Relays, Sensors and Switches

A/C Dual Pressure Switch

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

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

Reasons for abnormally low refrigerant pressure are an insufficient refrigerant supply or very low refrigerant temperatures. Reasons for abnormally high refrigerant pressure include extremely high refrigerant temperatures or an overcharging of the system during service.

A/C Compressor Clutch Relay

The A/C compressor clutch relay, when energized by the A/C amplifier, applies voltage to the compressor clutch. By energizing and de-energizing this relay, the A/C amplifier is capable of controlling air conditioning system operation.

A/C Condenser Fan Relay

The A/C compressor clutch relay, when energized by the A/C amplifier, applies voltage to the compressor clutch. By energizing and de-energizing this relay, the A/C amplifier is capable of controlling air conditioning system operation.

A/C Switch

The A/C switch, mounted on the heater control unit, signals the A/C amplifier that the A/C system has been turned on.

Evaporator Temperature Sensor

The evaporator temperature sensor is a thermistor which is a device that translates temperature into an electrical resistance. Therefore, thermistors are connected into electrical circuits to determine temperatures in critical areas. Thermistors are used in engine control systems such as Engine Coolant Temperature (ECT) and Intake Air Temperature (IAT) sensors. They are also widely used in air conditioning systems to determine temperatures at the evaporator core. If 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 seriously impedes the airflow through the evaporator and reduces the ability of the system to cool the vehicle's interior.

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