Intermittently working AC in a VW

[intx13]

The AC in my 2008 VW Eos (78k) is working intermittently (and not particularly well when it does). I did some testing today and got the following measurements, in this order.

(Ambient air ~85 F)
1. Engine OFF: Low side @ 88 PSI, High side @ 110 PSI
2. Engine ON, AC OFF: Low side @ 88 PSI, High side @ 110 PSI
3. Engine ON, AC ON: Low side @ 80 PSI, High side @ 115 PSI, Blowing ambient air
4. Engine OFF
5. Engine ON, AC ON: Low side @ 70 PSI, High side @ 200 PSI, Blowing slightly cool air (but within 5 minutes returns to 80/115 and blowing ambient)
6. Engine OFF
7. Engine ON, AC ON: Low side @ 55 PSI, High side @ 180 PSI, Blowing cool air (and stays that way)

A few miscellaneous notes:

• The compressor is clutchless, I think it's a Sanden PXE16 (or similar) from pictures. This does not appear to be the stock compressor for the car, but it does appear to be stock on newer, similar VWs. Maybe the previous owner had the original part replaced.
• The blowers all seem to work fine and come on as expected.
• The cabin air filter was pretty filthy so I replaced it, but with no apparent effect on cooling.
• I gave the car time to warm back up between each test above. Each test with the engine on and AC on, I let it sit for a good 5 minutes until the pressure reading stabilized.
• Although it would appear that the AC functioned better over time, it's not always in this order. Sometimes it'll blow cool, then the next time warm, then cool again, then warm. No discernible connection with ambient temperature or city/highway driving, etc.

I'd like to narrow down the problem. I imagine there's a couple possible culprits:

A. Sensors and compressor are all fine, but the ECU is driving an improper PWM signal to the compressor as a result of some electrical fault.
B. One or more sensors are faulty, so the ECU is getting bad information and driving the (working) compressor incorrectly.
C. The sensors are fine and the ECU is attempting to drive the compressor normally, but the compressor itself is faulty.

What should I test next? My gut feeling is that it's a problem with one of the pressure sensors, but I don't know how to proceed, or how to confirm that. Any input is appreciated!

[GM Tech]

Don't forget about air gap-- the next time it is blowing hot when it should be cold- look and see if compressor is turning- if not, tap it with a wooden hammer handle- if it snaps into action- you have a wide air gap, and a shim needs removed from in behind the clutch driver

[intx13]

Don't forget about air gap-- the next time it is blowing hot when it should be cold- look and see if compressor is turning- if not, tap it with a wooden hammer handle- if it snaps into action- you have a wide air gap, and a shim needs removed from in behind the clutch driver

This is a clutchless compressor.

Is there a way I can measure the PWM signal to the compressor to see how the ECU is trying to drive it? That would at least let me distinguish between compressor fault and sensor/ECU fault.

[bohica2xo]

VAG scan tool.

[intx13]

VAG scan tool.

Is there a way that doesn't cost as much as a new compressor? I have access to oscilloscopes and soldering equipment at the office. I was hoping I could just hook up some probes somewhere and monitor the signal on a scope?

[wptski]

VAG scan tool.

Is there a way that doesn't cost as much as a new compressor? I have access to oscilloscopes and soldering equipment at the office. I was hoping I could just hook up some probes somewhere and monitor the signal on a scope?

If it's a PWM signal it's nothing but a pulsed 12V at some frequency. You should be to tap into that easily and calculate the duty cycle. Actually might be more accurate than having some other device figuring it out for you. I've done that on a 4WD/AWD electric clutch system.

[intx13]

If it's a PWM signal it's nothing but a pulsed 12V at some frequency. You should be to tap into that easily and calculate the duty cycle. Actually might be more accurate than having some other device figuring it out for you. I've done that on a 4WD/AWD electric clutch system.

What do you think would be the best way to do this? I think to make a proper test I actually need the PWM signal to still drive the compressor. I could disconnect the signal at the compressor, jumper it, and tap it there, except that I'm not sure I can get the right wires down there into the engine compartment to mate with the compressor connector. Maybe it would be doable from underneath, not sure.

If I knew where it connected with the ECU I could measure it there. Do you think the PWM signal runs through a fuse? Unfortunately I don't have any manual or schematics on this car.

Worst case I could dice-and-splice the wires where they run through the engine compartment at a convenient location; I have heat shrink insulation, etc. so I could repair it acceptably, but still not ideal.

[bohica2xo]

Why are you so certain this is an electrical problem? Have you recovered the charge remaining & weighed it?

Sensors are typically thermistor pressure units - they fail open or shorted.

PWM control valves stick sometimes - usually open or closed.

You need a VAG compatible scanner to test the electrical end of the system. If the valve is sticking, it is posting codes. If the pressure sensor is failed,it is posting codes. Just because there is no big light on the dash glowing, there is a lot of data stored in the PCM / BCM

Hacking in to the wiring harness to look for a PWM signal - when you have no reverence values for the signal? Yikes.

Buy a subscription to AllData or Mitchell for the vehicle. They provide full factory wiring diagrams.

Buy a VAG compatible scanner if you plan to work on VW stuff. Thus one seems to do the job:
http://www.ross-tech.com/vag-com/index.html

But first verify that it is properly charged with refrigerant. Static pressures tell you nothing. One ounce or 16 ounces in a system will have the same static pressure.

.

[wptski]

What do you think would be the best way to do this? I think to make a proper test I actually need the PWM signal to still drive the compressor. I could disconnect the signal at the compressor, jumper it, and tap it there, except that I'm not sure I can get the right wires down there into the engine compartment to mate with the compressor connector. Maybe it would be doable from underneath, not sure.

If I knew where it connected with the ECU I could measure it there. Do you think the PWM signal runs through a fuse? Unfortunately I don't have any manual or schematics on this car.

Worst case I could dice-and-splice the wires where they run through the engine compartment at a convenient location; I have heat shrink insulation, etc. so I could repair it acceptably, but still not ideal.

A standard compressor clutch is fused but unsure if yours is. Fuse location info would be in your owner's manual and if you don't have that maybe you can download it from this site: http://www.volkswagen.co.uk/owners/manuals

You can "back probe" a connector, look that up if you don't know what that is. I have several wire piercing probes. Chasing wires can be a PITA and I've seen them change colors from the engine compartment to the PCM/ECM inside the vehicle. I had a schematic too but it was no help when the wires change colors. I used a toner instead of pulling the PCM and hoping to have pins marked/numbered and that worked.

[vwhaggis]

My EOS airconditioner work fine when the vehicle is moving but it turns off when it is waiting at lights travelling slow or its very hot 30 degrees and up (Sydney Australia). So I was wondering what it could be? A cooling sensor? The radiator fans?