A/C problem

Hi all,
Ran into the common A/C light won't stay on issue.
Checked the fuses, all okay. Had a mobile A/C guy come around today and after the re-gas( he said it was bone dry) the AC light comes on but the compressor does not seem to engage. The air blown is hot and the revs don't change when the AC button is pressed. He suggested a new compressor.
Is there anything else I could check before replacing the compressor?
I've changed one of these before on other cars, but on this one seems awfully tight in there(to reach the tensioner), is it a bumper off job? Are there any how to guides around?
Cheers.

A4snake said:

Have you checked the A/C relay, your fuse will test ok but if the relay isn't switching then the power will never get to the electromagnetic clutch on the pump, you can just check for a live feed at the A/C pump to determine this, if the wire to the pump doesn't go live when A/C button is pressed then you need to look at the relay/wiring. However if it is getting 12v but the pump isn't working, then that would suggest the pump is faulty or possibly a duff pressure sensor, but these tend to put a fault code in the system (no warning messages though)

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Thanks will do that today. I’ve replaced the sensor but the clutch still didn’t engage. Luckily got from amazon so I can send it back for a refund. I’ve seen one guy connecting a 12v probe to the live wire of the compressor to see if it engages but missing such equipment at the minute.

rum4mo said:

My January 2011 build date S4 AC compressor does not have a clutch, it is the newer variable displacement type as are most cars, fixed displacement with clutch on the drive pulley is old hat and wasteful on power/fuel, plus lets the system seals run dry if not used frequently.

Edit:- the best way round this issue will be to scan the AC module to see if it is logging faults that are inhibiting the running of the AC system, and even looking at live data.

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I did say to mysef that I need to invest in a scanning tool but nothing warranted it so far... any recommendations?

A4snake said:

Have you checked the A/C relay, your fuse will test ok but if the relay isn't switching then the power will never get to the electromagnetic clutch on the pump, you can just check for a live feed at the A/C pump to determine this, if the wire to the pump doesn't go live when A/C button is pressed then you need to look at the relay/wiring. However if it is getting 12v but the pump isn't working, then that would suggest the pump is faulty or possibly a duff pressure sensor, but these tend to put a fault code in the system (no warning messages though)

Click to expand...

I’ve managed to check and the multimetre reads 14.5 going in..I was expecting 12. As it’s getting 12v (at least) i’m going to go with a new compressor. Cheers for the reply

rum4mo said:

I'm guessing that by "sensor" you mean the fridge gas pressure sensor, I'd always only ever a genuine latest version of them because VW Group's supplier(s) just always seem to have problems making them work and keeping them working without causing a leak.

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You’re right, that was the pressure switch I was referring to. I got a lucas one from amazon but sent it back as it didn’t seem to be the culprit. For the compressor itself I found a DENSO one for 276 on autodoc. There’s some cheaper ones like Ridex for 180 but I thought I’ll go for the same brand as the original.

I’ll update just in case anyone else needs this.
Steps I took to identify the problem( AC LIGHT would stay on after refill but no cold air):
1. Checked all fuses, all ok no issue there.
2. Checked the pressure switch, ordered a new one from amazon, did not fix the problem so returned it for a full refund.
3.noticed that the central piece of the compressor was not spinning, denso 6seu14c compressors do not have a magnetic clutch so it should’ve spinned even if the compressor was not engaged. Checked with a multimetre and every time the ac button was pressed the condenser was getting 13-14V. So that narrowed it down to the compressor.
4. Ordered replacement compressor and dryer from autodoc, £270 for a denso, ecp quoted £400.
5. Had the gas recycled, and replaced the compressor.
There is a video on youtube of a guy doing an A6 one but he’s cutting corners. Make sure you take the oil out of the new compressor and put back a similar amount of what is was in the old one. Use a torque wrench to tighten everything back to specs. Usually 50% of the oil (Pag 46) sits in the compressor, in a 150ml system I’ve put back 75 ml.
It’s a pig of a job taking it out, you’ll need to be creative with the tools. L shaped allen keys helped.
I have to say that I did not manage to replace the dryer as one of the torx bolts for the condenser hoses sheared, to replace the dryer you will need the condenser out.
Had the system vacuumed and regassed.
A/C works fine.