Start your vehicle with the AC off and lift the hood. The electric cooling fan in front of the condenser core should be off. Then reach in and turn the AC on. You should here an audible click as the compressor clutch kicks in and the cooling fan should now be on. If the compressor is turning but the electric fan isn't, bad fan motor. If the compressor clutch is not engaging, you either have low system pressure, a bad panel switch in the dash, a bad relay, a bad fuse, or a bad clutch.
Many times the refrigerant recirculation system is fully charged and compressor running OK but the fan does not push enough air through the condenser which does not allow the refrigerant to condense into a liquid so it can boil through the suction throttling device (orifice tube/expansion valve). This causes poor cooling performance from the evaporator core which leads to increased high aside pressures which result in a safety shutdown of the compressor. Thinking the system is just low and adding more refrigerant compounds this problem.
Never add refrigerant unless it has been verified that all electrical and mechanicals are in working order first. The ONLY time you should add refrigerant is when the compressor begins to cycle on and off excessively
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