Air con leak finding.

I have often wondered -

when testing for leakage in the air-con system with a vacuum, could this not cause the flexible pipes to contract and possibly seal a small leak? Or does the outside pressure still find the way?

I understand the vacuum is necessary to remove moisture.

Would a high pressure test not be more appropriate, given that that is what happens when the system is working?

Hoses can take the vacuum in the same way that they take the pressure.
There is another reason for the vacuum test. If the car takes 750g of refrigerant, they have to evacuate the system before they refill it with the proper amount. There is no dipstick in an A/C system.

As far as I am aware, A/C leaks only show up when the system is under pressure....however, before work takes place the system has to be evacuated first to remove the toxic gas within.
After that, the refridgerant is introduced with a small quantity of oil to keep the compressor happy - some ultra violet dye can be added at the same time.
Once the system is refilled and switched on, any leaks show up a dramatic fluorescent green under U/V light....without the dye finding leaks would be almost impossible.
John

Correct John, also vacuuming removes moisture which is basically water. That's why there is a drier connected to the condenser, to absorb moisture.
Usually the first test will show if the system is low on gas. If it is low, that indicates a leak. So, the next step is to introduce the dye and then run it under pressure to find the leak. There's no point in re-gassing a system that is low on gas until the reason for the low gas has been found.

Am I right in saying that - say, for a system that has had a hose replaced - there would be no point in just pressurising the system and watching the pressure drop as you would not know where the leak was? You may test the new hose joints but there might also be other leaks.

It is only when the system is filled with gas and dye and working, that the leak, or leaks, can be found satisfactorily.

That's about it, really - if you just pressurised with air the leak could be anywhere and even the tiniest of leaks would disable the system very quickly. The environment may not approve though!
Firms such as Kwik Fit used to have an offer - they would regas an empty system, and if it failed within a very short time you didn't pay. Whether that included a dye and oil though I don't know, but it would have made sense if it did!
I've only replaced one compressor and three condensers - two failed due to collision and one due to a stress fracture so I don't have much experience. Heaven help me if I have ever to replace an evaporator - I'll trade that one on!
John

If the a/c can hold a vacuum for about 15 mins there wont be any major leaks . To check under pressure nitrogen can be used. check the pressure gauge to make sure no drop in pressure if it drops pressure you can round pipes ect with a leak detector fluid it will bubble up where leak is or some places have a listening device which you move around pipe work ect to hear the gas escaping.
The good thing about nitrogen is its safe to vent to atmosphere and it will also take out any moisture left in system
Main points of leaks are condenser evaporator or "o" rings where pipes are connected
Good luck

My system has been empty - various leaks - for more than eighteen months since I bought the car. I recall reading a post sometime from someone who claimed to be a specialist, saying that a system that had been empty needed more than just refilling because of also being open to the atmosphere ( pollution perhaps ? ).

Does anyone with experience agree with that, and what would this extra work/procedure be called ?

If it's unsure whether the system is leak free or not it can be filled with nitrogen and then pressure checked for a while before the correct refrigerant plus oil and dye is introduced.....kinder for the environment I guess!
If the system is just being checked, the old refrigerant has to be evacuated fully before more is introduced.
John

mointainwalker said:

My system has been empty - various leaks - for more than eighteen months since I bought the car. I recall reading a post sometime from someone who claimed to be a specialist, saying that a system that had been empty needed more than just refilling because of also being open to the atmosphere ( pollution perhaps ? ).

Does anyone with experience agree with that, and what would this extra work/procedure be called ?

Click to expand...

I don’t have personal experience but I’ve heard that the dryer part of the system will need replacing when a system has been ‘open’ for a while as it will be saturated with moisture.

mointainwalker said:

My system has been empty - various leaks - for more than eighteen months since I bought the car. I recall reading a post sometime from someone who claimed to be a specialist, saying that a system that had been empty needed more than just refilling because of also being open to the atmosphere ( pollution perhaps ? ).

Does anyone with experience agree with that, and what would this extra work/procedure be called ?

Click to expand...

Yes, that's correct. The most common refrigerant is R134a and the lubricating oil it uses reacts with water to form quite nasty acids which can corrode any steel parts in the system. As a result, there's a "receiver/ dryer" in most (I think maybe all?) systems. This is a filter, but also contains desiccant granules to absorb any tiny amounts of moisture that may find their way into the system. If the system has been open to atmosphere at any point, moisture will have found its way in and the granules will have been saturated. If there's any refrigerant pressure in it at all, then you might get away wit it, but if you poke either of the valves in the charging ports for a fraction of a second and nothing tries to escape, there will almost certainly be moisture in the system.

I've worked on a few aircon systems in the past, and the most common failures seem to be the condenser. It's right at the front of the car and is prone to stone chips. After that, it's hoses. (and the sealing of joints between hoses and hardware). If you want to replace a condenser/ receiver/dryer yourself, they're readily available and not that expensive:

https://www.autoairconparts.co.uk/

HOWEVER, if you expose a receiver / dryer to air for more than 20 minutes or so, you can saturate the granules so it's best to leave that until last, and then quickly evacuate the system. The problem is that if you find a leak, you don't have long before you've ruined your new receiver / dryer. For this reason, it's usually best to leave it to a pro, who will have all the necessary equipment. I'd tend to steer clear of Kwik Fit and the like. They use automated charging stations whereby the operative just enters the make and model of the car, and connects the hoses. The charging station does everything else - evacuation, vacuum test, pressure test, adds the right amount of refrigerant and oil, etc. They're fine if it just needs a bit of a top-up but not so good if the system actually needs parts replacing.

Thanks for both of the last posts. The specialist garage I used said that they had found a leak in the condenser using a sniffer. The replacement that I intend buying comes with a new dryer so that should take care of that.