hunterp27

Has anyone ever had to replace their AC High pressure sensor that's located on the front of the condenser? My AC compressor isn't kicking on and the scan at my INDY showed a failure at this component. The mechanic stated that if the pressure isn't right, then nothing's right and the compressor will stay in safe mode. Any opinions?
I replaced the part this evening but didn't have time to get the system recharged before the shops closed. Normally you would not have to recharge the system due to replacing this part but I had to replace an auxiliary fan and that required the whole system to be evacuated first. Now I need to get that squared away and hopefully I will have a working AC system again.

richard-tx

If the switch is shorted (closed) all the time then the compresssor will not engage as the switch is saying that the system has excessively high pressure. While I have not changed it on a A8, on some/many/most cars the high pressure switch can be changed without evacuating the system.

MP4.2+6.0

Not sure about the switch, but I would get it charged correctly. By weight; no sight glass stuff like days of yore. FWIW, my experience was system gave out without enough refrigerant. Symptoms were early on it felt like it was blowing somewhat humid air for a few minutes, then finally cooled off. As it got worse, hardly anything. Don't know what pressures were. I was chasing things like the exterior temp sensor--which did help a bit/for a while. That idea was pioneered by PaulW on his S8; he lives mostly on QW.

Anyway, got mine charged and night and day difference. Has worked perfectly ever since--3 years ago now. IIRC Charge is something like 630g. The report was mine was down to about half that. In "old" terms that would not be huge in absolute amount, but in newer systems with low capacities it becomes a big percentage. PaulW subsequently circled back, had same thing done, and had same good result.

hunterp27

well, the system has been professionally charged and we are back to square 1. No working AC, compressor is NOT turning on. Starting to think its a bad compressor. Still frustrated in Dallas......just a different issue now.

MP4.2+6.0

I think S8's have the Rube Goldberg auxiliary gear/spline based drive ("pardon my French") to run the compressor. Same as other FSI motors, including the 4.0T I now own. In any case, some longer shot ideas. You may need to head to dealer to get an accurate assessment.

I think it may still have the equivalent of what failed in my W12--a rubber buffering mechanism. If it thinks something is locking up, it tears out internally to protect the drive. On a belt motor, that means protecting against serp belt breakage that then takes out P/S, alternator, etc. On a gear drive set up, even more critical w/ oil pump, water pump, plus P/S. Compressors on D3's have no conventional "on-off" clutch that could otherwise just kick the clutch open (not engage) in a failure; D3 one works on a duty cycle. With a gear drive, I suspect something like that is still in there--I vaguely recall seeing some reference to it but moved on since my W12 is the conventional serp. belt driven set up like the 4.2 port injection motors. Don't see it in EKTA parts diagram offhand for V10 or 4.2FSI (which do look similar, and then different than 4.2 port or W12), so you would need to research it further. FWIW, my W12 specific post about it is here FWIW: https://www.audiworld.com/forums/a8-...otors-2886563/

You also have the possibility of something wrong in the basic gear drive. ...What could possibly go wrong as they say when seemingly underemployed Swiss watch makers are redeployed to engineer the auxiliary gear drive for this and most other FSI auxiliaries. Further trouble is, on gear drive I think you have to...pull the compressor to get at it. Seems like it defeats half the purpose of putting in protective mechanisms, if they fail separately and then involve same painful labor.

Look in Bentley about the S8 specific set up, assuming it is in there.

Finally, picture below is from the 4.2FSI SSP; I think S8 V10 is similar. Hence why my Rube Goldberg tag. People thought the cam chains were complicated on FSI's (and W12). Have a look.

hunterp27

MP4.2+6.0

would it terribly surprise you to hear that I had it at Audi Plano for this very issue and they couldn't diagnose it? They are the ones who told me that the aux fans were the issue and that they would keep the compressor from kicking on. So, Not fully buying into that I went ahead replaced both of them with a couple pre owned ones.....that didn't remedy it. Then we changed the high pressure switch after the diagnostic tool found a fault with it. That didn't work either. I am thinking its the compressor itself and the diagnostic tools aren't figuring it out. Its at an INDY near my house right now and they are checking everything they can think of............stay tuned and thanks for the info you have posted. Yes you are correct its a cam driven version and not a serp driven version. I have found a few used ones for a couple hundred bucks.

ooeddym

Wow, I love the idea of no belts to worry about but if something mechanically went wrong in that system boy would it be painful to fix.

MP4.2+6.0

Is issue new or been around? Bentley shows what I am talking about for S8--called Drive Plate and Overload Protection. On yours it is built into what looks like the front area of compressor surrounding the driven shaft. Trouble is you have to have compressor out to check it on S8 apparently. Basically if compressor is locked, it's compressor. But if that other component is now spinning free or itself messed up, then that. Gets tangled up though, because if compressor locks, then that self destructs to protect gear drive. I don't know either if that it is sold as a separate part like it is with the belt driven ones; didn't see it on the EKTA parts diagram broken out.

Reason I ask if issue has been around or is new/recent is whether you can look for other evidence it failed. If it was along time ago and many belly pan pulls or the motor pull and clean up, evidence would be gone. Anyway, on mine, besides the bearing bits it also spat out a bunch of rubber stuff from inside. At first I wondered what it was or worried it was serp belt or something. But as I turned pulley another piece of it fell out, plus it was painfully obvious by then pullley was seriously messed up somehow. I also found some of the rubber shards and sawdust type stuff below on the belly pan. It basically looks like the kind of shards found when a fan belt is getting munched, or some of the inside of a tire if you had to drive on it flat and it got torn up. Either way it likely still leads to pulling compressor unit unfortunately, but it gives you a better idea what part of it may be on the fritz. Then if it is that Overload feature, whether you can source that as its own part. And either way, it would tel you to focus on that area and pull the overall compressor unit.

Thus, look carefully right around compressor shaft area if visible where it goes into compressor main body. Look for some black spattered debris and dust that looks like it was spun out of the internals, sort of like off a well used power saw at blade guard or base, but here blackish, not wood, and not oily like a chain saw. And check belly pan below for debris if not touched or dumped already.

hunterp27

MP,
I think I am at the point of tearing the front end off of it again and taking the compressor off for inspection. I will certainly be on the lookout for the debris you aptly detailed/described. As far as looking in the belly pan for any forensic evidence, that ship has sailed long ago as I cleaned the belly pan thoroughly when the motor was out. It was covered in oil from the upper pan failure and had to be cleaned up. At that time the AC was working fine and I wouldn't have been looking for anything......even if I had, it's doubtful I'd have seen anything due to the amount of sludge and road debris that had accumulated. At any rate, this is in fact is a relatively new issue. I was simply driving down the highway at 70mph and the air went musty and damp all of a sudden. No noise, no lights, no flicker of gauges......nothin. It just went from working fine to blowin dank air. The pressure switch was in fact bad however as it was throwing a code every time on the diagnostic tools. Once replaced however, those codes are now gone and everything having to do with the HVAC system is reporting a clean bill of health. According to the diagnostics, every sensor, every output voltage and every reading is working as designed. That leads me and the INDY tech to one of two things:
1. Mechanical compressor failure
2. A less than adequate or no signal failure from the back of the HVAC controls in the center console to the Compressor.
At this point the Tech asked me if I could locate the wiring diagram for the HVAC controls so he can identify the correct wire that sends the signal to the compressor its instructions to turn on. His Bentley manual doesn't show this depth on the HVAC system. It shows it for everything else except this. Is this available to your knowledge? He's going to look thru some other avenues but I figured if it's available, someone here would have it.
We are going to start again in Monday and I'm going to begin taking off the front end (I can do this in my sleep now) to access the compressor. I find it hard to understand why there isn't some other way to tell if the AC compressor has suffered a mechanical failure. With all the sensors and self diagnostics these cars are equipped with, you'd think a mechanical failure would be easy to corner.
I really want to thank you in open forum for all the cycles you've given this mentally. Guys like you make this forum invaluable. If I didn't have this forum and the INDY tech as a neighbor, this car would have already suffered a suspicious car fire in a parking lot while at the grocery store.

MP4.2+6.0

You're welcome.

Actually, if it just up and died, look for the debris right around front of compressor now as you pull things apart. Your description of it up and dying is basically like mine when fully failed. First it was slipping when the rubber internal guts were pretty obviously tearing them selves up internally. A/C was erratic and I figured maybe another charge needed, or compressor was dying. Then I found pulley issues and when I went to rev motor, that was all folks. Rubber went flying onto my garage floor, and no more A/C.

You can't get any diagnostics out of compressor because it is probably just sitting there, if that's the issue. Either it is frozen, or the rubber based protection component self destructed and now drive is freewheeling disconnected form compressor, just like if you dropped a drive shaft. Hopefully issue is not in that whole drive gear system by the way. I'm thinking thee was a post where someone lost P/S on an FSI 4.2 and it was maybe in that whole drive/gear system? Also on yours, if that rubber protective thing let go, be wary of why. I can explain mine in that pulley bearing failed and whole component fell apart but compressor was fine. On yours though, it may go back to frozen compressor since I don't see any component between the gear drive and the compressor to fail. On other hand if compressor locked up, it may have sacrificed that coupling to avoid drive gear take off unit from chewing it self up.

Electrical: i looked in Bentley and all data and couldn't find either. I have kind of all purpose HVAC system diagnostic and components SSP from circa 2002. SSP 981203. It looks like late era D2. My C5 was same generation, and for HVAC it all seems very similar to D3, other than exact component locations vary some as you would expect. Skimming it the SSP seems to basically take an all purpose approach to explaining HVAC and then the typical components. It has some electrical diagrams, but only for subsystems and not vehicle specific in general. Send me a message though AW w/ an email if you need/want more info on it.