This term refers to two possible failures in an auto A/C system. On systems using a thermostatic expansion valve, a freeze up occurs when moisture that is in the system freezes at the expansion valve.

The freezing occurs because of the change in refrigerant pressures and temperatures as it flows through the valve. The expansion valve is the division between the high and low pressure sides of the A/C system. High pressure liquid refrigerant from the condenser is restricted at the expansion valve and the pressure is reduced. If the system has been contaminated with moisture, or the receiver drier can no longer hold all of the moisture in the system, that moisture travels down the liquid line with the refrigerant and freezes.

Typically, when this occurs, the A/C complaint is that the system works for some period of time and then cooling performance is lost. Once shut off for a short period of time, the A/C system can be started once again and will cool properly. What is happening is that the moisture at the expansion valve melts when the system is shut off. When it is started up again, the system will cool properly until the moisture freezes up the valve again.

A Freeze up also refers the formation of ice on the fins of the evaporator. With hot air passing over the evaporator, any humidity in the air will condense on the evaporator because of the sudden change in temperature. If low side pressure becomes too low, that humidity can freeze and restrict air flow over the evaporator.

The complaint is usually that the A/C system works well, but airflow stops after a short time of operation. When the system is shut off and then started again some time later, normal cooling performance is restored once again, until the humidity freezes up.

Depending on the A/C system, whatever valve or pressure sensing switch that is controlling the operation and cycling of the compressor has failed causing the compressor to pull the low pressure side of the A/C system too low. Therefore, humidity starts to freeze on the evaporator face when the system operates for any length of time.