Compressor HVAC: Description and Operation

Automatic Climate Control Unit
The automatic climate control unit receives battery voltage at all times through fuse 33. Battery voltage is also supplied through fuse 4 to the control unit when the ignition switch is ON (II). The control unit is grounded at G401.

A/C Compressor
Fuse 21 supplies battery voltage at all times to the normally open contacts of the A/C compressor clutch relay. When you turn the ignition ON (II), battery voltage is applied through the coil of the relay to the Engine Control Module (ECM). When you turn the A/C ON, the climate control unit sends a signal to the fan control unit. The fan control unit turns ON the condenser fans and sends an A/C request signal to the ECM. The ECM adjusts the idle speed (if the engine is idling) and grounds the relay coil, supplying battery voltage through the closed relay contacts to the compressor clutch, and the compressor runs. When the relay coil is energized, a signal is also sent to the Transmission Control Module (TCM) which adjusts the shift conditions for the additional engine load. The fan control unit will turn OFF the condenser fans and the "A/C request" signal to the ECM if the radiator fan control sensor detects a coolant temperature of 266°F (130°C) or higher. The middle pressure switch sends additional A/C load information to the ECM. The high/low pressure switch will open if the A/C system pressure gets too high or too low. High pressure could be caused by an obstruction or icing at the evaporator. Low pressure could be caused by a refrigerant leak.

Engagement of the compressor clutch is delayed for several seconds after you release the key form the START (III) position. The compressor clutch is also disengaged at wide open throttle.