Thread: No Air Con

My Air Con is playing up yet again. I have plugged into VagCom and get the fault :

00792 - A/C Pressure Switch (F129)
31-10 - Open or Short to Ground - Intermittent

When I drive the car with air con on, it will switch to ECON, I then press AUTO and it will switch the Air Con back on for a few mins then will go back onto ECON. It does this a couple of times then you can't take it of off ECON until you have erased the fault.

I ran a couple of tests on the pressure switch and the wiring using a jumper to see if the conpressor switches on (which it did). I thought it was the pressure switch that is faulty, so I brought another one. Guess what still didn't work. The only other reason is low on gas, so got it re-charged. Excellant it started working, well for a day. I have now got the same problem.

Has any body got any ideas on what I should do next???

(A6 2.5tdi 1999)

Any advice would be much appreciated.

There should be 2 pressure switches mate. 1 high pressure and 1 low. You may have changed the wrong one. Or it could be the wiring itself.

One thing you could try: disconnect that sensor and then see what fault is flagged. If it's F129 then you at least know it's the right one.

I thought there is only 1 pressure switch on this year car, thought it was changed around 2001.

Everything i have seen suggests there is only one. The switch i changed is the F129

I don't know about the car specifically mate, but most systems have 2 that's all.

It must be the wiring then.

Thanks for your reply. Have checked to see if I one or two pressure switch's and I have only got the one.

I have just sucked the gas back out of the system. I re-charged it a couple of days ago with 850g, I only managed to pull 720g out today. It looks like I have got a bit of a leak.

I am going to add the dye next week and re-charge the system to see if i can locate it.

Do you have any idea of where the most common area for leaks is?

What recovery kit are you using mate?

It's normally joints that go isn't it? Or a stone could have chipped the condensor.

I have no real practical experience by the way mate. I did a course 2 years ago and have not touched it since.

I just help out with the theory side as much as I can.

I don't know what the machine is, it is the proper one. I just borrow it from a mates garage.

I will have to insert the dye and spend some time in hunting for the leak.

Many Thanks for your help

Hello again, i added the dye to the air con and drove it for a few days. I
can't find any signs of leakage.
When I recovered the gas out of the system i was down by another 200 grams, and getting the pressure switch fault. (There is only one, have checked with Audi again)

I thought i might have a leak then in either the evap or the pump. If there is a leak in the pump then it shoud still hold the gas and i should of seen the dye anyway. I have put a camera down so i can see the evaporater and i can;t see any signs of leaking, but i wouldn't be 100% certain as you are only limited to what you can see.

I am now wondering if the dryer needs changing as i have been told this could be the poroblem.

Has anybody got any ideas??? any suggestions would be great, it's driving me mad now!!!!!

Cheers

Check the 30amp fuses located in tray ontop of the battery. Mine stopped working and it was one of these.

It has to be a leak mate, it's just you can't find it. A sniffer may be a better way of finding it though. Or look for drips on the floor to help identify a rough area.

Fuse's are Ok.

I have changed the dryer now and pulled a vac, over about 40 mins it had a very slight leak. I am going to re-charge it later and look around for a leak, I have now got a UV light so I might / hopefully find the leak.

Cheers for the comments

I didn't realise you didn't have the UV lamp already. It can be pretty had to see the dye without. I'm sure you'll find it though.

The next problem is the repair!

We lost the UV light. A new one has finally come in. Will post the find's hopefully later today.

I changed the Dryer and its seem's to be holding the pressure now, I can't find any leaks using the UV lights.

I am still getting the fault - A/C pressure switch. I have changed the control unit with another one that has the same part number. When it is on the unit keeps flashing for a couple of mins then stops but when the engine is restarted it will flash again (I didn't think it needs coding), also changed the relay for the A/C and the pressure switch again.

None of these things have fixed the problem. I have noticed that when i put my foot onto the gas pedal the relay click's on and off, (switchs the compressor on and off)

Does anybo dy have any ideas ?? we seem to off come to a dead end!!

Hmm really strange!

try reading the pressure switch when you give it the revs. I bet it goes within limits.

Just an update. Pulled up outside the garage yesterday to check air con. I had to move the car so i started it and then - engine was very loud and backfiring through the air intake. Turns out only 1 of the valves was opening, the little rods that open the valves (something to do with the cam shaft) had broken off. Have always thought the car should be quicker than it was, now i know why.

I am hoping that when the car is fixed it will sort the air con problem out. Reason is that when you boot the car it should switch the compressor off to give the car max power. If only one valve opening properly then the engine thinks it is being booted all of the time and keeps switching the compressor on and off.

Well this is what i hope anyway, shall put an update up in 3 - 4 days when car fixed.

Well i have now had all the work done on the car and it sounds much better. Whilst changing the belts we noticed that the air con tensioner was not set properly. We corrected this but the fault has come back again.

I spoke to Audi Master Tech's and they said it sounds like there is a blockage in the restrictor!! They have told me to hook it up to the air con machine and look at the pressures.

I am planning on doing this soon, has anybody heard of this??

What's a restrictor???

That's what I said as well. He told me it is located just before where the low and high pressure goes from the engine bay into the car.

It improves the circulation of the gas (or something like this), apparently my car has got one but this wasn't installed in cars from around mid 1999.

As I have a restrictor I have to add 850g + 50g, the same car without a restrictor add around 650g of gas.

Tried google and still have no idea what he's talking about. If you put any restriction in the system it affects the working of the system!

Plus the "point at which the low and high pressure enter the car" from the bay is rubbish. The evaporator section is all on the low pressure side.

I am sure that I have seen it mentioned on ElsaWin, I will try and find it and post what it says.

I have looked at ElsaWin and the restrictor is between the evap and the service points.

I have attached the removal instructions, still trying to locate what it does.

Repairing refrigerant circuit
Removing and installing restrictor






Removing

• ‒ Drain the refrigerant circuit

=> Air Conditioner with Refrigerant R134a

• ‒ Remove plenum chamber cover.
• ‒ → Separate the refrigerant pipe at the threaded connection -A- (tightening torque 15 Nm).
• ‒ Pull restrictor -B- out of refrigerant pipe using, for example, pointed pliers.

Installing
Installation is performed in reverse order, note the instructions and the tightening torque (see description of removal).
Notes:

• ◆ When replacing a restrictor note its colour and fit only one of the same colour (e.g. white or yellow), the different-coloured restrictors have different bore diameters => Page 87-263 and

=> Parts List
=> Air Conditioner with Refrigerant R134a

• ◆ → Ensure that restrictor -B- is inserted on the correct side (arrow on restrictor points towards evaporator).
• ◆ Check to see if O-ring seal -C- (7.5 mm; 1.5 mm) is correctly positioned in groove.
• ◆ Renew O-ring seal -D- (10.8 mm; 1.8 mm).

=> Parts List

• ◆ Close the open pipe connections.
• ◆ Restrictor must be positioned in refrigerant pipe without play
• ◆ Before fitting restrictor, apply a small amount of refrigerant oil to the O-ring seal in the restrictor.
• ◆ Install the refrigerant lines with no tension.
• ◆ Check the location of the refrigerant pipes to the evaporator, they must not be in contact with other components.

• ‒ Reinstall the remainder of the components that have been removed.
• ‒ Evacuating and charging the refrigerant circuit.

=> Air Conditioner with Refrigerant R134a

Well the positioning of it there sounds more logical but I still don't know what it does. They're not referring to the TEV are they? That's a restrictor by definition but it's normally called a TEV.

Any pictures?

They might be, never heard it called that before!! When I get a few mins I shall have another look at ElsaWin

Restrictor The restrictor forms a constriction. This constriction restricts the flow, thus separating the refrigerant circuit into high and low pressure ends. Upstream of the restrictor, the refrigerant is warm due to the high pressure. Downstream of the restrictor, the refrigerant is cold due to the low pressure. A strainer is provided upstream of the constriction to trap dirt. The strainer downstream of the constriction is designed to atomise the refrigerant before it enters the evaporator.

Yeah that's the TEV mate. Not normally the thing to break though, it's just a simple valve.

They were convinced that this is blocked and this is the cause of my problem.

Hopefully will drain the system Monday and strip it and look.

Fingers crossed it's sorted soon mate.

just a suggestion - you mentioned that you think you have a leak, but you cant find any uv traces. Is the condensor at the front of the car i.e in front of the water cooling radiator?? If it is, it could have been damaged by a stone and have a small leak in it. You could whip the font bumper off and have a good look at it with the UV lamp or get a hold of an electronic leak detector to help find the leak.

colinra,

We had the bumper of couple of weeks ago i did have a good look but couldn't find any traces of a leak.


Hopefully going to check this restrictor in a few days.

Staz,

whilst doing another output test on the Auto Hvac system I noticed the fan that keeps the air con rad cool was not working. I have brought a new one plugged it in and it works so I am going to fit it on Monday.

I done a bit of research on internet and found out that if the fan doesn't work then the pressure gets too high and switches the compressor off. (sounds like my problem)

Hopefully this was the cause of the problem the whole time!!

Really?! lol yeah mate that'll be it. I have to say I would have thought it would flag a fault code up though.

If the condenser fan stops working the refrigerant doesn't get condensed back into a liquid and then there's a build up of pressure until the cut-out goes.

Well hopefully i have got there in the end. Will be changing on Monday. Will post the results in a couple of weeks. Cheers for your help.

No worries mate. I hope it's sorted finally!

My high pressure fault was with the orofice tube/restrictor. It was clogged with what looked like grinding paste as the compressor had previously failed and a washer had broken up and got itself ground up and mixed with the oil.

I cleared it and had aircon for a short time, Tho i suspect i didnt get as much crap out as i should have as it's gone again.
Is there anyway to flush the system to remove all the crap in the system prior to refilling?

Originally Posted by Byzan A4

My high pressure fault was with the orofice tube/restrictor. It was clogged with what looked like grinding paste as the compressor had previously failed and a washer had broken up and got itself ground up and mixed with the oil.

I cleared it and had aircon for a short time, Tho i suspect i didnt get as much crap out as i should have as it's gone again.
Is there anyway to flush the system to remove all the crap in the system prior to refilling?

Yes mate. Need to get them to do a full recovery and then deep vacuum. It may take more than one go though if it's that clogged. It'd probably be easier just to replace the TEV.

Originally Posted by danny104

When I drive the car with air con on, it will switch to ECON, I then press AUTO and it will switch the Air Con back on for a few mins then will go back onto ECON. It does this a couple of times then you can't take it of off ECON until you have erased the fault.

Originally Posted by danny104

Staz,

whilst doing another output test on the Auto Hvac system I noticed the fan that keeps the air con rad cool was not working. I have brought a new one plugged it in and it works so I am going to fit it on Monday.

I done a bit of research on internet and found out that if the fan doesn't work then the pressure gets too high and switches the compressor off. (sounds like my problem)

TBH econ mode switching is sure fire wire to check the fans as 1st call as its the most well known reason for econ to keep coming on even when turning aircon on, there's been number of posts/threads mentioned about econ mode constantly on even when ac turned on & alot have been down to fan fuses & fan faults. TBH at least its led you to the other issues & saved the car, lol.

Another thing which happened to my S3 99 was a think was pressure switch/valve went so I had no cool air, this was located behind the fire wall in engine bay in front of the passenger side on a silver pipe, once we changed this it allow to work again, just a side not to add if hasnt been resolved & worth checking, £30 odd part & easily changed.

Staz,

I have fitted the fan. I had to do some things under the car and guess what I noticed.

A nice green (from air con dye) compressor. I think I have just found the leak. Hopefully will be fitting it in the next couple of days.

The dye wasn't there the other day though. Never minds least I have found it at last.

Is the fan working ok? I hope the day you fix it is sunny mate

Not sure yet, as I am in the process of replacing a dammaged bracket for the cam belt, so the car is in bits again, another expence.