FAO Dave N - Audi A4 TDi AC Fault - DL
Dave
I wondered if you'd be kind enough to look over a fault for me on a '98 A4 TDi? Many thanks in advance.

Symptom - No cooling, no cold air delivered to passenger cabin. AC has been strong in operation throughout the cars life. System has been in constant use during the winter. Not sure if the customer is capable of telling if it was indeed working during the colder months...so it could have been down for a while.

System swept, vacuumed and charged, 700g R134a - there was no loss of gas in the first instance.

Compressor clutch activates, engine note loads down slightly (indicating effort is being taken by the compressor). Condensor fan runs.

There is no temperature difference on either the high side or low side pipes - both remain at a static ambient temperature.

I have system pressure/ambient/cabin temperature readings on my desk at work, I'll post them to this thread as soon as I can.

I haven't fault-read the car yet, I know the HEVAC system reports to my scan tool. Might give that a try later today, but I think it's more a/c system related..

I suspect a system blockage or an internal compressor fault - what do you think?
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FAO Dave N - Audi A4 TDi AC Fault - Number_Cruncher
I would look at the restrictor on the inlet of the evaporator. I had one block up on an Astra, with similar symptoms.

Number_Cruncher
FAO Dave N - Audi A4 TDi AC Fault - DL
That did cross my mind.......doesn't look like a nice job to do either!
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FAO Dave N - Audi A4 TDi AC Fault - DL
System pressures/Temperatures as promised


Ambient Temp 15.3

Vent Temp 20.3 (It was hot outside but in a cool workshop)


Pressures recorded with system running....

Low Side - 1.8bar (Flickering slightly)

High Side - 4 bar (Steady)


Pressures recorded with system off....

Low Side - 4.4bar

High Side - 4 bar
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FAO Dave N - Audi A4 TDi AC Fault - Dave N
I reckon the compressors shot, and probably blocked the orifice tube at the inlet to the evaporator. However, normally you would expect to see the low side in a vacuum if there is a block, but with a variable displacement compressor everything gets a bit screwed up.

You only have to lift the battery out to get at the join that contains the orifice tube, although if it's a diesel it's a big battery and you have to remove a few extra bits like the shroud first.

Did you get any gas out of it first off?

If the orifice tube is clear, then make sure it has been put in the right way round. If that's ok, then it's the variable valve in the compressor holding the compressor at minimum displacement. Let us know what you find.

FAO Dave N - Audi A4 TDi AC Fault - Dave N
Sorry, read your post a bit better this time, and can see you got gas out of it.

If the tube is clagged up, then it's compressor, tube, accumulator, and damn good flush time (and keep your fingers crossed).
FAO Dave N - Audi A4 TDi AC Fault - DL
Blimey Dave, thanks for that, it confirms what I was dreading.

I am aware of compressor trouble on these cars - I hope mine stays alive for the forseeable future!

If I see metallic particles at the orifice, blocking it, it is safe enough to assume a compressor failure, yes?

Hmmmmmmm flushing - we've got a flushing kit but never used it, let alone even unpacked it!
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FAO Dave N - Audi A4 TDi AC Fault - Number_Cruncher
I've never had any cause to look closely at the workings of a variable displacement A/C compressor - how is the displacement controlling valve configured?

I suspect that the actuating valve is acted upon by the high pressure gas. As the pressure at the output of the compressor increases, the displacement of the compressor is reduced - the valve restrained by a spring perhaps? Does the displacement of the compressor ever reduce to zero?

Number_Cruncher
FAO Dave N - Audi A4 TDi AC Fault - DL
I was pondering the same - it's over to Dave N!

I think you could be on the right lines, N-C
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FAO Dave N - Audi A4 TDi AC Fault - Dave N
Any metal at the orifice tube means the compressors toast I'm afraid. I guess it's possible for just a few bits to get caught that have been floating around from manufacture, but once it starts to get obstructed, then the lack of oil flow back to the compressor only increases the rate of wear. If I were you, after flushing and repairing, I would have the car back after a week or two and pull the tube again just to make sure everything is as it should be. The problem is the passageways in the condensor and evaporator are so small nowadays it's difficult to be sure it's all clean, so it's always worth a second look.

The variable valve actually works with the inlet pressure, as pressure and temperature are always related. Therefore, if it knows the inlet pressure, then it also knows the temperature of the evaporator. So if it sees a high pressure, it knows the evap temp is also high, so it increases the stroke of the compressor, and vice-versa. At minimum stroke the compressor is only consuming about 200watts and moving very little refrigerant, so it's doing virtually nothing. But on some cars this has led to problems, as little refrigerant movement also means little oil movement, which means it's crucial to have a good oil return path back to the compressor. I think this may be the reason why some cars like the passatt, A4 and Sharan/Galaxy seem to have recurring compressor failures.

The latest variable compressors fitted by VAG (to name one) are externally controlled by a thermistor on the evap that works an electrically controlled swash plate in the compressor. They also do not have a clutch, so the compressor cannot be disengaged at all. They do though have a 'breakaway' rubber drive in the event of compressor seizure. Of course fault finding is a nightmare, so total compressor replacement is the only course of action in the event of malfunction.

Sometimes the variable swashplate mechanism stops working, and can sometimes be resurected with plenty of revs, but this is usually shortlived. GM (Delco) have been making a variable compressor for years, and on these you can buy just the valve and replace it in situ. On the Sandens used by virtually every other manufacturer though, you have to replace the whole compressor.

Dave, have you got the Autoclimate flushing kit?
FAO Dave N - Audi A4 TDi AC Fault - DL
Very interesting info, Dave, many thanks.

Yes, got the Autoclimate Kit, it's in a big box which has never ben opened!
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FAO Dave N - Audi A4 TDi AC Fault - Number_Cruncher
Very interesting info, Dave, many thanks.


I'll second that! Thanks Dave.

FAO Dave N - Audi A4 TDi AC Fault - Dave N
When you come to flush the condensor, don't try and undo the pipes at the condensor end. They'll probably be corroded on and you'll need a new condensor and pipe set. Flush it through from the other ends of the pipes.
FAO Dave N - Audi A4 TDi AC Fault - DL
Ha, yeah, been there on mine, had to angle-grind the union off when I had to replace a low-slung hose that got damaged. Nearly had to accet defeat and replace the condensor!

8 months later on and it's still OK........
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FAO Dave N - Audi A4 TDi AC Fault - DL
Dave - When you say from the other ends, do you mean at the compressor end?
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FAO Dave N - Audi A4 TDi AC Fault - Dave N
Yeah, one end goes to the compressor, the other ends up near the orifice tube connection. You can use those autoclimate hose adaptor thingys to go on the pipes. For the large pipes you may have to open out the 'mouth' on one of the adapters to get it to fit.

IIRC I did the condensor and pipes as one piece, the evaporator and pipes back to the accumulator as another, then just the short piece of pipe from accumulator to compressor as another. Then reverse the connections and do all over again. It takes a while, but does a good job.
FAO Dave N - Audi A4 TDi AC Fault - DL
Many thanks Dave, we'll see how things progress.

The customer might end up leaving it as-is.....hopefully!
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FAO Dave N - Audi A4 TDi AC Fault - DL
Sorry to bring an old subject back to life - but a progress report!

Dave - Would you mind looking at the image www.rovercarhospital.co.uk/images/a4aircon.jpg

This item is covered in alloy swarf - I take it this is some kind of filter and the expansion valve itself?

I fear the worst - compressor failure.

Your comments would be appreciated.
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FAO Dave N - Audi A4 TDi AC Fault - Dave N
That's the orifice tube, basically a very small hole with a filter screen to protect it. As you can see it's a classic case of compressor failure.

Time to get your flushing kit out and flush everything till it comes clean - then flush once more.

Depending on the customer, it may be a good idea to replace the condensor as it has extremely small passageways, so flushing may not always get everything out. Be careful of the connector at the bottom of the condensor, it's usually seized solid meaning you'll need to buy a new pipe as well.
FAO Dave N - Audi A4 TDi AC Fault - DL
Thanks for that info Dave - confirming my worst fears.

So, ideally, I'm looking at Compressor, Condensor and Reciever Dryer replacement - with a thorough flushing. I guess cleaning the filter/orifice tube would suffice?

What is the system flushed with? A special flushing agent or similar - or R134a itself?

Go knows how I'm going to quote for the job.....I really don't want to do it, being honest!
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FAO Dave N - Audi A4 TDi AC Fault - elekie&a/c doctor
I think I would be quoting about £800-£1000 to do a job like this on a car of this age.As Dave says you will probably find the pipes to the condensor will not come apart without wrecking them.If you do take on the Job ,then good luck,and make sure you tell the customer that he will probably be back next year to have the leaking evaporator replaced!!!
FAO Dave N - Audi A4 TDi AC Fault - DL
Including parts in that price (Compressor, RD etc etc)?
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FAO Dave N - Audi A4 TDi AC Fault - Dave N
Just replace the orifice tube as well, they're only about a pound.

Certainly it needs a comp and accumulator, and it's your/your customer's call on the condensor. And if I were you, I would also take a look at the orifice tube again after a couple of weeks, just to be sure some more rubbish hasn't found it's way back there, ready to block the refrigerant (and oil) flow over the winter when he doesn' realise it isn't cooling any more.

I guess it's around about a £1000 mark to do a decent job, which isn't too bad considering just a compressor from the dealer is something like £650.

Just as a thought, are you sure it holds gas ok, before you go to the expense of fixing everything else?
 

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