A5 Air Conditioning thread

xracerbob

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As I intruded on the very informative A4 A/C thread with my questions it was suggested we start one for the A5's. Basically, my AC no workie and I'm trying to figure out why. It appears some of the issues are common to both series of cars (fans), but a lot has changed.

So to review- here are my symptoms- Neither fans operate when tha AC is turned on. I checked the common brush problem with the drivers side and all appears to be good. The clutch is always engaged (apparently this is how this series of compressor works). System pressure was checked- 52 psi low side, same high side with AC on and 45 degree ambient temp. All fuses are good.
 

DanG144

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In this series of AC systems, VCDS is one of your best friends.

By doing the Engine module OUTPUT TESTS, you can check your fan operation.

By going into the HVAC module you can get pressure and control current data, get all kinds of sensor data, and probably do some output tests, too. ( I am guessing at the output tests, as I do not have a model like this to work with.)

This series of compressors has a fairly high failure rate on the electrically controlled compressor displacement control valve.

According to VW you can only replace the compressor, but this might not be the final word.
 

jetta 97

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xracerbob said:
As I intruded on the very informative A4 A/C thread with my questions it was suggested we start one for the A5's. Basically, my AC no workie and I'm trying to figure out why. It appears some of the issues are common to both series of cars (fans), but a lot has changed.

So to review- here are my symptoms- Neither fans operate when tha AC is turned on. I checked the common brush problem with the drivers side and all appears to be good. The clutch is always engaged (apparently this is how this series of compressor works). System pressure was checked- 52 psi low side, same high side with AC on and 45 degree ambient temp. All fuses are good.
It is to cold to turn a/c. This compressors works different way then old ones. When high pressure start raising then your fan should start working. When high pressure is low fan should not start working.
Wait till get more hotter and check it ,like on 85 F.
 

Mach1

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Defroster should work down to below freezing..

AC compressor will come on when the defroster is energized..


Like Dan said..vag-com is your friend..
 

DanG144

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Yeah, at the temperature and pressures specified the AC should not have been interlocked out by either temperature or pressure interlocks.

You only need to be above about 35 degrees to reach the setpoints, and 40 degrees to be reliably above them.
 

xracerbob

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I found it it wasn't working on a day when it was mid 70's a couple weeks ago. I have tried the defrost setting as well, and no go. Hope to have it on a vag-com tomorrow.
 

oilhammer

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Just to clarify: the A5 has no compressor clutch, they spin all the time. They use a duty-cycled control valve integral with the compressor, controlled by either the fan control module or the ECU (not sure which, Dan will have to chime in here :p ). However, the ECU should have the actual command portion, just the actual load of the circuit is done via the fan control module.

I would see if the ECU is getting an A/C request in the data blocks first.
 

DanG144

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oilhammer said:
Just to clarify: the A5 has no compressor clutch, they spin all the time. They use a duty-cycled control valve integral with the compressor, controlled by either the fan control module or the ECU (not sure which, Dan will have to chime in here :p ). However, the ECU should have the actual command portion, just the actual load of the circuit is done via the fan control module.

I would see if the ECU is getting an A/C request in the data blocks first.
Dead on, Brian.

The high pressure sensor feeds the AC Control Module a pressure signal. The ACCM turns that into a demand current that is sent to the AC Compressor regulator valve. This controls the displacement of the compressor, which in turn raises or lowers discharge pressure.

The Engine Control Module/Unit (ECU) just gives permission to the ACCM to load the compressor at all (this is one of the VCDS OUTPUT TESTs). The ACCM decides how much to load it.

The fans are controlled from the ECU. A signal is sent to the controller built into the large fan. This controls the speed of both fans. The ECU will turn on the fans based on signals from the coolant temperature sensor or the sensor on the radiator, or a request from the ACCM. The Bentley calls this system the "Coolant Fan, 2 speed wth thermal switch" but the speeds are really infinitely variable over a range. (Testing this is another VCDS OUTPUT test.)

Normally you see the fans kick on based on AC system demand after about a minute of operation. To test these fans without VCDS, you need to run the AC system at an idle for about 2 minutes. They should turn on by then. The fans should run together, and at the same speed.
 

oilhammer

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Excellent post, thanks! Interesting to fill in the gaps in my working knowledge of the A5's HVAC/cooling fan system.

So ECU requests "air-con", and the ACCM determines the compressor loading via the PWM valve in the compressor, but beyond an on/off request, the ECU is essentially out of the loop on compressor control. I'm sure it probably has some full-load request to the ACCM to back compressor loading off under certain conditions.

Curious, does the ACCM and ECU communicate on the CAN bus or do they have their own dedicated line?
 

DanG144

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It looks as if the ACCM and the ECU communicate over the CAN bus.

If by PWM valve you mean a Pulse Width Modulation controlled valve, I think that is wrong. I think the valve is simply sent a current/voltage not a pulse width signal. It has been a while but I think the VCDS gives you the control current in a measuring block.

The pressure signal from the high pressure sensor, on the other hand is a PWM or duty cycle signal.

The ECU does all its signalling to the fan control module via dedicated lines, not over the can bus.

I would expect the ECU to forbid compressor loading in high load situations, delay after starting the engine, and probably in overtemperature situations as well. But this is only conjecture.
 

xracerbob

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OK- Got my car scanned and here is what came up:

00898-Control circuit a/c compressor
010-open circuit short to plus

So we poke around a little and the connection on the back of the compressor is there but with a slight wiggle comes off in Jason's hand. So no signal is getting to the compressor to do anything. I check with VW and this is an integral part of the compressor and not replaceable seperately. I check with aftermarket A/C parts sellers and they say the same. So its a new compressor for me. I decided to buy a new oem instead of rebuilt ($437.00). The rebuilt was only $75 less, so I decided the difference wasn't worth it. It will be here Friday.

Thanks to Jason (TDIJetta99) for his help and the scan.
 

oilhammer

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Great, glad you found it. Just goes to show once again the best first step in diagnosis of any system on a modern automobile is to use a scan tool and exploit the car's own OBD. That's what it is there for! :)
 

funoutdoors121

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DanG144 said:
In this series of AC systems, VCDS is one of your best friends.
By doing the Engine module OUTPUT TESTS, you can check your fan operation.
By going into the HVAC module you can get pressure and control current data, get all kinds of sensor data, and probably do some output tests, too.
Dan, any idea how to use VCDS to get the freon pressure?
I went into the HVAC module, and in Advanced Measuring Values, there are many parameters that can be looked at but I couldn't find the refrigerant pressure.
(I'd like to find out if my system was fully recharged when the condenser was replaced)
Thanks!
 

DanG144

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Can you go back into VCDS, HVAC module, measuring blocks, then look in the upper left corner of the screen and tell me the name of your label file.

Then I can help you out. The pressure is going to read out in BAR (almost certainly) so you have to multiply by 14.5 to get psig.

Remember you cannot tell how much refrigerant is in the system by the pressure alone. But you can tell if it has some liquid refrigerant in it.

Dan
 

funoutdoors121

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Label file is: 1K0-820-047.lbl
I am not sure on how to use the measuring blocks function, just trying to learn how to use VCDS.
Thanks for any info/tips!
 

xracerbob

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When we were in the HVAC module the system pressure came right up and verified the earlier test we did with the gauge system. It is indicated in bar and not psi.
 

DanG144

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Go to VCDS, HVAC, MEASURING BLOCKS, and put the number 3 in the pull down box on the left. Then hit GO. It will read some parameters for you.

This is measuring block 3, and it has 4 fields,
the first is coolant (I would have chosen "refrigerant") pressure in BARS.
The second is fan speed actual,
the third is fan speed specified,
and the 4th is engine speed.

This group makes sense as the pressure is what determines fan speed (or at least is one of the fan speed inputs.)

About 14.5 psi per bar. You can find out what it should be with the engine off and everything static (say off for 30 minutes) for the given ambient temperature. This is best measured before running the car for the day, otherwise the temperature is impacted by the heat of the radiator and other vehicle components.
Refrigerant pressure vs temperature if you use this table, then you can skip the multiplication.

If I recall correctly pressure will be up near 200 psi (call it 12 to 15 bar) when the system is running well and is loaded. This does vary with ambient temperature (lower when colder, higher when hotter.)
Check this pressure with the cabin fan on fast speed, and the engine at 1500 rpm, after 2 to 5 minutes read the parameters.

Put the air to the center dash outlets and expect the temperature to be about 35-45 F or so.

(Please do not take this as gospel, this is best effort at memory and using the A4 Bentley manual - the A5 manual gives no expected readings.)
 

funoutdoors121

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Thanks so much, Dan! Sent you a PM.
OP, sorry for semi-hijacking your thread...debating about posting in the VCDS thread or on this one, but since my issue was an AC question, and the tool to answer it is VCDS, it was a tough call ;)!
TDI Club ROCKS!!!
 

DanG144

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I checked my wife's 2010 Jetta Climatic system tonight.

I could not get measuring blocks to work on the HVAC module. But Advanced Measuring blocks works. I was running 10 to 14 Bar at 1500 rpm, fast cabin air speed, full cool setting.

And it was labeled "refrigerant pressure".
 

socrates42

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does anyone have the VCDS scan tool in the boston area, it would be great to know if it is in fact low refrigerant before going to get it recharged. Also, does anyone know if you can do the recharge yourself? or is it a dealer/shop only thing?
 

oilhammer

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socrates42 said:
does anyone have the VCDS scan tool in the boston area, it would be great to know if it is in fact low refrigerant before going to get it recharged. Also, does anyone know if you can do the recharge yourself? or is it a dealer/shop only thing?
Personally, I think the only PROPER way to charge a modern automotive system is to evacuate whatever is in there, pull a deep vacuum on the system for a minimum of an hour, and recharge with the full amount on the underhood label. "Topping up" is a guessing game at best, given the nature of the system with regards to variation of pressures during operation which is normal.

I know there are plenty of people that use the crappy little tap-can thingies, and I have pity on them. :cool:

But if the system has lost charge anyways, it has a leak that needs to be found and remedied.
 

DanG144

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I agree with oilhammer.

There have been folks on here who have overpressurized their systems by going solid, lifting relief valves, blowing pressure transmitters. And they actually came off lucky. If the relief valve fails, compressors can blow apart causing serious damage.

This is too likely to happen with a single gauge (or no gauge) cheapie filling rig.

Even with a good set of dual gauges (if you do not evacuate the system), you do not know when you have the proper amount of refrigerant in the system, and can still end up overfilled. With the high pressure gauge you can stop before you get abnormal pressures, but having too much freon in the sytem for the amount of oil is sure death for a compressor. It will work great for a while.

But, looking at your system operaton with VCDS before taking it in for maintenance would not hurt anything.
 

birkie

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oilhammer said:
Personally, I think the only PROPER way to charge a modern automotive system is to evacuate whatever is in there, pull a deep vacuum on the system for a minimum of an hour, and recharge with the full amount on the underhood label.
+1
One of by best loading dock finds when I was a student at Cornell was an old laboratory vacuum pump, basically a 1970s version of this. That thing could pull a vacuum down to .0005 mm hg :D. It allowed me to do much of the AC work myself at home when a rock nicked the condenser open on my '98 Jetta. As I learned along the way, a deep vacuum is a must if the system has been exposed to the environment (i.e. moisture)
 

xracerbob

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OK- New compressor installed and I took a shot at recharging my self. I borrowed a good set of the dual gauges and I have a small vacuum pump so I drew a vacuum the best that I could, which was about 15 in/hg. I think you are supposed to go more than that but my pump is probably not up to it. I then put in 1 small can of R-134. I could only get the pressure up to 2.5 bar (70 degrees ambient). It stayed there and I did get cold air from the system. Both fans working!

The new compressor was pre-oiled, you could see quite a bit in one of the ports once I took off the protective cover. I had the PAG oil but didn't add any because none came out when I took the old compressor off.

Question is, should I add a second can to get the pressure up to where it was before which was 3.5 bar at 45 degree ambient?

I really wanted to get this sorted out this weekend since no shops were open.
 
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