Compressor Controls

The Compressor for the Air Conditioning System is driven by the Engine by means of a Belt Drive and the A/C Compressor Clutch. The Clutch allows the Compressor to be disengaged when Air Conditioning is not required.

When the Ignition Switch is in RUN, Battery Voltage is supplied to the Heater and A/C Control Module, the A/C Compressor Clutch Relay Coil, and the normally open A/C Compressor Clutch Relay contact. In the Heater and A/C Control Module, Battery Voltage is also applied to a detent contact on the temperature control dial for operation of the Water Valve (Suburban/Utility with gasoline engines only).

The A/C Compressor Clutch Coil must be energized to drive the A/C Compressor. The Clutch Coil is energized when the Clutch Relay is energized. Voltage is applied from the relay's contact on DK GRN (59) wire to the Clutch Coil. Diode D104 in the harness is reverse-biased and prevents current flow bypassing the Clutch Coil to ground. When the Relay Coil is de-energized, the collapsing magnetic field of the Clutch Coil forward-biases the Diode and dissipates the energy from the magnetic field.

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 only) or the Vehicle Control Module (VCM) (Gasoline only) provides this ground path through contact-equivalents in the module. These modules may stop compressor operations while otherwise enabled under certain operating conditions as described in the Driveability and Emissions sections of this manual.

The VCM or PCM modules enable A/C Compressor operations when a voltage input is received from the Heater and A/C Control Module on the DK GRN/WHT (762) wire. Two switch contacts in series between the Heater and A/C Control Module and the VCM/PCM module can isolate this voltage. The A/C High Pressure Cutout Switch monitors the compressor high side pressure. When this pressure exceeds a limit, the switch contacts open to stop the compressor and reduce the pressure. This switch does not operate unless conditions are unusual. The A/C Compressor Cycling Switch monitors compressor low side pressure to provide freeze protection. When the low side pressure reaches a limit, the switch contacts open. When the low side pressure reaches a limit, the switch contacts open. When the pressure recovers, the contacts close. This switch operates normally to cycle the compressor while operating.

The Heater and A/C Control Module places voltage on the LT GRN (66) wire when the Ignition Switch is in Run, the Front Blower Switch is not in OFF, and either the A/C Push-button Switch has been depressed or the mode Control Dial on the module is at the defrost detent. The A/C Push-button Switch is a latching switch which allows a light emitting Diode (LED) on the switch to be illuminated when the module enables compressor operations from the switch. The mode control Dial Detent Switch does not light the LED but does operate the compressor during defrosting operations to dry the air and prevent fogging.

The Heater and A/C Control Module's temperature control dial detent switch operates the Water Valve (Suburban/Utility with gasoline engines only). With the Ignition Switch in RUN, adjusting the dial to a low temperature setting applies a voltage on the GRY/BLK (1596) wire and energizes the valve's solenoid. This closed the valve which isolates Coolant Flow to the Heater Core.