Compressor Clutch Circuit Diagnosis - HVAC System - Manual

CIRCUIT DESCRIPTION
The compressor for the air conditioning (A/C) system is driven by the engine by means of a drive belt for all engine applications. The clutch allows the compressor to disengage when the A/C is not required. When the ignition switch is in RUN, battery voltage is supplied to the following:
- The HVAC control module
- The A/C compressor clutch relay coil
- The normally open A/C compressor clutch relay contact

The A/C compressor clutch coil must be energized in order to drive the A/C compressor clutch. The clutch coil is energized when the clutch relay is energized. Voltage is applied from the relays contact on CKT 59 (DK GRN) to the clutch coil. The diode in the harness is reverse-biased. The diode prevents current flow from 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. The energy from the magnetic field dissipates

The A/C compressor clutch relay is energized when a ground path is provided for the relay coil on CKT 459 (DK GRN/WHT). The PCM/VCM provides the ground path through the module only when the module receives an A/C request signal from the HVAC module. The PCM/VCM modules may stop the A/C compressor operation while otherwise enabled under certain operating conditions as described in the powertrain engine controls sections.

When the PCM/VCM receives the A/C request signal from the HVAC module through CKT 203 (LT BLU) through the A/C high pressure cutout switch and CKT 762(DK GRN/WHT), the PCM/VCM will source circuit 459 to ground. This closes the relay contact and supplies battery voltage to the A/C compressor clutch coil. The A/C compressor high pressure cutout switch, located at the rear of the compressor, monitors the compressor high side pressure. When this pressure exceeds a preset limit, the switch contacts open in order to interrupt the request signal from the HVAC module. The A/C compressor disengages. The A/C low pressure cycling switch monitors evaporator low side pressure in order to provide freeze-up protection. When the low side pressure reaches a preset limit, the switch contacts open. The PCM/VCM monitor the open contacts. The ground to the A/C relay is removed. The A/C compressor disengages. When the pressure recovers, the low pressure switch contacts close. The normal operation of the A/C compressor resumes.

The A/C automatic recirculating switch is a secondary high pressure switch, located at the lower right front of the A/C condenser, that will close when high side pressure is detected. The PCM/VCM monitors this switch operation. The PCM/VCM will place the A/C air delivery system in a recirculating mode until the high side pressure returns to normal.

The HVAC control module consists of the following components:
- A 6 way blower switch
- A rotary air delivery switch
- An air mix rotary control switch
- An air source discrete push buttons that select between outside and recirculated air
- An A/C compressor discrete request button and a rear defrost discrete push button with a built in timer

The HVAC control module also has LEDs (light emitting diodes) for the A/C request and defrost that will illuminate when selected.