Your air conditioning system is essentially a close-loop pump system. There are two sides to the system, separated by the compressor and the expansion valve. The compressor pumps refrigerant in a gaseous state into the high-pressure side of the system. The refrigerant cools in the condenser, which is located in front of the radiator, converting into a liquid. The liquid refrigerant makes it way to the expansion valve, which is just before the evaporator, both of which are located inside the vehicle. Heat inside the vehicle, passing through the evaporator, transfers into the refrigerant, turning it back into a gas, which returns to the compressor.
The entire system only works if there is the correct amount of refrigerant in the system. There are a couple of access ports which allow for pressure testing and system charging, but otherwise the system is sealed. Damage or corrosion can allow refrigerant to leak, leading to partial or complete loss of air conditioning function. Usually, pressure sensors will disable the compressor if there is insufficient refrigerant pressure, so don’t immediately assume that the compressor or clutch or electrical system is at fault. If turning on the air conditioning doesn’t cool the interior of the vehicle, refrigerant loss could be the problem.
Manifold Gauges – This tool can be expensive and may be available for rent at a local auto parts store. The gauges connect to the ports on the lines of the air conditioning system. Red connects to the high side, and blue connects to the low side, and the two aren’t interchangeable. Open all the valves except the center valve on the head to equalize the pressures and get a reading of the system at rest. If you want to watch the system while it is functioning, close the valves on the head, but leave the valves at the connections open. Before disconnecting the manifold gauges from the vehicle, be sure to close the red connection valve first and open all the others to allow the pump to draw as much of the pressure back into the vehicle. When the gauges read as low as possible, perhaps 20psi, close the rest of the valves and remove the gauges from the vehicle.
DIY Air Conditioning Top-Off Kit – This kit contains a can of refrigerant and a special hose to connect to the low side of the air conditioning system. It should also have a basic red-green-red pressure gauge on it that helps you to keep from overfilling the system. The basic gauge can also help you determine if you have a leak. The refrigerant can may also come with a UV-sensitive dye for future leak checking. Note: Some DIY Air Conditioning Top-Off Kits come with a leak-stop or sealing agent. I do not recommend using these, as they often seal up things that ought not be sealed, such as the expansion valve, a critical component in the system, and can also contaminate the recovery machines that your technician uses, which are also very expensive to repair.
Ultraviolet Air Conditioning Leak Detection Kit – This kit contains at least two very important parts, an oil infused with UV-sensitive dye, and a UV light. There should be a hose to connect to the low side of the system, and possibly a pair of yellow safety glasses, which help identifying when the UV-sensitive dye fluoresces at the point of leakage.
Halogen Air Conditioning Leak Detection Tool – This tool uses a sensor to detect R-134a refrigerant escaping from a hole in the system. It is very expensive, and may or may not be available for rent at a local auto parts store.
First, run the engine and turn on the air conditioning system medium fan speed, full cold, and air conditioning “on” and listen for the compressor to engage. If the compressor does not engage, check fuses, but do not condemn the compressor or electrical system. Check the pressures first with a set of manifold gauges. You can probably rent these at the local auto parts store. Some refrigerant top-off kits may also include a basic pressure gauge with a red-green-red indication. With the system at rest, that is, with the car off for at least ten or fifteen minutes, the manifold gauges should read the same amount of psi as the ambient temperature. You can allow a few pounds more if the engine bay is hot off the highway. The basic gauge should be in the green zone.
If the pressure is too low, you most likely have a leak in the system. If there is no pressure in the system, you may need more than one can of refrigerant in order to do any testing. Perhaps it is best to leave it to the technician in this case, since they have the equipment on hand to quickly perform the following procedures.
If there is enough pressure in the system to run the compressor, inject the UV-sensitive dye into the system and run the engine with the air conditioning on full cold, full fan, with the windows open. You may have to run the system for a few minutes to detect a large leak, or even a few days for a small leak. Put on the yellow glasses and shine the UV light on all parts of the air conditioning system, including hoses, lines, the compressor, condenser, and evaporator if you can access it. Common leak spots include pretty much any joint where two components are connected.
The condenser in front of the vehicle is particularly vulnerable to damage from road debris, as well as corrosion. O-rings in the joints can also dry out or be damaged due to poor assembly. The evaporator and expansion valve are typically the most difficult to access, but it isn’t unheard of to find a leak due to a bad o-ring or corrosion here either. Save the most difficult-to-access parts for last to save time.
If there isn’t enough pressure in the system to run it, you’ll have to add more refrigerant until it runs. If you add one or two cans and still can’t get the system to start, you’ll be better off bringing it to your trusted technician to take a look. On the other hand, if you have access to a halogen air conditioning leak detection tool, or ac sniffer, you can use this to find where the refrigerant is escaping. R-134a is heavier than air so, with the engine off, point the probe around suspect areas and below joints where there are possible leaks. The lights and sound on the tool will increase in intensity with the amount of refrigerant detected. Home in on the most intense source, and this is most likely your point of leakage.