Bowie Anderson

So I am wondering if I can change this pulley without disconnecting or recharging the rest of the system. I had to replace the power steering pump (OE bearings failed at 212k) and I'm doing belts, water pump etc...the only pulley I failed to purchase was this one because the compressor was just replaced a few years ago. I just dont like the way this one looks. Any thoughts? I have an 04 A8 with 4.2 belt driven compressor

MP4.2+6.0

Yes, pulley is replaceable as a separate part. But also costs several hundred dollars; dealer only AFAIK. I would first assess/ask about the A/C compressor. When was it replaced? With new, or rebuilt? Was pulley really new if it was rebuilt? Or more likely, just transferred over from old compressor or a parts wash and quick black cosmetic spray paint job if part of a rebuilt one.

From having had this specific part fail on my W12, make sure you understand its construction and then whether you have issues that might correlate. What is is NOT is something like the idler pulleys. It has no bearings in center. The pump is what is ALWAYS spinning and then has the bearing support. Instead, what it really is consists of at its center is a sacrificial one way clutch or bushing made out of rubber. That is what is inside that quasi square looking large center area under the cover plate. It helps buffer the pump demand changes and engine vibrations and all, but mostly it is ready to break apart if the pump ever locks up. Goes back to pump is always spinning--it is not an old USA clutch type set up. The theory is that avoids the serp belt from burning up with a pump lock up failure, in which case you would otherwise immediately lose alternator, power steering, etc, and end up at side of road. In my case, over a few days it went from groan that made me first suspect water pump, to squealing cat in bad heat. Once it started spitting out pieces of rubber and I could see it was not smooth/was wobbling, then when I pulled serp belt off I knew it was gone. Fortunately for me it was an internal pulley failure and not the pump. Several years later and with new pulley assembly, the original compressor is still fully operational.

In your case with area open, I would just grab it physically and spin compressor, try to move it off center axis with hand force etc. If it isn't loose or wobbly feeling, I would leave it. Get out the black spray rocker can if look is important.

Did you do the idler pulley directly to its left by the way? With some of those you pull off the black center trim cap and then realize it is just another pulley held there by a center bolt. Or, the hydraulic tensioner that is under the main crank pulley, at least on my 2000 A6 4.2? Crank pulley/vibration dampener has to come off and you need good access like this with front clip off to get at it more easily. Your call, but if I do the higher mileage really deep dive belt change, I go after those in addition to the mechanical tensioner, the basic bulleys, water pump, thermo, etc.

Bowie Anderson

Thanks! I appreciate the info. I can't remember for sure if the compressor was rebuilt or new, I'd have to dig up the paperwork for that. I generally like to use new though. It was repaired in the summer of 15 I think. From the looks of it, either way, I think they used the old pulley part. I can spin it with my hand easily, and it doesn't have any extra play front to back. I guess I will leave it like you said. I haven't had any AC problems except for the fan on top which I replaced last summer. The wheel on this compressor is just rusty and has chipped off in a few places. I dont really care about it being black, but I dont want to have to take this front clip off again for this. I had a pulley look rusty on my Cadillac, I kind of ignored it and it failed, costing me a water pump, which is why I asked.

Yes I am replacing both of those tensioners and everything else I can get to while I have it open. I aim to get another 100k out of this thing

Bowie Anderson

And, my memory failed me. The compressor wasn't replaced. It was the evaporator, drier, and orifice tube. Still cost me $2500 though, ugh

MP4.2+6.0

FWIW, I replaced mine in W12 with clip fully in place. Not easy and it had disintegrated not pieces, but still got it out. I would regard mine as sort of a fluke. Hasn't come really on the board over the years. When I had my issue, I had to track down what it even was. Found it finally on some VW boards, where it seems to be somewhat more common.

Alos while in there and with front clip off, look critically at the front engine torque mount. The one in front of the main mount above the alternator area on the USA passenger side. Seems to fail pretty commonly with time and miles. Should be way easier to get at with things disassembled already to this sort of level. Look for big cracks--or even split in two--or any signs of dark brown looking hydraulic type oil that has oozed out around and especially directly below it. The oil tends to permanently stain parts below when it is blown.

If you are really going through things, I would consider replacing engine coolant sensor if it has not already been done. On a 4.2, that's a "when" it will fail type part, not an "if". On a 4.2 it is buried deep down in the hoses in back of motor.

Bowie Anderson

Also while in there and with front clip off, look critically at the front engine torque mount. The one in front of the main mount above the alternator area on the USA passenger side. Seems to fail pretty commonly with time and miles. Should be way easier to get at with things disassembled already to this sort of level. Look for big cracks--or even split in two--or any signs of dark brown looking hydraulic type oil that has oozed out around and especially directly below it. The oil tends to permanently stain parts below when it is blown.

This is what you are talking about here, right? I haven't seen any black oil yet but it looks pretty worn out

This is the mount you mean, correct?

I'm guessing it shouldn't be worn away like that...


If you are really going through things, I would consider replacing engine coolant sensor if it has not already been done. On a 4.2, that's a "when" it will fail type part, not an "if". On a 4.2 it is buried deep down in the hoses in back of motor.[/QUOTE]

So that brings me to another topic I have been looking at...I search and lurk around here quite a bit lol. I am not sure exactly where that sensor is located. I know that I have a coolant leak back there, and I could see it when I changed out the CV joints. It looks like it is a plastic tube, and if i recall correctly, I could only see it with the CV dust guard off. Any idea how to get to it cleanly? I am confused by what I see on the fourm, and I have been using ALLDATA but that only gets me so far...here is what I can see at the moment..

MP4.2+6.0

Yes, that is the torque mount in the picture. Does not look blown, but it does look like it is getting old. A lot easier to do now than later if you have parts budget for it. Oddly, cheaper from the online Audi parts dealers than the usual indy places when I checked.

Not sure what that tube is in the picture. Almost looks like some kind of vent/overflow tube rather than a water line. Or maybe wiring harness sheathing? See if you get any 4.2 owner input. The coolant sensor is higher up. On my C5 it was on a crosspipe that ran between the two sides of the "V." It might be in a flexible line in D3 4.2. A pic or two has been posted in last year or so IIRC and you search.

Be suspicious of the oil cooler pipe on the passenger side of the motor. Another when not if on older/higher mileage 4.2's. Search on it for more info. Not fun to change. Not sure though the front clip disassembly helps much on it though. Maybe others can comment. But anything in coolant system is more efficient to tackle together for antifreeze renewal, bleeding, etc.