe compressor operation and voltage at pin T14/3. If the voltage at pin T14/3 returns to 11 volts with the idle speed above 2500 rpm and compressor operation resumes, then a throttle basic setting procedure is needed and must be performed with a factory-compatible, by-directional scan tool. Note: A loss of throttle basic settings will keep the compressor from activating.

If all previous tests have passed, check the T14 connector pin T14/10 for 12 volts. This pin is the output signal to the compressor clutch coil. If all the other tests have passed and there is no voltage at pin T14/10, this indicates a faulty fan control module.

As you can see, turning on an A/C clutch is not a simple function on late model vehicles, and systems that you might not associate with an A/C problem can stop you and your customer from keeping cool. Be sure to check Direct-Hit's Hotline Archive section for more diagnostic procedures and tips."

Posts: 3
Registered: 05/30/07
Re: 2001 VR6 A/C Troubleshooting
Posted: Jun 1, 2007 3:08 PM   in response to: BikerFry in response to: BikerFry
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Then again, a multimeter that could test duty cycle costs a lot more than the 53 dollar sensor. I would be surprised if it's the pressure sensor, as the fans turn off if I pull the harness off. Then again, VW could have designed it so that the fans still run even though the pressure sensor was holding the A/C compressor off...

Posts: 1
Registered: 06/03/07
Re: 2001 VR6 A/C Troubleshooting
Posted: Jun 3, 2007 2:56 PM   in response to: BikerFry in response to: BikerFry
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I have same problem like at BikerFry but I unable find the cooling fan wiring diagram Volkswagen BORA 2000

Posts: 2
Registered: 06/08/07
Re: 2001 VR6 A/C Troubleshooting
Posted: Jun 8, 2007 11:19 PM   in response to: BikerFry in response to: BikerFry
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If there is even a tiny amount of salt on your roads ring out your c/clutch harness back to the 4? pin connector near the battery enclosure. The clutch positive wire likes to rot off there where it enters the connector. Just fixed one today as a matter of fact. Seen quite a few. Let me know what you find. Rgds, Ken

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