Compressor Controls

With the Ignition Switch in the "ON" position, system voltage is applied to the Dual Pressure Switch through the HEATER Fuse. The Dual Pressure Switch closes when the pressure in the A/C system is between 193 kPa (28 psi) and 3137 kPa (455 psi). Voltage is then provided to the A/C Condenser Fan Relay coil.

The A/C Condenser Fan Relay coil is grounded by the A/C Amplifier, provided the A/C Amplifier has not received a low temperature signal from the Evaporator Thermistor.

When the relay is energized through the Dual Pressure Switch, battery voltage is applied to the A/C Condenser Fan through the A/C Fuse and the contacts of the relay. Since the A/C Condenser Fan is permanently grounded at G108, the coolant fan operates as long as the relay is energized.

The A/C Amplifier monitors the Evaporator Thermistor mounted inside the evaporator housing. The Evaporator Thermistor converts evaporator core temperature into a resistance value. Using this resistance value, the A/C Amplifier can sense when evaporator temperature drops below 0°C (32° F). When this occurs, the A/C Amplifier removes the A/C Condenser Fan Relay coil ground (as well as the A/C Clutch Relay coil ground). This prevents evaporator frost and ice-up which reduces air flow and reduces the unit's cooling capacity.

Whenever the pressure in the A/C system drops below 193 kPa (28 psi) or rises above 3137 kPa (455 psi), the Dual Pressure Switch opens, the A/C Condenser Fan Relay is de-energized, and voltage to the A/C Condenser Fan is interrupted.