Compressor HVAC: Description and Operation

A/C AMPLIFIER
A/C system operation is controlled and monitored by the A/C AMPLIFIER. The A/C AMPLIFIER monitors A/C SWITCH position, engine coolant temperature, and the presence of an engine cranking signal. If engine temperature is above 108°C (226°F), or if the starter is cranking the engine, the A/C AMPLIFIER will not permit A/C system operation. The A/C AMPLIFIER also monitors the EVAPORATOR THERMISTOR mounted inside the evaporator housing. 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 and a ground to the A/C VACUUM SWITCHING VALVE when the A/C system operates at low engine speed.

IGNITION SWITCH
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 also applied to the DUAL PRESSURE SWITCH and the A/C AMPLIFIER through the IG Fuse with the IGNITION SWITCH in the "ON" position. The DUAL PRESSURE SWITCH remains closed as long as refrigerant pressure is between 207 kPa (30 psi) and 2641 kPa (383 psi). With the DUAL PRESSURE SWITCH closed, voltage is applied to the A/C (COMPRESSOR CLUTCH) RELAY.

A/C COMPRESSOR CLUTCH RELAY
The A/C (COMPRESSOR CLUTCH) RELAY is grounded through the A/C AMPLIFIER to G201, provided that the A/C AMPLIFIER has not received a low temperature signal from the EVAPORATOR THERMISTOR, a high coolant temperature signal from the A/C COOLANT TEMPERATURE SWITCH, or an engine cranking signal.

When the A/C (COMPRESSOR CLUTCH) RELAY is energized, voltage is applied through the A/C Fuse and the closed contacts of the A/C (COMPRESSOR CLUTCH) RELAY to the coil of the A/C (COMPRESSOR FAN) RELAY and the A/C (COMPRESSOR CLUTCH) RELAY. Since the A/C (COMPRESSOR FAN) RELAY is permanently grounded at G104, the relay energizes, the contacts close, an voltage is applied to the A/C CONDENSER FAN MOTOR.