AC issues

B6/7 S4/RS4
#1

So my AC has been acting weird.

About 2 years ago my compressor crapped out and was replaced, been good since. Last time out in May I noticed it wasn’t blowing cold air, figured all the standing still over winter especially had accelerated the natural loss of refrigerant. It wasn’t hot out so I wasn’t too bothered back then.

After I got back to the car last week, I took it for an AC “service” with refill which involved the car being hooked up to a system for about an hour. Was told only 125 grams of R134A remained of the 440 which were spec’d. Figured that was it, and took off. AC seemed to work fine as I drove away, but shortly afterwards it turned out that it only cooled with the engine at idle or below 1500 RPM or so anyway. Took it back but they had no ideas other than to tell me to take it to an Audi dealership instead (something I wanted to avoid as they’ve all been clueless in my experience)

Time ran out so I figured I’d try and get it fixed somewhere along the way of our planned trip southwards. No fun lacking AC when it’s 35 degrees C out, obviously. The problem seems to be even worse in hot weather. It will cool a bit at idle, sometimes, but gives up pretty quickly. The weird thing about this is that it’s usually the other way around - ACs tend to work better with sufficient airflow to cool down and condense the gas and when the charge is borderline it would only work when moving.

As far as VCDS is concerned, the compressor is ‘ON’ even when it’s blowing hot air, FWIW. I’m uncertain whether something like a dodgy pressure sensor cutting the AC would then show it as “OFF” or not.

I’m not particularly familiar with the compressors in our cars, in particular the clutch and if it could be slipping under load.

Just asking if anyone’s had something similar as I’ll need the most possible details and luck to give anyone around here (in fcking Croatia) a hope in hell of getting it working. If I can find anyone in a shop who speaks English.

#2

No codes stored right?

I would like to see the readings for the measuring value blocks channels 1, 10, and 11 at idle and then while moving with the AC off and then on in both circumstances.

You do probably have a leak.

#3

The only codes there are the two missing front seat heater circuits (swapped seats to Recaro CS) and one fault for the recirc flap. Which still seems to kind of work FWIW.

I made some logs, but I was standing still as driving would have meant giving up my heard earned parking spots. I kept the revs up to about 2K a few times which is more than enough to make the AC quit.

The first part of the log I had the AC off as you can see. First idle, then revs for a little while. Then let it settle and turned the AC on. Slightly cooler air came, but I wasn’t in any risk of freezing as you can see from the temps.

Looking at the pressure and evap temp (and simply holding my hand out) it’s obvious that even at idle it drops in and out and never gets really cold. When I bring up the revs it drops out and doesn’t come back. At idle it works a little bit.

The log: http://s000.tinyupload.com/index.php?file_id=09969027209984682056

#4

Thank you for sending me a PM so that I got a notice to look at this. I had to look at it on my computer since it wouldn’t load on my phone.

A few things stand out to me. Before I get to that then I should tell you that it would be better if you could put a fan in front of the car blowing air on the AC condenser and radiator if you are going to test it without driving the car. I tested my car for the KC system and a bunch of customers’ cars that way. The fan blows off the AC condenser to get some air movement since the two radiator fans are not enough for more than two to three minutes before heat soak sets in.

  1. The compressor amperage seems right between 0.64-0.65 so the compressor is getting power with the AC on.

  2. The compressor duty cycle is lower than I would like with the AC on. It is in the mid 60% but I usually see low 70% like 72-73%.

  3. The high pressure switch signal follows the circuit pressure correctly according to what I have seen. It is a 5V pentiometer so the percentage reading is based off the circuit pressure.

  4. The most important reading is the circuit pressure. Which is why I have separated this notation from the rest. It starts off around 7 bar with the AC off which is relative to the ambient temperature and seems right for 29-30°C. If it was at or below 2 bar then the compressor would not come on due to little refrigerant in the system. If the pressure goes to like 18-20 bar with the AC on then the compressor duty cycle will go down to shutting off maybe to reduce the pressure from causing an explosive leak. With the AC on your circuit pressure gets to somewhere between 8-12 bar normally with a high of 14, which is too low. With that high of an ambient temperature the circuit pressure should get up to around 16-17 bar and hold there pretty steady from what I have seen with my car. Just as an aside, I tested my S4 in around 60°F/15°C ambient temperatures and the circuit pressure started around 5 bar and got up to 14 bar so that is why I said that the refrigerant pressure is relative to the ambient temperatures.

  5. Going of the evaporator outlet and different ambient temperature readings. The evaporator outlet temperature stays pretty close to the ambient temperature reading which is showing that the AC is not working correctly. From a numbers perspective, the ambient temps were like 29-30°C and the evaporator outlet temps range from 29-22°C mostly with a low of like 17°C. In similar conditions my evaporator outlet temps got down to 13.5°C compared to 30°C ambient temperatures and stayed there.

  6. The rest of the readings are simply there to see that the HVAC blower fan is running, the engine coolant temps don’t get too high, and that the idle speed increase request is on due to the AC being on.

In my opinion, I think that your compressor has crapped out or failed. It is occasionally building high side pressure but not consistently like it should. These AC compressors use a wobble plate to vary the displacement since the shaft is always driven by the spur gear drive unit of the engine and thus by extension the timing chains. The AC systems in these B6/7 V8 cars are about as simple from a design standpoint as possible and there really isn’t much that can go wrong with them.

Going off on a tangent. My S4’s AC compressor died again in early July. When it failed it really bothered me because it had been almost three years to the month since I had replaced it before. So I did research and I found that all the MPI timing chain V8s (B6/7 S4s, V8 Allroads, and early C6 A6s V8s), 4.2 FSI V8s (RS4s, Q7s, 2007+ A6 V8s, 2007+ D3/D4 A8s, S5 coupes, R8s), and 5.2 FSI V10s (C6 S6s, D3 S8, and R8s) all use the same AC compressor. The part number is different on some of them but it is the same part physically. So all of those cars will eventually have the AC compressor fail and I am starting to see more RS4 guys having issues so I guess that it just takes time. Think about this though, the V8 R8 uses the same AC compressor right but the front if the engine is right up against the back of the cabin so how do you replace it? That is right, the engine and transmission have to be removed to replace the AC compressor. At least the V10 R8 has the compressor on the transmission side so it only takes a few hours to replace rather than a week of labor!

#5

Thanks for that detailed reply! I had figured the compressor to be the most likely culprit as the behavior doesn’t make much sense for other issues.

This compressor has only been in the car for about 2 years actually. I bought the car and the compressor died a few months in. Bought a new Denso compressor and it worked well since then. So maybe two years and a couple of months, 15K miles if that and it rarely had to work hard.

Edit: airflow doesn’t seem to matter as it behaves exactly the same at high speed, I’ve tried going 100 mph+ in Germany and coast for a minute at the time and the result is similar to standing still and idling. I will say that I can’t notice any extra fans running with the AC on vs off.

#6

Just a theory:

Original compressor failed, sending trash downstream, clogging the orifice tube > compressor was replaced but not the orifice tube > orifice tube remains clogged, restricting flow. It’s a cheap thing to check before plopping down $ for a new compressor…

#7

Thanks. I’ve left the car with a local VAG shop that seemed quite good. Gave them a spare new compressor (only 300 euros from Denso) because it makes sense to avoid overpriced OEM parts given the odds that the compressor is going to be the issue more often than not.

After they disassembled the front end of the car they noticed the aux belt was in pretty bad shape, so that will be replaced while they’re at it. It’s their problem to fix now, hope to have it back tomorrow. I was lucky to know a local guy here from way back who made some inquiries with car people where best to go with it. Would never have found a suitable place on my own, Google was pretty useless.

#8

Just for the record, and to increase the daily post count here by 50% for today, my AC compressor has gone to the big parts bin in the sky yet again.

Noticed a metallic “bad” sound with the window open that changed a bit with revs, sounded like a broken PS pump but my steering was fine so checked the AC and it’s a goner.

3 AC compressors in 3.5 years now, but this one only lasted about 5000 miles. System was flushed repeatedly after the last failure with just about every part replaced dryer and filter included.

Only good thing is that the car is back “home” in Russia so it’s 350 for the compressor and another 150 to get it back in and the system recharged.

I remember now why 10 year old cars that were once expensive are such a bad idea, you’d think I would have remembered from when I had the RS6 but apparently not. Only picked the car up 4 days ago, had to replace a leaking PS pipe and deleted the aux rad on the passenger side that was leaking as well. Still getting occasional random cylinder misfires on cold starts also, probably the injectors but who knows.

I’ll get it fixed and put the car up for sale. Getting too old for this crap, it was fun 10 years ago when I’d help my buddy mechanic with the wrenching but now I just want stuff to work and the car is surplus to what I actually need here anyway.