Compressor HVAC: Description and Operation

With the engine running, the GENERATOR applies Battery voltage through the normally closed contacts of the Restart Relay to the coil of the Heater and A/C Relay. Since the coil of the Heater and A/C Relay is permanently grounded at G102, the relay is immediately energized. With this relay energized, voltage is applied through Fuse E-2 to the TRIPLE SWITCH, the A/C SWITCH, and the coil of the A/C Compress Relay.

As long as refrigerant pressure remains between 193 kPa (28 psi) and 3867 kPa (386 psi), the TRIPLE SWITCH remains closed, allowing Battery voltage to reach the open contacts of the A/C Thermo Relay.

When the A/C SWITCH is pressed to ON, the switch contacts close and voltage is applied to the coil of the A/C Thermo Relay. In order to energize the coil of the A/C Thermo Relay, the coil must be grounded through the closed contacts of both the A/C THERMOSTATIC SWITCH and the BLOWER SPEED SELECTOR SWITCH. The contacts of the A/C THERMOSTATIC SWITCH are closed as long as evaporator core temperature remains above 3°C (37°F). The contacts of the BLOWER SPEED SELECTOR SWITCH are closed with the switch in any position except "OFF."

With the A/C Thermo Relay energized, Battery voltage is applied to the open contacts of the A/C Compress Relay and to the ENGINE CONTROL MODULE (ECM). This voltage applied to the ECM acts as the A/C Request signal and informs the module that A/C system operation is desired.

When engine conditions permit A/C system operation, the ECM provides a ground to the coil of the A/C Compress Relay. Because the coil of the A/C Compress Relay is receiving voltage from the Heater and A/C Relay, this ground energizes the relay, closes its contacts, and applies voltage to the A/C COMPRESSOR CLUTCH.

If, during A/C system operation, the ECM receives an input informing it that engine coolant temperature is greater than 120°C (248°F), engine speed is less than or equal to 400 or greater than or equal to 5800 rpm, the throttle is wide open (GSi only), there is rapid throttle movement (GSi only), or that power steering pressure is greater than 569 psi (GSi only), the ECM will interrupt the ground circuit for the A/C Compress Relay. This action de-energizes the relay and stops A/C system operation until normal operation is restored. The Restart Relay, when energized during starter motor operation, also stops A/C system operation by interrupting the circuit between the GENERATOR and the Heater and A/C Relay, making A/C system operation impossible during engine cranking.

If engine coolant temperature reaches 110°C (230°F) and vehicle speed is above 32 km/h (20 mph), the ECM will cycle the A/C COMPRESSOR CLUTCH every 15 seconds. In this manner, the compressor load is partially removed from the engine, allowing engine coolant temperature to decrease. When engine coolant temperature falls below 110°C (230°F) or vehicle speed falls below 32 km/h (20mph), the ECM stops cycling the A/C COMPRESSOR CLUTCH and full A/C system operation resumes.