The A/C compressor is belt-driven. The A/C compressor's pulley rotates whenever the engine is in operation. The A/C compressor only operates when the A/C compressor clutch is energized. In a gasoline engine vehicle, the vehicle control module (VCM) energizes the clutch upon A/C request input to the control module. The powertrain control module (PCM) in a diesel engine vehicle energizes the clutch upon A/C request input to the control module. The control module energizes the clutch through the A/C compressor enable relay, which handles the current load of the clutch.

The A/C compressor enable relay receives two sources of voltage. A/C fuse provides voltage at all times to the normally-open contacts of the A/C compressor enable relay through CKT 1240 (ORN). When the ignition switch is in the RUN position, IGN-E fuse provides power to the coil of the A/C enable relay through CKT 639 (PNK). When the A/C compressor enable relay is energized, the normally-open contacts close in order to apply voltage through CKT 59 (DK GRN) to the A/C compressor clutch. Because the A/C compressor clutch grounds through CKT 150 (BLK), the coil energizes when voltage is available.

Diode D102 is reverse-biased when voltage is available through the A/C compressor relay. The reverse bias prevents current from flowing through the diode to ground. When the A/C compressor clutch de-energizes, the diode is forward-biased, allowing the magnetic field of the coil to collapse without causing damage to the A/C compressor relay.

The A/C compressor enable relay energizes when CKT 459 (GRN/WHT) provides a ground path for the relay coil. The PCM on a vehicle with a diesel engine or the VCM on a vehicle with a gasoline engine provides the ground path through contact-equivalents in the module. This ground connection is open under normal circumstances when the control module senses the A/C request voltage in CKT 762 (DK GRN) from the heater and A/C controller. The control module may stop the compressor operations under certain operating conditions as described in Engine Controls for specific engines.

The heater and A/C controller mode switch receives power from HTR-A/C fuse 12. CKT 141 (BRN) provides this voltage when the ignition switch is in the RUN position. The A/C request voltage is available in CKT 66 (LT GRN) when the mode switch is in the following positions:

The A/C request voltage is normally present at the input terminal for the VCM or the PCM when you select air conditioning. Either one of two switches responding to conditions in the air conditioning refrigerant system interrupts the A/C request voltage. The A/C compressor low pressure cutout switch provides for the normal cycling of the compressor in order to maintain refrigerant low-side pressure. The A/C compressor high pressure cutoff switch serves a protective function by stopping the compressor when the high-side pressure exceeds the limits. During normal operations, the A/C compressor clutch periodically cycles on and off depending upon the cooling load required. Very frequent cycling may reflect the lack of adequate refrigerant in the system. Refer to Relays and Sensors Description .