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VW MKIV-A4 TDIs (VE and PD) This is a general discussion about A4/MkIV Jetta (99.5-~2005), Golf(99.5-2006), and New Beetle(98-2006). Both VE and PD engines are covered here.

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Old September 14th, 2011, 18:18   #1
brett.knutson
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Default AC Compressor won't engage

Hopefully someone can help me out... Due to a military deployment, my '02 Jetta TDI sat for several months without being driven. When I returned, it started right up (after a jump start) and ran perfectly fine, except no AC.

The console button illuminates, the cabin fan blows, and the radiator fans both operate, but the compressor doesn't engage.

I ran battery voltage to the compressor and the clutch does click.

I tested the coolant temp sensor and the fans will run properly (both low and high speeds).

The high pressure switch connector has 12 vdc at pins 1 and 3.

After all those tests, could it still be the Fan Control Module? Or, is there another test I can try, without having any pressure gauges?

Should I try to add a can of R-134?

Any assistance or recommended gurus in the Northern VA area would be greatly appreciated...
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Old September 14th, 2011, 18:26   #2
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You can bridge the pins on the high pressure switch and see if the compressor clutch clicks in...

With the compressor not running, you ambient pressure should be around 100 PSI. If you check this and the pressure is next to nothing, you may have a leak and your coolant may have leaked out. Your electrical testing seems like it has covered most of the bases.
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Old September 14th, 2011, 18:28   #3
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I don't have pressure gauges... Which pins on the HPS do I bridge?
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Old September 15th, 2011, 08:40   #4
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Don't these systems have a low pressure switch to keep the compressor from running with no refrigerant in the system?
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Old September 15th, 2011, 08:45   #5
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Quote:
Originally Posted by chromeBuddha View Post
You can bridge the pins on the high pressure switch and see if the compressor clutch clicks in...
NO NO NO!

This is not a 1987 Chevy we're talking about.

Usually if the fans come on when you turn on the a/c, then the FCM is getting the go ahead from everything that it needs to turn on the a/c, but is just not sending power to the clutch coil. I would replace the FCM with one from a reputable vendor. NOT ebay.
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Old September 15th, 2011, 11:18   #6
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Quote:
Originally Posted by gquenstedt View Post
NO NO NO!

This is not a 1987 Chevy we're talking about.

Usually if the fans come on when you turn on the a/c, then the FCM is getting the go ahead from everything that it needs to turn on the a/c, but is just not sending power to the clutch coil. I would replace the FCM with one from a reputable vendor. NOT ebay.
The bridging of these pins to test the high pressure switch is in the Bentley manual. I will get the section and page # if that will put you at ease.

Brett,
I believe it would be pin 1 to 2, meant to confirm in the manual last night but was whipped. Will check it tonight.

In re-reading, I meant the pins on the connector, not the switch. Bridging anything on the switch with it disconnected wouldn't accomplish much...
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Last edited by chromeBuddha; September 15th, 2011 at 11:23.
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Old September 15th, 2011, 12:54   #7
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most likely...the A/C clutch coil is dead.
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Old September 15th, 2011, 14:52   #8
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The bridging of the pins is for models before May of '99, which had a dual hi lo pressure switch on the high side, and four pins.

Do not try to bridge a 3 pin high pressure sensor used after May of '99. At best you will blow the fuse.
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Old September 15th, 2011, 16:12   #9
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I don't think I am getting voltage to the compressor coil at all... It will definitely click when I apply 12v directly from the battery but nothing when I have the car running w/ ac 'on'.

Sooo, how confident are you guys it is the FCM? How much would one be, and where is a good place to get one? I would like to get this resolved tonight if possible.
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Old September 15th, 2011, 16:22   #10
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Quote:
Originally Posted by brett.knutson View Post
I don't think I am getting voltage to the compressor coil at all... It will definitely click when I apply 12v directly from the battery but nothing when I have the car running w/ ac 'on'.

Sooo, how confident are you guys it is the FCM? How much would one be, and where is a good place to get one? I would like to get this resolved tonight if possible.
If the coil is good as you suspect, and when you turn the ac on and off the radiator fans turn on and off, then your pressure sensor is good and the only thing it can be is the FCM. You can always get one at the dealer if you're in a hurry but you'll pay a lot. Www.autohausaz.com www.boraparts.com or www.idparts.com are all good
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Old September 15th, 2011, 17:22   #11
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The fans seem to always run on low speed, but if I jumper at the coolant sensor they will go to high speed... How can I test the pressure sensor?
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Old September 15th, 2011, 19:05   #12
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I just pulled the FCM off, it looks fine. No corrosion, nothing melted, all connections clean and dry. Is there a way to test it?
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Old September 15th, 2011, 19:23   #13
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Quote:
Originally Posted by brett.knutson View Post
I just pulled the FCM off, it looks fine. No corrosion, nothing melted, all connections clean and dry. Is there a way to test it?
1. Turn the ignition to on, but do not start the engine.

2. Turn the a/c on. Do the radiator fans come on low speed?

3. Turn the a/c off. Do the radiator fans turn off?

If so, that means that the FCM is getting all the right signals to power up the clutch, but it is just not sending it power because it is broken. You can not tell by looking at it.
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Old September 15th, 2011, 20:19   #14
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the fans come on even with the a/c off, and stay on for awhile even after the key is turned off. SOunds

I just bought one from boraparts for $56... wish me luck.
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Old September 16th, 2011, 04:41   #15
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If the fans are coming on with the AC off, and staying on with the key off, check your fuses for your system.

You have lost one of the power signals. Bad fuse or bad wire.

Fuse 15, I think.

I did not post this earlier, because some of your wording seemed to suggest you had already found it, but have you done the troubleshooting
http://forums.tdiclub.com/showthread.php?t=242699 found in post #1 of this thread?
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