Compressor Controls

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 (Gasoline Engines) or by the Powertrain Control Module (Diesel Engine) upon A/C Request input to the control module. The control module energizes the clutch through the A/C Compressor Clutch Relay which can handle the clutch's current load.

The A/C Compressor Clutch Relay receives two sources of voltage. The A/C Mini Fuse provides voltage at all times to the A/C Compressor Clutch Relay's coil through the GRN (1240) wire. When the Ignition Switch is in Run, the IGN E Mini Fuse provides power to the A/C Compressor Clutch Relay's normally-open contacts through the PNK (639) wire. When the A/C Compressor Clutch Relay is energized, the normally-open contracts close to apply voltage through the DK GRN (59) wire to the A/C Compressor Clutch Coil. (For the 7.4L Gasoline Engine, this voltage may also be used to operate the Auxiliary Engine Cooling Fan). Since the A/C Compressor Clutch Coil 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 Clutch Relay. This prevents current from flowing though the diode to ground. When the A/C Compressor Clutch Coil is de-energized, the diode is forward biased allowing the coil's magnetic field to collapse without causing damage to the A/C Compressor Clutch Relay.

The A/C Compressor Clutch 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.

The Heater and A/C Controller's 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 Cutoff 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 Coil 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.