HVAC: Compressor Control

A/C system operation is controlled and monitored by the A/C Amplifier. The A/C Amplifier monitors A/C Switch position. The A/C Amplifier also 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 1°C (34°F). When this occurs, the A/C Amplifier stops A/C system operation. This prevents evaporator frost and ice-up which reduces airflow and reduces the unit's cooling capacity. The A/C Amplifier also gives an idle-up signal to the Engine Control Module (ECM).

With the Ignition Switch in the "ON" position, system voltage is applied to the A/C Amplifier through the HTR Fuse, Blower Speed Selector Switch, and A/C Switch when both switches are in the ON position. System voltage is applied to the A/C Amplifier through the IG Fuse with the Ignition Switch in the "ON" or "START" positions. The Dual Pressure Switch monitors A/C system pressure and remains closed as long as refrigerant pressure is between 196 kPa (28 psi) and 3137 kPa (455 psi). With the Dual Pressure Switch closed, voltage is applied to the A/C 1 Relay (Compressor Clutch).

The A/C 1 Relay (Compressor Clutch) is guillotined through the A/C Amplifier to G201, provided that the A/C Amplifier has not received a low temperature signal horn the Evaporator Thermistor, or a high engine coolant temperature (ECT) signal from the ECM.

When the A/C 1 Relay (Compressor Clutch) is energized, voltage is applied through the A/C Fuse and the closed contacts of the A/C 1 Relay (Compressor Clutch) to the coil of the A/C 2 Relay (Condenser Fan) and to the A/C Compressor Clutch. Since the A/C 2 Relay (Condenser Fan) is permanently grounded at G103, the relay energies, the contacts close, and voltage is applied to the A/C Condenser Fan Motor.