AC compressor - engaged when interior temp set to HI???

[Lander91]

Took the six out to stretch its legs/blow out the cobwebs yesterday and as I usually do when I return home I left it idling it with the heater on HI to help cool things off. After a few minutes I came back and could smell something burning. It wasn't the smell of oil, coolant or trans fluid, or brake pads. Nothing was leaking.

Note: the AC wasn't blowing cold and I've suspected a slow leak but given the drive was the first in good. warm weather I haven't been concerned about a lack of cooling AC.

So I drove it into the garage to have some darkness and no wind to see if and where the smoke was coming from. There was smoke, which I traced to the AC compressor area, I touched the compressor and received a burnt finger. How hot? A bit of WD1 (spit) on my finger resulted in an instant sizzle when touching the compressor again!

I've ordered new seals for every junction in the circuit as well as the AC line strainer as a matter of maintenance and/or repair depending on the source of the leaking refrigerant.

Google has stated that a lack of refrigerant can overheat a compressor, though I would think that the clutch wasn't engaged when I'm asking for heat - correct?

Scan has revealed nothing.

[Shoppinit]

You’d want AC for demisting so I think it’s normal that it’s on when the heating is on max.

[Shoppinit]

The compressor clutch on my XC90 hari-kari’d a couple of years ago. It wasn’t immediately obvious what it was because the alternator and a whole bunch of other stuff failed at the same time. It smelled of electrical burning.

I’d have thought that the AC wouldn’t attempt to engage if the system pressure was too low, but maybe the RS6 doesn’t have a sensor for that, whereas the Volvo does.

[srichards]

There is a pressure switch. I only know that as mine failed and the AC didn't work until I had it changed.

[Shoppinit]

Yeah, there's a high pressure switch, but there might not be a low pressure sensor.

[Shoppinit]

The high pressure switch might still give interesting information. You can log it in group 001 of the HVAC. It's a PWM output so it'll be a % value. It'll give a very rough indication of the pressure on the high pressure side.

I don't see any information about a low pressure side sensor, so I don't think there is one.

/oh, that's interesting. Looks like the climatronic guestimates the pressure in the system from the PWM signal from the high pressure switch and displays it in group 001 too.

I haven't been able to connect to my HVAC with the Vagcom since about 2011 so I'm not overly familiar with what's going on in the parameters.

[Lander91]

You’d want AC for demisting so I think it’s normal that it’s on when the heating is on max.

Hmmm interesting point, thanks Shopp (as always). I'll log the high pressure value and the clutch engagement during HVAC operation and see what happens. I'm crossing my fingers that if I have to replace the compressor its not going to be a complete ball ache due to its handy location.

[Shoppinit]

Haha. How long have you owned an RS6? Should know better by now.

[Lander91]

I know I know, but we're all masochists at heart given we own these things. I figure I'll tempt Murphy knowing full well what I'm in for, as opposed to tip toeing around in counter clockwise circles while throwing salt over my shoulder

[Mɐʇʇ]

Might be wrong, but I think you can do it from below with the lock carrier in service position. I certainly grazed my knuckles on it a lot when I replaced my power steering pump.

[Shoppinit]

I have an inexpensive kit for checking pressure / refilling. It's the only way to know for sure how much gas is in there. Something like this:

https://www.amazon.co.uk/R1234yf-Rechar ... B09PH6ZJ2H

[Shoppinit]

You might get away with just replacing the clutch on the compressor. This is what I did to fix my Volvo. Didn’t even have to open the system. I did buy a whole new compressor though to harvest the clutch. It was cheaper than buying the clutch on its own.

Don’t know if access will allow that on the RS6. It was bad enough on the Volvo.

Although in your case, you’ve almost certainly got a leak to you’ll have to trace that anyway.

[Lander91]

Thanks gents. I'll quickly confirm the engagement on HI then get the leak test done, fix the leak and replace the strainer, regas then cross fingers .

[Shoppinit]

Put some UV tracer in there while you're at it. For next time. You'll thank yourself.

I successfully argued a warranty issue on the AC drier once because I traced the leak to it with a UV lamp where the dealer failed.

[Lander91]

Got into the scuttle box on the weekend. Replaced the orifice tube/screen and 3 seals that I could access. Check out that orifice tube! I don't think it's the smoking gun, potentially it's that way as a result of the system overheating and scorching itself after repeatedly being topped up (repeatedly getting low on gas). Anyway, as an FYI the procedure of replacing the orifice tube is easy.

De gas the system
Remove the scuttle panel
Unbolt the coolant res and secure aside
Remove the hex bolt (8mm I think) from the bulkhead high & low pressure line block
Loosen the high pressure junction that is below the coolant res (22 & 24mm spanners)
Remove the detached piece of high pressure line
Needle nose pliers to extract the orifice tube found in the section of pipe
Clean pipe and junctions - well!
New orifice tube is a challenge to get in, new version has 2 orings on it. Constant careful pressure pops it in (reference how far in the replaced part was in)
Put it all back together with new seals.

Cross fingers and regas. (I haven't done this step yet)