For 1990-2009 cars

### Air Conditioning (A/C) Refrigerant Pressure Switch

The air conditioning (A/C) refrigerant pressure switch is located along the right front inner fender. The A/C refrigerant pressure switch is in the liquid pipe between the receiver/dryer and the evaporator. The A/C refrigerant pressure switch acts in order to cycle the compressor on and off under conditions of abnormally low or abnormally high refrigerant pressure.

If the pressure drops below 193 kPa (28 psi) or rises above 3137 kPa (455 psi), the A/C refrigerant pressure switch opens. The A/C refrigerant pressure switch signals the A/C compressor control module to shut down the compressor.

An insufficient refrigerant supply or a very low refrigerant temperature can cause an abnormally low refrigerant pressure. Extremely high refrigerant temperatures or an overcharging of the system during servicing can cause an abnormally high refrigerant pressure.

### A/C Relay (Compressor Clutch)

The A/C relay (compressor clutch) applies the voltage to the compressor clutch when the A/C compressor control module energizes the A/C relay (compressor clutch). By energizing and de-energizing the relay, the A/C compressor control module is capable of controlling the A/C system operation.

### A/C Relay (Condenser Fan)

The condenser fan receives the voltage from, and is activated by, the A/C relay. The condenser fan runs continuously as long as the A/C compressor is engaged.

### A/C Switch

The A/C switch is on the heater control unit. The A/C switch signals the A/C compressor control module that the A/C system operation has been requested.

### A/C Refrigerant (Evaporator) Temperature Sensor

The A/C refrigerant temperature sensor is a thermistor that translates temperature into an electrical resistance. Thermistors are connected into electrical circuits in order to determine the temperatures in critical areas. Thermistors are used in engine controls systems such as engine coolant temperature (ECT) and intake air temperature (IAT) sensors. Thermistors are used in A/C systems in order to determine temperatures at the evaporator core. If the evaporator core temperatures drop to about 0°C (32°F), frost and ice will start to form on the evaporator fins. Ice formation on the evaporator fins will seriously impede airflow through the evaporator and will reduce the ability of the system to cool the vehicle interior.

In order to prevent ice formation on the evaporator fins the system incorporates a thermistor that is electrically connected to the A/C compressor control module. When the evaporator temperatures drop to 0°C (32°F) the A/C compressor control module reads the corresponding thermistor resistance as a system cutout signal. When the signal is received, the A/C compressor control module shuts down the compressor until the evaporator core temperatures increase to 3°C (37°F).