Intermittent Air Conditioning Problem - Did Bentley Test, Now What?

[CardFan]

Just picked up my 05 A4 last week with a known ac issue (occasionally blows hot air). Used car dealer I bought it from said it needed a compressor. I went through the Bentley test procedure using vag-com to log data (when it was blowing hot air). The compressor seemed to check out based on the a/c valve current and duty cycle readings, but the refrig. circuit pressure didn't rise much compared to when the compressor was switched off (econ button on). Today I logged with vag-com while the air was blowing cold, and the refrig. circuit pressure was noticeably higher. Here are the logs.

Log while blowing COLD air
A/C Valve (N280) A/C Valve (N280) High-Pressure Refrigerant Circuit
Current Duty Cycle Signal (G65) Pressure
A % % bar
0.648 66.3 44 14
0.644 65.9 44 14
0.652 66.3 44 14
0.648 65.5 44.5 14
0.648 65.9 44.5 14
0.648 65.9 44.5 14
0.648 65.9 44.5 14
0.644 65.9 44.5 14

Log while blowing HOT air
A/C Valve (N280) A/C Valve (N280) High-Pressure Refrigerant Circuit
Current Duty Cycle Signal (G65) Pressure
A % % bar
0.644 66.7 28 8
0.644 66.3 29 8
0.644 66.7 29 8
0.648 67.1 29 8
0.648 67.9 29.5 8
0.648 67.9 29.5 8
0.652 67.5 29.5 8
0.652 68.3 29.5 8

Also, the evaporator outflow temp sensor seemed to match up with the hot and cold air scenarios above, i.e. I didn't see temps from it to make me think it's throwing off the system.

So I'm confused on how I should move forward. Could the compressor still be bad, or is there something else that could be causing the lack of circuit pressure?



And the obligatory pic of the car.....

[downhillracer]

possibly a bad or sticking expansion valve.

[CardFan]

Well I don't see that there is a separate "expansion valve" in the system. The equivalent seems to be the N280 A/C Compressor Regulator Valve. The N280 valve looks like it's also a part of the compressor and can't be changed separately. Is that correct, or am I missing something?

[old guy]

The A4 doesn't use an expansion valve. It uses a restrictor valve. Locate the high pressure access valve and follow the high pressure line toward the firewall. About an inch or so past the valve you will see a union in the high pressure line. About three inches past the union you will see a small kink or indentation in the high pressure line. That indentation is what holds the restrictor valve in place. To replace it you open the union and remove the restrictor.

When the AC is on and working properly you should be able to feel an extreme temperature difference from one side of the indentation to the other (hot on the side coming from the compressor and cold on the other side). You need to check your high side and low side readings to see if the restrictor is working properly. Checking the high pressure side will require specific high pressure AC test equipment. You can check the low pressure side with a cheep AC gauge (such as what comes with a refill kit). Depending upon the ambient temperature the low side should be anywhere from 25 to 45 psi. If you leave the low pressure gauge attached and turn off the car you should see a slow but steady increase to well over 100psi on the low pressure side as the system equalizes. For safety reasons I would not let the gauge go much over 80-90psi before removing. It is only intended to measure the low pressure side when the AC is operating. If the low pressure side equalizes below 90 psi you probably just need to add a little R-34a.

[rbnarcos]

My car does the same thing it seems that it blows hot air when it's really hot outside. Then when the temp. cool like at night time it blows cold again. Hope you figure it out, please post what you find out so I have a route to try on my car.

[diagnosticator]

The N280 is replaceable, but I don't think that is the problem. Besides the advice "old guy" provided, the temp control air flap servo motor may not be working right. You can reset all of the servomotor position adaptations by opening mb 001 and entering Basic Settings Mode. Wait for all the servo position values to settle at 000 then close the controller. This often remedies the temp control issues, at least temporally. Record and clear any DTCs in the controller.

[CardFan]

Thanks for the tips guys. I didn't get a chance to mess with it this past weekend, and the car's in the shop this week for some body work, so I'll post up my findings this weekend.

Also, old guy, just curious what the difference is between checking the low side pressure with the car running and without it running as suggested? Does checking it when it's not running provide a more accurate result?

[old guy]

Also, old guy, just curious what the difference is between checking the low side pressure with the car running and without it running as suggested? Does checking it when it's not running provide a more accurate result?

No it does not. And any AC tech reading this thread is already cringing at my advice. for the AC to work properly you need a pressure differential between the high side and low side. That differential changes considerably depending upon the ambient temperature. Most guys don't have access to AC testing equipment but they can easily get their hands on a low pressure gauge as they are available with most any AC recharge kit. The low pressure gauge was not meant to be used on the high pressure side, and in fact the high pressure access valve is a different size so you cannot inadvertently hook it up wrong.

When the AC is operating you can see high pressure readings over 300psi on the high side so it isn't something you want to mess with without the right equipment. When you turn the AC off the pressure will equalize throughout the system and the low side will continue to increase as the pressure equalizes. It won't equalize anywhere near 300psi since it isn't being compressed but it can go well over 100psi depending upon the ambient air temp. That is why I recommended watching the gauge closely and remove it if it gets near the limit of the gauge (typically between 70-80psi). If the system equalizes somewhere below 80-90psi that is a pretty good indication that you are low on freon. This procedure should by no means be considered an acceptable replacement for a proper high/low pressure evaluation but it can help in figuring out why your system isn't cooling properly.

Disclaimer: Do this at your own risk! "For off-road use only"

[Charlescrs6]

Good to see you got the car man. I'm having some AC issues myself now.. Black USP looks great

[CardFan]

Good to see you got the car man. I'm having some AC issues myself now.. Black USP looks great

Yeah I did. I'll probably regret it but oh well. Car was in a little worse shape than they described (imagine that), but I got it anyway. Had some hood damage they didn't disclose over the phone, so I'm getting that and the bumper fixed, then getting the wheels fixed next week (lots of curb rash). And they informed me of the a/c issue once I got there. Anyway, thanks for your help with the process. I saw your a/c thread. Sounds like a vag-com would really help if you can get your hands on one. Good luck figuring it out.

[1BAD4DR]

Im having very similar issues with my A/C. Here is the deal:
I can make it work anytime I want, I drive off but i push the gas all the way to the floor for a quick few seconds and cold air is now coming out.
I get to a light and its a 50/50 chance warm air starts to come out.

So all I really need to do is drive off and smash on the pedal for a quick sec. and I got A/C.....

Would this be mean my compressor is done with?

OP try this it works for me.

[CardFan]

Im having very similar issues with my A/C. Here is the deal:
I can make it work anytime I want, I drive off but i push the gas all the way to the floor for a quick few seconds and cold air is now coming out.
I get to a light and its a 50/50 chance warm air starts to come out.

So all I really need to do is drive off and smash on the pedal for a quick sec. and I got A/C.....

Would this be mean my compressor is done with?

OP try this it works for me.

My problem is actually slightly different. When the a/c blows cold, it stays cold (doesn't go back and forth with hot also). Sometimes when I drive it the air either takes 10 plus minutes to get cold or never gets cold. I'll try your trick though if I get a chance.

[CardFan]

1BAD4DR, I tried your suggestion and it worked for me as well, at least most of the time.

I finally got a chance to hook up a cheap gauge to the low pressure side. I ended up busting 2 different gauges in the process. If I hook a gauge up while the system is blowing hot air, the gauge (which only read to 80 psi), pegged the needle on the wrong side of the zero stop and never moved again. I didn't know this is what happened the first time. I got a second gauge and made sure the system was blowing cold air before I connected it. With the car still running, the gauge showed pressure within the correct range (per the gauge/bottle info). I turned the car off and disconnected the gauge when it started to go past 80 psi (per old guy's suggestion). I started the car again, made sure cold air was blowing, then hooked up the gauge again, and the needle shot around to the zero stop. I checked the vents and hot air was coming out.

So it doesn't seem like I'm low on R134.

Next I did the reset is basic settings mode that diagnosticator suggested. I've driven the car once since doing this, and system blew slightly cool air (didn't get as cold as it was). Obviously I need to give it more time to see what the reset does, but since it's not blowing as cold now I'm guessing I should add some more R134 (some did leak out while connecting/disconnecting)? Or could that be more of a product of the reset? I need to get a hold of a third gauge if I need to read any more pressures lol. Any suggestions on how I should move forward?

[fastfastcorrado]

CardFan:

I have a very similar issue with mine. If you don't mind let me tell you what happened.

This winter someone in a pickup truck lost their crock pot on the interstate and it flew straight into my lower grill. No joke..
The body shop replaced the condenser with an aftermarket as I could not get my insurance to put an OEM in.

Now when the car is at operating temperature and at idle (760 to 780 RPM's) and I turn on the A/C, I do not get any cold air until I blip the throttle to raise RPM's to about 1200.
After that the cold air starts to flow. And it will continue as long as I don't turn the A/C off again.. I can let it sit there at idle for 20 minutes and get freezing cold air.

If I start a cold car and the idle starts up at 1100 RPM's or so then the cold air flows immediately.. if when the RPM's then drop to 780 or so and I turn the A/C off and on again then I don't get any cold air until I rev the engine slightly.

I took the car back to another location of the same body shop chain to have the A/C refrigerant checked and they told me it was about 3 oz light and topped it off but still have the same behavior. My local Audi Service advisor told me it is the compressor but I am not so sure since it stays cold even at idle for as long as I want it to.. I don't have to keep the RPM's up.

So it sounds to me like we have the same issue.. is there any chance there has been A/C some components replaced? I can tell you when I turn on the A/C the compressor does kick on but the circuit pressure does not raise from 5 bar... as soon as I tap the pedal to about 1200 RPM's it will climb up to 10 or 11 bar.. ? I did all the same things as you and I was really hoping to post something that would help you resolve this but I'm afraid I'm stuck where you are.

[lyleswk]

CardFan:
This winter someone in a pickup truck lost their crock pot on the interstate and it flew straight into my lower grill. No joke.

Sorry for the damage to the car, but that is hilarious. I can just hear the conversation with the insurance company...

[CardFan]

fastfastcorrado: I just picked the car up a month ago, but oddly enough it does have some damage to the condenser. The condenser is pushed in a bit at the upper half, but just ever so slightly. The hood was also slightly pushed in, and the grill looks like it had been replaced. I'm not sure how the condenser damage could be related though, but I have no idea at this point. Btw, nice choice for your second ride. :) I need to get mine out of the garage, but this Audi's keeping me to busy to touch it lol.

[fastfastcorrado]

CardFan:

I don't know if the condenser has anything to do with it either but it's the only thing that was replaced and up until then the A/C worked flawless.. I'm talking with my insurance company about putting an OEM on since they said they warranty all parts.. Ah yes.. the SLC.. I love it still. I am the second owner and bought it in 1995. If it would stop raining I'd drive it more.. it sounds crazy but I think since I've owned it it has only seen rain a dozen times or so and it has never ever been in the snow. I see you have a G60, so you probably have the same love/hate relationship with your car? :)

If and when I can get the OEM condenser and it fixes things I'll post up and let you know..

Sorry for the damage to the car, but that is hilarious. I can just hear the conversation with the insurance company...

I laugh about it too but not when it happened.. I actually searched Craigslist hoping someone would post looking for a lost crock pot! One thing with talking to the insurance company.. if they ask if it was moving or sitting in the middle of the road tell them it was moving.. makes a difference of whether it's a collision or comprehensive claim and I originally told them it was sitting in my lane..so I have to pay my deductible. I first told the claims rep it was a turkey fryer, I think they thought I was nuts! At least I got a new bumper cover so I don't have those front plate holes anymore.

[CardFan]

Yep, definitely a love hate relationship with the G60. :)

Thinking about this more, is it possible that the systems were just not seeviced properly after the work was done? I have no idea if work was even done on my ac system, but it's possible.

[old guy]

From what you described it sounds like you have adequate R134a in the system. For some reason your compressor is kicking out. If the compressor isn’t running the pressure will equalize in the closed system until the high side/low side read the same. When the compressor kicks on it pulls down the low side and increases the high side.

Another possibility is that you have too much R134a in the system. When this happens the compressor can’t generate the pressure differential necessary for the system to operate properly. However, unless this problem started right after the system was serviced I doubt that is the issue in your case.

Sounds like it’s time to take it to a competent AC shop.

Good luck!

[fastfastcorrado]

At least for me, I think you're right. I'll probably have it looked at soon, i just want to see what my insurance co. will move forward if there is an issue with the condenser or maybe something else that should have been replaced. The car sat for about 10 to 12 days this winter with a hole in the condenser - I'm not sure if any moisture of some dirt got in the system. After work today the A/C acted the same, I had to rev it to get cold air.. but everytime since for the rest of the day, it kicked right on. I even let it sit for 3 hours.

Maybe it is overcharged... mine worked fine before the crock pot incident. And whenever I turn on the A/C, the compressor always turns on.. just the pressure doesn't build up until a slight increase in RPM.

thanks for all the advice everyone.. If it keeps acting up I'll take it in.

[fastfastcorrado]

Here's an update. I took my car to the dealership today to have them look at the A/C issues. I was told by the service manager that they were able to duplicate the issues I am experiencing and while it is strange, the system is working as it is supposed to. They contacted someone at Audi (NA?) and was told that most likely the A/C controls are not seeing the proper 13.7 volts needed to activate the system.. ? That is the design intent of the A/C so that you do not draw too much power if it is not sufficient. So when the RPM's are quickly increased the voltage requirement is met. They said the alternator is supplying the proper voltage so the first impression is that the my non-OEM battery is possibly to blame... can that be true? The battery is only 5 months old, I got it from Batteries Plus and it is the from what I can tell, the same battery that is in my Passat Wagon with a 2.8L V6...and that A/C works perfect.

I did notice that when I don't get cold air, the electric fans do not kick on at full speed but are at low speed.. the compressor is engaged, the RPM increase request is made, the N280 valve is energized but the G65 high pressure switch does not change.. just like the logs from the OP.... So I'm starting to think that it might either be the high pressure switch, the fan control module (J293?) and that the dealership didn't do much more than move my car, see the A/C works when you rev the engine once and wash it...

I do not have any codes at all when I scan the car so I'm still puzzled. I actually charged the battery for 20 minutes and then started the car with the charger still on... still no cold air so I don't think it is the battery... does anyone have any ideas? thanks.

[road_tripper]

my system was overcharged with r134a; it worked OK until the temps here started crossing 100, and then the dreaded ECON came on and hot air out the vents.

Once the outside air temp dropped (morning or evening), the AC worked. No codes were ever thrown.

Removing some of the r134 from the system now has it working "OK", but the passenger vents never get as cold as the driver vents (about 10 degree difference).

[CardFan]

fastfastcorrado: I have a Monday appt with a local shop for my car, so I'll let you know what they say. The PO of my car said they weren't having ac issues when they traded it in. When I bought it the battery was dying, and I put a duralast in. So while I'm not sure that the battery is the issue, I have recently changed mine as well.

[fastfastcorrado]

CardFan - I changed mine this past winter, just before the condensor was replaced so there are two variables at work.. about the same for you as it sounds.
Do you observe the same characteristics as I explained above? If it is the battery that would be something!

[CardFan]

I believe mine are similar, but not exactly the same. When I'm getting hot air, sometimes a quick rev or two does the trick, and sometimes it doesn't. When it doesn't, revving to close to redline usually does the trick, but even that doesn't always work. I haven't paid attention to the fans, but will check them out to see what they are doing. For the voltage issue, I wonder if there is something we can log or measure with a meter? Guess I may try to look at that more tonight.

[fastfastcorrado]

I've noticed that sometimes I need to get the RPM's up above 2000 to 3000 sometimes but that has been as bad as it's gotten.. I don't know if there is anything in the A/C control we can look at with the VAG or not.. I might get a muti-meter and test the alternator output and battery output when the A/C is on but not cooling. The weather is nice this week so I'm driving the Corrado everywhere for now so maybe this weekend I can do that!

[CardFan]

Well searched through the Bentley last night but no luck finding anything useful. My car still is not throwing any codes related to this, and I checked blocks 26 and 27 from the control module (they are empty). I am definitely getting the feeling that whoever figures this out is really going to have to know the system operation. The Bentley didn't seem to have the necessary level of detail on how it works (or I missed it).

[fastfastcorrado]

I found in ElsaWin under the heating and air conditioning:
Always perform basic setting (function “04”) after replacing an air conditioner component. Then read out the DTC memory, eliminate any faults displayed and erase the DTC memory (functions “02” and “05”).

It doesn't really say what function "04" is, I haven't check basic settings to see if there is something there with a VAG but I'm curious so I may before the week is over.

Same for me though, no codes.. when you get a chance observe your fans and see if they kick on low speed when the A/C is on but you are only getting ambient air temps... then when you get the cold air to start see if the fans have activated at high speed..

[diagnosticator]

Function 04 is when the Basic Settings is run with the measuring block 001 showing, it resets all of the servo motor position values in the HVAC controller. Once started, allow all the values to go to 000 before exiting Basic Settings mode.

Another possible obscure cause of the delayed A/C cooling that resolves with increased RPM, is if the rubber compressor drive blocks in the drive pulley are slipping. Since the compressor is driven 100% of the time with the engine running, and is internally controlled between 0% and 100% output, if the rubber drive blocks in the compressor drive pulley are loose and slipping, that can cause the delayed output and the symptoms described with the cold air output only after blipping the throttle.

[CardFan]

I ran the basic settings reset after I checked the r134 level. I'll try to check out the fans as well. With my car, I've found that as long as I leave the ac on I almost always get cold air without having to rev the engine. I started leaving it on all the time after I ran the basic settings reset, so I don't know for sure but the reset may have helped. Guess I'll have to induce some warm air again. :)

[diagnosticator]

See my edit above....

[fastfastcorrado]

thanks diagnosticator, I did already tried to reset the servo motors, a few times actually, once with the A/C on blowing cold air.
The Audi dealership told me they don't suspect the compressor to have any issues, I don't know how much they dove into looking at it though.

Like CardFan, if I leave the A/C on I always get cold air, and most times, if it is left on when I turn the car of, the next time I start the car it blows cold right away.. it only seems to occur when I turn on the A/C at idle.
If it's just some strange voltage/battery issue I can live with that, but my CPO expires in 5 months so if this is something that will become major I need to nip it now..
thanks for all the input.

[diagnosticator]

Yeah, the compressor itself is not likely the problem, but the compressor drive pulley rubber blocks are not often considered so that should be ruled out too. Since it is consistant with the symptoms requiring the engine idle speed blip to get the cold air to start.

[CardFan]

Dropped my car off at the shop today for Monday's appointment (since I'm headed out of town for the week) and they already looked at it today. They said the r134 level was only about half of what it should have been, and they evacuated and refilled. And of course it worked fine for them after that. I won't be back until a week from tomorrow, so it'll be a while until I can report back on whether or not it's a successful fix. I also forgot to ask whether or not they tested/found any leaks when they called, so I'm waiting to hear back on that as well (they still have the car). Crossing my fingers......

[acesmeg]

I've had the exact same problem for a while on a B5 A4. After idling at a stop light for a few seconds I can feel the change as the compressor turns off, then the vents start spewing warm and humid air. I can blip the throttle over 1k and the A/C will come back on. Sometimes it will come back on its own if I wait a couple minutes while idling.

Topping up the r134 level did not help.

I never use the auto climate control mode, only manual. I wonder if that's confusing the controller somehow.

[fastfastcorrado]

I think the problem CardFan and i am experiencing is different in that once the a/c systems starts generating cold air it does not stop unless we turn the a/c off. So at idle during a traffic stop or similar the system still blows out cold air.

I took mine out today and since it was in the low 90's I just set it to auto and drove, it worked flawlessly and was very cold, but it was too hot today to play around with the controls.

They said the r134 level was only about half of what it should have been, and they evacuated and refilled.

If that turns out to be it and I hope it is, then I may have to bring it back and ask the dealership to check the level this time.. so far I have only had the body shop that did the front end work check it.

[CardFan]

Well, I've only driven the car a couple times, but so far so good. I had one scenario where I could pretty consistently get it to blow hot air before, when it starting it up after work (hottest part of the day) and the system had been off the last time the car ran (i.e. didn't run the a/c driving to work in the morning). Basically every time I did this before it would blow hot air until I revved it a few times. I did it both yesterday and today (post shop visit), and it started blowing cold air very quickly both days when I turned the a/c on. So I'll give it some more time before I declare it a fix for sure, but it looks like it worked. Also, the shop did test for leaks and found no problems.

[03 A4 Avant]

so was topping up the refrigerant the fix?

[CardFan]

so was topping up the refrigerant the fix?

Evac and refill appears to be the fix. The r134 level was half of what it should have been. System appeared to be fine though using a cheap refill kit gauge (even though level was low).

[fastfastcorrado]

I'm glad to hear a recharge fixed your issue.. mine doesn't look to be that easy. I had the system checked again and it was determined to be full. Two things I now wonder about. First, I put a multimeter to the battery to check that and it was fully charged at 12.7 volts, when I start the car to check the alternator output it max's out at 13.8 volts. Now if I turn on the A/C, stereo, rear defrosters etc the alternator output drops to 12.4 to 12.6, so it appears to struggle to keep up with the demand. My wife's Passat will hit 14.2 volts under the same conditions. Maybe this is where the lack of 13.7 volts that the module is looking for is coming from - per the dealerships feedback.

Diagnosticator - you mentioned the rubber blocks of the drive pulley, I pay attention for that now and when the cold air and pressure circuit build, I hear a squeak or rubber slipping sound for about the first 2 or 3 seconds after turning on the A/C.. if I don't hear that sound the cold air requires the throttle blip.. is that sound normal at first? Maybe I need the compressor looked at also?

thanks