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.
The battery supplies voltage to the following components:
| • | The HVAC control module (when the ignition switch is in the RUN position). |
| • | The A/C compressor clutch relay coil (when the ignition switch is in the RUN position). |
| • | The normally open A/C compressor clutch relay contact. |
In the HVAC control module, the battery supplies voltage to a detent contact on the temperature control dial for operation of the water valve on the Suburban/Utility with the gasoline engine only.
To drive the A/C compressor clutch, the A/C compressor clutch must be energized. The relay contact on CKT 59 (DK GRN) supplies voltage to the clutch coil. Diode D104 in the harness is reverse-biased and prevents current flow bypassing the clutch coil to ground. The collapsing magnetic field of the clutch coil forward-biases the diode and dissipates the energy from the magnetic field when the relay coil is de-energized.
The A/C compressor clutch relay energizes when CKT 459 (GRN/WHT) provides a ground path provides for the relay coil. The powertrain control module (PCM for diesel vehicles only), or the vehicle control module (VCM) provides the ground path through contacts-equivalents in the control module. The control modules may stop compressor operations. The modules may enable compressor operations under certain operating conditions.
The VCM or the PCM enables A/C compressor clutch operations when a voltage input is received from the HVAC control module on CKT 762 (DK GRN/WHT). The 2 switch contacts between the HVAC control module and the VCM/PCM can isolate the voltage. The A/C compressor high pressure cutout switch monitors the compressor high side pressure. The switch contacts open in order to stop the compressor. This reduces the pressure when the pressure exceeds the limit. The switch does not operate unless conditions are unusual. The A/C compressor cycling switch compressor monitors low side pressure in order to provide freeze protection. The switch contacts open when the low side pressure reaches the limit. The contacts close when the pressure recovers. The switch operates normally in order to cycle the compressor while operating.
The HVAC control module supplies voltage through CKT 66 (LT GRN) when the ignition switch is in the RUN position. The front blower switch is NOT in the OFF position. The A/C push-button switch is depressed or the mode control dial on the module is the defrost detent. The A/C push-button switch is a latching switch which allows a light emitting diode, or 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 in order to dry the air and prevent fogging.
The HVAC control module's temperature control dial detent switch is used to control the water flow into the heater core, in the Suburban/Utility vehicles with gasoline engines only. With the ignition switch is in the RUN position, adjusting the dial to the coldest setting applies voltage on CKT 1596 (GRY/BLK) and energizes the water shut off valve's actuator. The actuator, ground at G105 through CKT 150 (BLK), when energized, opens sending engine vacuum to the water valve. The applied vacuum closes the valve isolating the coolant flow to the heater core providing maximum cooling.
