Automotive Air Conditioning Repair|Leak Detection

Automotive A/C differs from most household units in that the components are spread out across the vehicle and connected together with various hoses and lines, whereas household a/c units have all the components contained in a small package with everything fairly easy to get to.
As I stated in a previous post the number one problem is the a/c is low or out of freon. I also said that the a/c unit does not use up freon, if it is low, there is a leak.
Freon leaks on an automotive a/c system can be very difficult to locate. Some of the most common areas that tend to leak is the front seal on the compressor, the high or low pressure safety switches and fittings where lines go together.

If the a/c system is completely out of freon you can put pressure on the system with nitrogen and check all the fittings, lines and switches with soapy water. The front seal on the A/C compressor is another matter. If you're going to be doing a lot of A/C work, a good investment is an electronic leak detector. This device has an opening in the end of a wand that allows air to pass across a sensor. It goes from a slow beep to a steady beep when freon is picked up, much like a metal detector. If the system is using R-12 refrigerant, a much cheaper device can be used to detect leaks. This is a halide torch. Propane is used for a flame that is burned under a copper element, air is drawn over the element through a tube, the color of the flame will change to bright blue if the air contains halogenated refrigerant. There is no chlorine in R-134 so the torch won't detect it. The system will have to be at least partially charged with a freon containing chlorine to use the halide torch.
One other way to find a leak is pressure testing. This can be especially useful for determining a leak in an evaporator. The evaporator is almost impossible to get to for leak checking without disassembling the dash, so it's good to know the leak is there before you start taking things apart. I usually separate the evaporator from the system, block off the lines and pressurize it with nitrogen. Do not over pressurize it, 100 to 150 psi is good. Let it sit and see if the pressure leaks down. It's not a good idea to use compressed air to pressurize an a/c system or even to blow out the lines. Compressed air contains moisture and can ruin an a/c system. Freon is not good for pressure checking because the pressure can change considerably with temperature changes. Nitrogen is the best choice for pressure testing an a/c system.
Once the leak has been located and repaired we can finish up the job. Pull a vacuum on the system. If it's been open for any length of time, out of freon completely, or air was used to blow the system let the vacuum run for several hours. Overnight won't hurt anything, we have to make sure all the moisture is out of the system. the vacuum pump should pull the system down to 29 hg and hold it there. If not, you either still have a leak , the vacuum pump is weak or there is a lot of moisture still in the system. Do not recharge the system until you can get the vacuum down to this and hold it.
Charging the a/c system by wieght is the only sure way to know it has the exact amount of freon it should have.
After I have pulled a vacuum on the system I connect my freon to the gauges , bleed any air from the line and charge the system from the high side with liquid until the proper amount of freon is in the system, or until the system balances with the freon tank and won't take any more. If the proper amount of freon is reached before the tank and system balance, do not put more in it. Do not ever charge the system on the high side with the engine or compressor running. Do not attempt to add freon to the high pressure side after the compressor has been started up as the pressure could still be high enough to burst the freon can. Only use this method for recharging when the system is under a vacuum and not running. Alternately you can charge the system on the low side with gas freon. Don't put liquid in on the low pressure side as this can cause the oil to migrate from the compressor and can cause compressor damage on startup. After the gauges have been turned off and no more freon can go in, start the car, turn on the A/C. If the proper amount of freon was not put in the system previously it can be added to the suction side as vapor at this point.

Next post retro-fitting an R-12 a/c system with R-134