Had A/C re-gassed but still not working

If the condenser is dead you will have no gas/pressure.

I went through a similar experience on my VAG car (Octavia vRS). Started off with the pressure switch, no good. Moved on to the compressor but a nitrogen test showed it working. Ended up being a pin hole in the condenser, not too expensive but I think I was about £150-£200 in by the time I fixed it.

I saved cash by fitting a new condenser myself.

You are right regarding the switch, it sits on a Schrader type valve that self seals when you remove the switch.
 
Any decent competent refrigeration engineer will be able to test the system properly for you.

As mentioned earlier, when re-gassing an AC system the old refrigerant is taken out by sucking the old liquid out and putting the system under vacuum for a while.

HOWEVER

That will not find all faults or possible leaks.
The vac test should be left for at least an hour preferably more, as shorter times will not spot the majority of small leaks, it will only prove you have a major leak somewhere if the pressure raises fast in less than an hour or so.

The best way to continue after that, if the vac test shows no pressure rise, is to then pressure test the system with nitrogen, and leave it pressurised for at least a couple of hours, this is almost the only way of detecting minor pin hole leaks from either the condenser (most common) which is located by the front of the car along side the cars main radiator, or the evaporator less common but does happen) which is located within the main HVAC unit inside the car.

It will also test all the pipes and connectors in the system as well, as ideally a competent engineer will add a dye into the nitrogen that can be seen with ultraviolet light so leaks will be more evident.

Only after all of that and the replacement of any parts necessary should new refrigerant be installed.

If you have any leaks of any kind then pressure sensors and temp sensors will read wrong until leaks are sorted and refrigerant is replaced, as they are setup to work properly with refrigerant at the correct pressure.


Your usual high street garage and all the kwik fit type places will almost never test a system thoroughly like this as it takes too much time and they do not want their expensive AC pumps attached to a single car for that amount of time when they can be doing quick re-gasses in 30-45 mins and charge people more that way.

You need to go to a competent independent garage who knows what they are doing and will take the necessary time and effort.
 
I don't think a system leak is the issue here, as if it was a pin-hole leak, the A/C would have worked immediately after the re-gas, which it didn't.

I have a feeling it's a faulty outside temperature sensor, as the mechanic told me it was reading -40 degrees, and from reading online, if this is malfunctioning, this can cause the A/C to fail.
 
Is your car's display showing -40 as an outside temp ?

The AC system uses the same sensor as the cars display.

Even a temp as low as that would not stop the AC system from working at all.
 
If it's climatronic, then I would replace the outside temp sensor as that will indeed affect the system. If it's reading -40c "outside" then it's never going to blow cold air as it thinks it's too cold.
 
Entai said:
Is your car's display showing -40 as an outside temp ?

The AC system uses the same sensor as the cars display.

Even a temp as low as that would not stop the AC system from working at all.
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I don't believe this to be true, looking on another forum re. somebody's Skoda:

"And take note, the outside temp sensor for the aircon (readout in the Climatronic control panel, if fitted) is not the same as the one for the trip computer and engine ambient temperature (readout in clock display)"

Swifty55 said:
If it's climatronic, then I would replace the outside temp sensor as that will indeed affect the system. If it's reading -40c "outside" then it's never going to blow cold air as it thinks it's too cold.
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From what I have been reading it appears this is correct, as the issue is with the outside temp sensor, not the ambient sensor.
 
With VAG cars if the outside temperature is under 4c the aircon compressor won't operate, as you have posted above on some models they have two outside temperature sensors one for the aircon the other for the dash readout.

If you have VCDS/VAGCOM you can run a diagnostic check on the compressor to check if it is working and also see what parameters aren't being met blocking the compressor from running.
 
Yeah -40c is the default that all(?) temperature sensors go to when they're faulty or there's an open circuit. Sounds like a cheap second hand temperature sensor may be the key to fixing it!
 
Dangerous Dave said:
With VAG cars if the outside temperature is under 4c the aircon compressor won't operate, as you have posted above on some models they have two outside temperature sensors one for the aircon the other for the dash readout.
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That seems really daft, is that only on VAG models then ?

How are you supposed to clear the windscreen if you cannot have the heater blowing hot conditioned (read dried) air onto it ?

Every car I have ever had the AC (Air Conditioning) compressor works fine at all temps down to -30 which is the lowest I ever saw in a car.

You can have air conditioned hot air blowing out of your vents, so the compressor still needs to run.
 
Entai said:
That seems really daft, is that only on VAG models then ?
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Nope. I know Lexus cars do the same, although their limit is 6ºC, I believe.

The thinking is that there is no real benefit to airconning if the air is already chilled to below 5 degrees. The air
is already as dry as the aircon can get it, precisely because it's as cold as the aircon can get it!

"Aircon works for demisting at higher temperatures as it condenses the moisture carried on in warmer air. If the air is already too cold to carry too much water vapour, there isn't any in it to dry out using the
aircon. Ergo it is designed not to come on unnecessarily"
 
Gaygle said:
I'm pretty sure every pure air conditioner (including ones on buildings) stop working if the ambient temperature is below 4-5c to stop it from being damaged.
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How can it get damaged ?

The most common air conditioning refrigerant R134a boils off at -26 °C (give or take a bit), so anything above that temperature and the system will work perfectly well, a well maintained system should be able to take heat out of a system and deliver chilled air down to -20 to -25°C easily, there is no reason for it not to work.

My Ford Focus worked fine down to -10°C a couple of years ago in North wales, defrosted and demisted the windscreen in no time without having to switch on the heated front screen.
 
A quick Google would suggest it's due to the evaporator in the system freezing when the ambient air is near 0c so the system is disabled above this to prevent damage from ice buildup.

I have had no problem in winter clearing windows in cars without aircon fitted so I wouldn't take that as a reason to think the compressor is running in winter. Another quick Google shows plenty of Ford owners advising the aircon is disabled when the ice icon is displayed on the dash around 4c.
 
I believe its due to stop the evap coil freezing, if you have cold air going over the coil i.e 0c this will mean your refrigerant will be boiling at approx -10c which would cause ice build up and blocking air flow across the evap coil and this would lead to liquid flood back to the compressor and damage this (the low pressure switch should cut out) - also you probably wont be able to get the head pressure high enough.

Gaygle said:
I'm pretty sure every pure air conditioner (including ones on buildings) stop working if the ambient temperature is below 4-5c to stop it from being damaged.
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Split systems which require to work at low ambients (mainly condenser) would have a low ambient kit which would vary the fan speed, also the evaporator would have a heat load i.e pc's and people.

Our Air-Cooled chillers operate down to -10c before shutting down on low ambient.
 
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Entai said:
How can it get damaged ?

The most common air conditioning refrigerant R134a boils off at -26 °C (give or take a bit), so anything above that temperature and the system will work perfectly well, a well maintained system should be able to take heat out of a system and deliver chilled air down to -20 to -25°C easily, there is no reason for it not to work.

My Ford Focus worked fine down to -10°C a couple of years ago in North wales, defrosted and demisted the windscreen in no time without having to switch on the heated front screen.
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It will damage the compressor due to liquid returning to it and they don't have crankcase heaters to stop refrigerant mixing with the oil at lower temperatures. They are not designed to work in low ambient temperatures, even if the refrigerant is capable of it. We don't often see minus 40 in the U.K. and trying to spec a system that is able to operate with ambients of -40 to +30 isn't exactly practical. A system is designed with an ambient temperature average, any higher and pressures start to increase making the system less effective. When the ambient drops the pressures lower and the evaporator will start to freeze as the evaporating temperature starts to drop - this is when liquid returns back to the compressor due the lack of air flow. The small car compressors are not designed to pump liquid and will seize.

With your example of R134a which is a low pressure refrigerant anyway to achieve a evaporating temperature of -25 the back pressure is just above a vacuum, the tipping point for that compressor to start running in a vacuum is non existent. Imagine how long that compressor will work for? This is why different refrigerants exist for different temperature applications. Different size systems, compressors etc.
More research is required if you don't understand the effect ambient temperatures have on refrigeration systems - why do you maintain a clean condenser? To aid the the heat transfer from the flowing refrigerant for the cooler ambient air to absorb, raise this ambient and less heat is absorbed from the system... It's the basics.
 
mattyy said:
It will damage the compressor due to liquid returning to it and they don't have crankcase heaters to stop refrigerant mixing with the oil at lower temperatures. They are not designed to work in low ambient temperatures, even if the refrigerant is capable of it. We don't often see minus 40 in the U.K. and trying to spec a system that is able to operate with ambients of -40 to +30 isn't exactly practical. A system is designed with an ambient temperature average, any higher and pressures start to increase making the system less effective. When the ambient drops the pressures lower and the evaporator will start to freeze as the evaporating temperature starts to drop - this is when liquid returns back to the compressor due the lack of air flow. The small car compressors are not designed to pump liquid and will seize.

With your example of R134a which is a low pressure refrigerant anyway to achieve a evaporating temperature of -25 the back pressure is just above a vacuum, the tipping point for that compressor to start running in a vacuum is non existent. Imagine how long that compressor will work for? This is why different refrigerants exist for different temperature applications. Different size systems, compressors etc.
More research is required if you don't understand the effect ambient temperatures have on refrigeration systems - why do you maintain a clean condenser? To aid the the heat transfer from the flowing refrigerant for the cooler ambient air to absorb, raise this ambient and less heat is absorbed from the system... It's the basics.
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I understand all that, and am not expecting a system capable of -40°C to +30°C in a car, but to be able to go down to -4 maybe -6 outside ambient is perfectly within the systems capability, even without crankcase heaters, and at those temps the refrigerant would still be a gas going to the compressor so no chance of hydraulicing.

The evaporator though is inside the car, and within the main HVAC unit so on a cold morning almost everyone will have the heater on, which will mean hot air (on most modern cars the coolant within the engine rises in temp fairly rapidly once the engine is ruining so hot coolant will be delivered to the HVAC within a few minutes), is being put across the evaporator so there will be minimal chance of icing up surely ?

Yes pressures will drop slightly in those initial couple of minutes, but again I do not see how they will fall enough to make the system inoperable, yes it will be slightly less efficient but pressures will increase quick enough once the heater is warm and the internal ambient has raised a degree or two, it will still be well within the spec of the refrigerant and will not be low enough that liquid will be returning to the compressor.
 
The other issue will be the discharge pressure, you will not have any way of controlling the airflow across the condenser when travelling at motorway speeds or town driving, which would impact the discharge pressure intern would impact the suction pressure.

Also at those temperatures the mass flow would be greatly reduced and the TEV may not be able to shut down enough to stop flooding back.
 
Majority of TEV's on vehicle systems have an extremely small aperture so even when fully open do not allow a huge flow at best of times.
 
I have managed to locate the outside temperature sensor, and this is the one which informs the trip computer display, and also affects the A/C, as the A/C light would not come on when I unplugged the sensor. It appears this is working completely fine. Is there another sensor located outside the cabin under the front bumper, or is it the cabin air temperature sensor I need to be locating? It's a pain in the arse a it's the newer shape of A3, there haven't yet been enough failures of the system to see any tutorials/videos. It seems parts are difficult to source as well. May well be a breakers yard job I think. If anybody in the Manchester area has access to a VAG-COM scanner I'd be eternally grateful if I could borrow it to narrow down which sensor it is that is failing!
 
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