Air conditioning troubleshooting

xaeryan · Apr 9, 2013

Only used it once or twice over winter, now that it's warm again it no longer blows cold. When I hit the button, no change in idle or any clutch engagement noises, no fan noise, nothing.
Had the pressure checked and it's up near 100psi, so it is charged. Checked fuse 16 and it seems fine.

Prior to taking it to a pro, are there any tips on what to check? Does VCDS have any test procedures?

powerfool · Apr 9, 2013

There is a clutch in the AC compressor that can malfunction and it can be replaced by itself... there is a seller on eBay that pushes many of them. If you have no fan noise, the blower could be bad or a fuse for the blower could be out.

Redneck Truck · Apr 9, 2013

First of all, do NOT take it to a "pro" unless they're recommended on here. As I found out tonight, I had the same symptom; the A/C compressor was not plugged in. First, disconnect the electrical connector on your compressor when the A/C should be on and see if you have voltage. If not and if the A/C light on the dash is illuminated, it's probably your fan control module. There's a whole writeup on here about how to diagnose the system, but one thing is sure - if you take it to Firestone, etc, you'll be buying a new compressor, condenser, evaporator, receiver/dryer, lines, expansion valve, 2 fans, blower motor, and THEN the fan control module.

Good luck!

gquenstedt · Apr 9, 2013

If your radiator fans don't come on at all, it is usually one of two things. First, check the fuse box on top of the battery for any signs of melting. If you don't see anything melted, turn the a/c on and then wiggle the big green fuse on the left.

If you don't get any results from that, the next likely culprit is the pressure sensor. Do an advanced search for threads started by Dang144 he has an excellent how to for diagnosing fans and a/c

Joe_Meehan · Apr 10, 2013

I agree that you should get a recommendation before choosing someone to do work on your AC. Most cities will have either radiator shops (like Florida) or A/C shops like Maine.

These are generally the best bet. Local do it all shops are seldom your best choice.

Good Luck

gforce1108 · Apr 10, 2013

This should have what you need... has worked very well for me in the past.
http://forums.tdiclub.com/showthread.php?t=242699

3800rpmDiesel · Apr 10, 2013

I would check your clutch coil first - easy to check, cheap to repair, common failure point, and makes sense in this case as you can't hear/feel the compressor kick on....

You can buy the coil by itself (separate from the clutch) for around $90 from Roseland up here in Canada - not sure where you can get them in the US....

xaeryan · Apr 10, 2013

Thanks guys!

So far I've had a chance to check the fuses above the battery - no burn marks, all look good and I did check voltage at the top to at least make sure they are getting voltage (11.6V).
Confirmed my interior blower fan works for all 4 settings, and the AC light does come on when pressed. The radiator fans and compressor do not come on at all.

Next I'll try chasing down voltages and the various connections mentioned above - thankfully it's gonna be cooler the next few days.

JohnnyChimpo · Apr 10, 2013

Redneck Truck said:
- if you take it to Firestone, etc, you'll be buying a new compressor, condenser, evaporator, receiver/dryer, lines, expansion valve, 2 fans, blower motor, and THEN the fan control module.

Ouch. I work for Firestone but I was able to diagnose mine correctly when it quit last year. Turned out the wiring at the lower left corner of the radiator right next to the fan control module was melted and shorted. That's okay, most of the techs who work for me couldn't have diagnosed it.

Redneck Truck · Apr 10, 2013

Yeah, you're definitely the exception rather than the norm. What is your target ticket price at your store, about $2500?

JohnnyChimpo · Apr 10, 2013

Redneck Truck said:
Yeah, you're definitely the exception rather than the norm. What is your target ticket price at your store, about $2500?

Oh man, you must have really got stroked once. Hope you're joking! But you are correct about one thing- I've been a service manager for 6 years but still won't let my techs work on my car.

xaeryan · Apr 13, 2013

Ok, so the compressor IS getting voltage with the AC turned on (around 9.8 volts with fan setting 4). I'm not seeing how to test the clutch coil from staring at the compressor (I still have the plastic guards obstructing a good portion of view), so any references welcomed...

But first, some questions - if the compressor failed, would that be reason for the radiator fans not to turn on also? Or should they still be coming on anyway and maybe the compressor isn't the root cause? It doesn't look like the compressor has any other connections other than power/ground, so I'm assuming it doesn't look at any other parameters other than voltage before turning on... but whether the fans do is my real question here.

xaeryan · Apr 13, 2013

UPDATE: Jumped the compressor with a full 12 volts and it definitely kicks on (I can hear the click of the clutch) and the idle drops a bit.

xaeryan · Apr 13, 2013

UPDATE 2: No voltage at the fans when the AC is on.

gquenstedt · Apr 14, 2013

Ok. So, for the fans and the clutch coil to receive voltage when you request a/c, several conditions must be met. Most notably:

1. The fan control module must be receiving current. This is why I said wiggle the big green fuse on the left in the box on top of the battery. That is the fuse that gives power to the fan control module.

2. Next, the fan control module must have signals from several different sources all giving it the green light to turn on the a/c. If any one of those signals is missing, no a/c. The one that usually is missing is from the pressure sensor which is screwed on to the high side line close to the firewall. There is a procedure to test this, but it requires a multimeter that can measure hertz.

3. The fan control module itself can be bad, but I doubt this. Every time I've seen those fail, they still will send power to the fans, but not the clutch coil.

xaeryan · Apr 16, 2013

gquenstedt said:
Ok. So, for the fans and the clutch coil to receive voltage when you request a/c, several conditions must be met. Most notably:
1. The fan control module must be receiving current. This is why I said wiggle the big green fuse on the left in the box on top of the battery. That is the fuse that gives power to the fan control module.
2. Next, the fan control module must have signals from several different sources all giving it the green light to turn on the a/c. If any one of those signals is missing, no a/c. The one that usually is missing is from the pressure sensor which is screwed on to the high side line close to the firewall. There is a procedure to test this, but it requires a multimeter that can measure hertz.
3. The fan control module itself can be bad, but I doubt this. Every time I've seen those fail, they still will send power to the fans, but not the clutch coil.

Good info, thanks. Does anyone have the procedure for testing the pressure switch? I have a friend who's handy with an oscilloscope... My searches don't find anything.

Also, I've found that the ambient sensor under the cowl and the radiator temp sensor are some of the items inline with this circuit that feeds the FCM... anything I'm missing?

gquenstedt · Apr 16, 2013

http://pics.tdiclub.com/data/517/A4_Air_Conditioning_Troubleshooting_Rev_7.pdf

Basically, with the sensor plugged in, and the system on, pierce the white wire coming out of the sensor with a safety pin. measure duty cycle between that wire and ground.

xaeryan · Apr 17, 2013

That document... is EXCELLENT!
It should be standard response for any posts starting with, "my AC..."

gforce1108 · Apr 17, 2013

xaeryan said:
That document... is EXCELLENT!
It should be standard response for any posts starting with, "my AC..."

I agree - it was listed in the link I posted for you last week

xaeryan · Apr 17, 2013

gforce1108 said:
I agree - it was listed in the link I posted for you last week

Haha, well how about that... I musta missed it first time through. *chases shiny red ball*

xaeryan · Apr 22, 2013

Pressure sensor checked out fine, so now I'm trying to get at the FCM for testing... I've got the battery tray out, but I can't for the life of me get the clips to release on the FCM connectors... Any hints before I hulk smash them? I'm pressing back on the tab that I can see.

Redneck Truck · Apr 22, 2013

years of dirt and grime buildup will do that. push connectors on further before pulling release tabs back. may have to try a few times. get ready to scrape your knuckles/whack your elbow - they'll come off suddenly.

xaeryan · Apr 23, 2013

Redneck Truck said:
years of dirt and grime buildup will do that. push connectors on further before pulling release tabs back. may have to try a few times. get ready to scrape your knuckles/whack your elbow - they'll come off suddenly.

Got it finally. Tested everything up to the duty cycle steps. Everything checked out except resistance on the low AND high speed fan leads, which was .311 ohm to ground for both (doc says they should be 9ish ohm). Not sure I understand what that means. I powered the low speed lead as a test, fans both work. Can anyone interpet this?

DanG144 · Apr 23, 2013

If the fans run in slow speed with power applied, then you are back to checking detailed circuit interlock status.

Have you done steps 1-10 on page 4 and 5? Your statement above is a bit ambiguous, as there are several places that mention checking the duty cycle.

xaeryan · Apr 23, 2013

DanG144 said:
If the fans run in slow speed with power applied, then you are back to checking detailed circuit interlock status.

Have you done steps 1-10 on page 4 and 5? Your statement above is a bit ambiguous, as there are several places that mention checking the duty cycle.

Sorry, yep steps 1-10 on page 4 are exactly the ones I just completed. (I had someone with an advanced multimeter check G65 previously)
Step 3 is the one I got .31 ohms for both tests - I rechecked it a few times.

xaeryan · May 9, 2013

So can someone recommend where I can have this diagnosed? At this point, I've given up trying to find free time to continue troubleshooting.

lovingHDTV · May 12, 2013

I must say thanks to everyone here. Another successful self-diagnosis based on the great information on this forum. My A/C cooling fans were not coming on, had to re-seat the coolant fan fuse and now everything is working!

dave

GaryBrandon60 · Nov 18, 2016

Air conditioning troubleshooting tests the filling level of the warm up system. Prove the push. To reduce air from the system. Test the temperature limiter and involving cables...

GREENMK4TDI · Jul 17, 2018

xaeryan said:
So can someone recommend where I can have this diagnosed? At this point, I've given up trying to find free time to continue troubleshooting.

Did you ever figure out that issue? im having the same exact issues as you and I've done everything that was said in the previous posts and I haven't had any luck.

Air conditioning troubleshooting

Active member

Veteran Member

Gone, but not forgotten

Veteran Member

Top Post Dawg

Top Post Dawg

Veteran Member

Active member

Member

Gone, but not forgotten

Member

Active member

Active member

Active member

Veteran Member

Active member

Veteran Member

Active member

Top Post Dawg

Active member

Active member

Gone, but not forgotten

Active member

Top Post Dawg

Active member

Active member

Veteran Member

New member

New member