HVAC Compressor Controls (Manual)

The A/C Compressor is belt driven and the A/C Compressor's pulley rotates whenever the engine is operating. The A/C Compressor only operates when the A/C Compressor Clutch has been energized. The clutch is energized by the Vehicle Control Module (Gas) or by the Powertrain Control Module (Diesel) upon A/C Request input to the control module. The control module energizes the clutch through the A/C Compressor Enable Relay which can handle the clutch's current load.

The A/C Compressor Enable Relay receives two sources of voltage. The A/C Fuse provides voltage at all times to the A/C Compressor Enable Relay's coil through the ORN (1240) wire. When the Ignition Switch is in Run, the IGN-E Fuse provides power to the A/C Compressor Enable Relay's normally-open contacts through the PNK (639) wire. When the A/C Compressor Enable Relay is energized, the normally-open contracts close to apply voltage through the DK GRN (59) wire to the A/C Compressor Clutch. Since the A/C Compressor Clutch is grounded by the BLK (150) wire, the coil is energized when voltage is applied.

Diode D102 is reverse biased when voltage is applied from the A/C Compressor Enable Relay. This prevents current from flowing though the diode to ground. When the A/C Compressor Clutch is de-energized, the diode is forward biased allowing the coil's magnetic field to collapse without causing damage to the A/C Compressor Enable Relay.

The A/C Compressor Enable Relay is energized when a ground path is provided for the relay coil on the DK GRN/WHT (459) wire. The Powertrain Control Module (PCM, Diesel Engine) or the Vehicle Control Module (VCM, Gasoline Engine) provides this ground path through contact-equivalents in the module. This ground connection is provided under normal circumstances when the control module senses the A/C Request voltage from the Heater and A/C Controller on the DK GRN/WHT (762) wire. The control module may stop compressor operations while otherwise enabled under certain operating conditions as described in the Driveability and Emissions sections for the specific engine.

The Heater and A/C Controller Mode Switch is powered from HTR-A/C Fuse 12. The BRN (141) wire provides this voltage when the Ignition Switch is in Run. When the Mode Switch is placed in the Defrost, Blend, Bi-Level, A/C, or MAX A/C positions, the A/C Request voltage is applied on the LT GRN (66) wire.

The A/C Request voltage is normally present at the input terminal for the VCM or the PCM when air conditioning is selected. This voltage is interrupted by either of two switches responding to conditions in the air conditioning refrigerant system. The A/C Compressor Low Pressure Cutout Switch provides for normal cycling of the compressor to maintain refrigerant low-side pressure. The A/C Compressor High Pressure Cutoff Switch serves a protective function to stop the compressor if the high-side pressure is excessive. During normal operations, the A/C Compressor Clutch should cycle on and off periodically depending upon the cooling load required. Excessively frequent cycling may reflect the lack of adequate refrigerant in the system.