An automotive air-conditioning system requires many components to function properly. When a failed component is replaced, typically, the system must be recharged with refrigerant. When a mechanic attempts to refill the system with refrigerant, most commonly R134a or R12, and the system will not accept a full charge, there can be several causes.
The air-conditioning system in a vehicle has a high- and a low-pressure side. Both sides require specific pressure ranges: the low-pressure side requires 20 to 25 psi; and the high side requires 200 to 225 psi with R12, and 225 to 250 psi with R134a. When there is a clog in the air-conditioning system, these pressures are reached without the charging station reaching the proper refrigerant level; at this point, the charging station will not allow any more refrigerant to enter the system.
The air-conditioning compressor is responsible for creating the pressure in the system, as well as circulating the refrigerant through the system. When the compressor fails, the refrigerant no longer circulates, also causing the system to be unable to pull in any new refrigerant.
The clutch is part of the pulley on the front of the compressor. It is a electromagnetic clutch that engages the compressor when needed. When this clutch fails, it can no longer activate the compressor, leading to the system not circulating refrigerant or accepting any new refrigerant. Fortunately, the clutch can typically be replaced without replacing the entire compressor.
All air-conditioning systems have a pressure switch. This switch measures the psi of the refrigerant and allows the compressor clutch to engage or disengage. When this switch fails, the system goes into a fail-safe mode that does not allow the clutch to engage on the compressor. This failure to engage prevents the compressor from pressurizing the system or drawing any new refrigerant into the system.
All air-conditioning systems have two service ports -- high- and low-pressure. These ports have a small valve inside of them to allow refrigerant to go into the system, with the proper tool attached, but remain sealed when the tool is not attached, to prevent leakage. These valves can become damaged from improper servicing or simply age. When these ports are damaged, the refrigerant cannot enter the air-conditioning system.
