There's a number of possible causes of failure to cool, but none of them are do-it-yourselfer repairs. First the problem needs to be properly diagnosed, then the system has to be discharged. Special equipment is needed to recover the expensive refrigerant. Once the problem is corrected, the system has to be pumped into a vacuum for at least half an hour. That causes any moisture in the system to boil at 77 degrees so it can be sucked out. Failure to do that will leave water droplets circulating with the refrigerant, and it will freeze at the port where it gets real cold. That will stop the flow of refrigerant, and stop the cooling. Water also mixes with refrigerant to form an acid that will corrode the metal parts. That will result in leaking evaporators and condensers.
There is also a filter that is supposed to be replaced each time the system is opened for service. Recharging the system requires equipment with a scale and gauges to know when it is fully charged. Leaving it under-charged can result in poor cooling, and over-charging can allow liquid refrigerant to spill into the compressor and destroy that.
Also, refrigerant is extremely dangerous to work with. It can cause frostbite and blindness. Professionals wear gloves and safety glasses, and the smart ones wear a face shield.
I applaud anyone who wants to know more about their car and learn to do some of the repairs, but working with air conditioning systems isn't the place to start.
Wednesday, June 25th, 2014 AT 3:41 PM