They must do a leak test otherwise they wouldn't know you had one? They won't be able to tell you were from but that is a different job and way beyond their capabilities. ATS, or at least my local one when the missus took advantage of their cheap deal, use the programmable machine as they don't really understand what it is they are doing. When I did my full FGas course to allow me to install domestic AC systems, I asked what the difference was between the full week course I was doing and the 1 day automotive course. I was told that the automotive course only teaches how to do it and not why you are doing it that way or how the system works. I paid an additional exam fee so I could do both exams and get both qualifications. Compared with the FGas exam, the 20 question automotive exam was so simple although they allow 30 minutes to complete it, I did it in under 5 minutes!
The process is to use a recovery machine to slurp out any remaining refrigerant (if there is any in there in the first place), pressurise the system with Nitrogen at 10bar and leave it for a minimum of 30 minutes to check that the pressure hasn't dropped, release the Nitrogen then vacuum out to 1000 microns (0.019 psi), then refill with the correct quantity of refrigerant along with oil and dye (if you can be bothered). In comparison on a domestic/commercial system it is pressure tested at 42 bar for a minimum of 60 minutes and vacuumed to 150 microns (0.002 psi).
However, other than having at least one person that has sat the exam so they have the required qualification, the ATS/KwikFit, etc. places use a machine to do it. They connect the hoses, tell it how much refrigerant the car they are doing needs (or leave it at the default 600 grams as refrigerant has shot up in price and it will still work after a fashion with a short charge), press go and leave it to get on with it. If the machine detects a drop in pressure on the Nitrogen test, it stops and tells them there is a leak, so that is what they tell the customer. They've probably forgotten everything they were taught on the course as they don't need to know it, the machine does it all for them. What they don't think about is temperature differences and a car with a slight leak will still pass the test. The Nitrogen bottle is at ambient, you drive your car in so the engine is hot, Nitrogen is put in at 10 bar but for every 10 degrees C increase in temperature, the pressure will increase by roughly 0.5 bar, or 7 psi. So the pressure actually increases and as long as it doesn't leak more than the 0.5 bar that is taken up by the thermal expansion of the gas, the machine is perfectly happy. If the car is cold and the Nitrogen bottle is standing in the sun, the opposite happens. The pressure drops as the temperature falls and the machine says you have a leak when you haven't.
Peterborough, Cambs
- '93 Range Rover Classic 4.2 LSE, sold
- '97 Range Rover 4.0SE, in Oxford Blue with a sort of grey/blue leather interior sold as two is plenty.....
- '96 4.6HSE Ascot - now sold
- '98 4.0SE in Rioja Red
'98 Ex-Greater Manchester Police motorway patrol car, Range Rover P38 4.0, in Chawton white - the everyday car
All running perfectly on LPG
- Proud to be a member of the YCHJCYA2PDTHFH club.