For 1990-2009 cars

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

The pressure switch of the air conditioning (A/C) refrigerant 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 the following conditions:

• | Abnormally low refrigerant pressure

• | 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. The following conditions can cause an abnormally high refrigerant pressure:

• | Very high refrigerant temperatures

• | Over charging the system during service.

### A/C Relay (Compressor Clutch)

The A/C compressor clutch relay applies voltage to the compressor clutch when the A/C compressor control module energizes the A/C compressor cluch relay. By energizing and by de-energizing the A/C compressor clutch relay, the A/C compressor control module controls the A/C system operation.

### A/C Relay (Condenser Fan)

The condenser fan receives voltage from the A/C condenser fan relay.The A/C condenser fan relay activates the condenser fan. 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 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 connected to electrical circuits in order to determine 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 ice will start to form on the evaporator fins. Ice 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 from forming 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 signal in order to shut down the system. When the A/C compressor control module receives the signal, the module shuts down the compressor until the evaporator core temperatures increase to 3°C (37°F).