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Posted: 4/18/2010 3:34:15 PM EDT
My a/c compressor won't kick in on my 01 Jetta TDI anymore and it's getting hot down here.

It started acting up and wouldn't come on one day then it worked fine the next for a couple days and now it won't do anything.

Checked voltage at the compressor and I think I had like 3.x volts. OK, went up to the pressure switch and I had 12v on 1 of the 3 wires, decided to try a pressure switch for the hell of it. NO JOY

I haven't had to drive it in a couple weeks but I'm gonna have to start again so I looked up a schematic on Autozone.com. I figured it out for the most part and it looks like a typical overcomplicated system. The pressure switch wires appear to go to the ecm along with the clutch wires.

The schematic is not the best and not quite what I'm used to looking at, so does anyone know any common problems, or maybe can you tell me how the system works?

It's looking like rather than running the power or ground for the clutch through the pressure switch the ecm takes inputs from the pressure switch and it tells the compressor to engage. Am I right?
Link Posted: 4/18/2010 4:30:06 PM EDT
Link Posted: 4/18/2010 4:59:10 PM EDT
Honestly it was a few weeks ago when I checked the voltage at the compressor clutch so I don't really remember but I want to say I got that with the plug pulled off because the spoons that I have won't fit in the vw connector. I didn't think about the clutch circuit having a relay on it but that could very well be, I'll have to look at that tomorrow.
Link Posted: 4/18/2010 5:01:52 PM EDT
Get the pressure in the system checked before you go looking for electrical issues.
Link Posted: 4/18/2010 6:19:07 PM EDT
I haven't checked how much freon it's got in it but It probably wouldn't hurt just to be sure. It's been working perfectly up until just a few weeks ago when it petered out on me.

I don't think it's a lack of freon though because it was a good hot day when it wouldn't come on and the day after when it would come on it was about the same temp. Also if that was the case you'd think it might not want to work at first when it's cold but it would try to kick in when the engine compartment heated up and made the freon in the system expand.

I just put a new compressor/expansion valve/drier and flushed it last august or so.
Link Posted: 4/18/2010 6:49:20 PM EDT
Link Posted: 4/18/2010 6:49:48 PM EDT
[Last Edit: 4/18/2010 6:51:16 PM EDT by oneshot1kill]
Link Posted: 4/18/2010 6:56:00 PM EDT
Originally Posted By oneshot1kill:
The short answer is YES, the control unit does send a signal to the clutch...long answer, diagnostic tip and system overview below.

VW's 'Climatronic' A/C system has OBDII capability built into the system. I always begin any VW A/C diagnosis with a quick Scan for any trouble codes. Although not every issue will trip a code, the more serious problems will. The head unit in the dash also contains the A/C control module, and when you have a stored trouble code the display will flash when you first turn it on.

The way the system works is the control module gets inputs from various sensors in the system and based on a pre-set range -or when specific conditions are met- the control unit sends a signal to actuate electronic components in the system...such as the A/C Compressor Clutch.


Mine doesn't have any kind of fancy climate control with temperature setting, just a temp knob, fan, vent selector, recirculate and a/c buttons. Does that still apply?

I do have an engine light on but it's been on because I gutted the EGR cooler. I guess I'll have to get someone with a scanner doublecheck that too.

Link Posted: 4/18/2010 8:54:15 PM EDT
Link Posted: 4/19/2010 7:41:29 AM EDT
[Last Edit: 4/19/2010 7:45:02 AM EDT by cityboy172]
I would start with checking the system pressure. My 04 has a leak in it that started about
80,000 miles ago. I have to recharge about twice a year to get through the Indiana summer.


EDIT- DO NOT resort to jumping any thing out until you know there is enough refrigerant
in there. You don't want to buy another compressor.
Link Posted: 4/21/2010 7:01:25 AM EDT
Considering that you DO have voltage at the AC comp clutch connector , just not ENOUGH voltage ;

The very first thing to do is to pull the cover off of the battery , open the small fuse panel which is mounted on top of the battery , and check the condition of the 3 green 30 amp fuses at the right rear corner of the panel.

Chances are , you will find one of the fuses (the far left one IIRC) severely corroded , and the fuse panel burned/melted in the area of the fuse contacts.

I've replaced literally HUNDREDS of those fuse panels for that very same problem ; AC inop , low clutch apply voltage.



The next most common cause is a failure of the "cooling fan control module". This is a combined relay/control unit , mounted on the driver side frame horn directly under the battery tray , which is directly responsible for providing voltage to the fans AND the AC comp clutch.


If neither of those are the problem , you may have a severe voltage drop on the wires going to the AC comp clutch. The assembly comes as part of the "alternator wiring harness" , but the affected wires can be replaced by overlaying new wires while reusing the original terminals and connector housings.
This is generally the LEAST probable cause of failure , but it does happen occasionally.
Link Posted: 4/21/2010 7:47:12 AM EDT
Thanks a lot!!!! That's the kind of info I was hoping to get. I haven't touched it this week but I should have time to look at It when I get home today.

I will say though that after working on construction equipment all day it's hard to get motivated to work on my own junk.
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