Circuit Operation

CIRCUIT OPERATION
When the operator requests A/C operation by pressing the A/C switch in the HVAC control, or by turning the defrosters ON, a signal is sent to the Powertrain Control Module (PCM) requesting A/C operation, on circuit C103.

The PCM interprets this request and supplies a ground path for the A/C compressor clutch relay (coil side), providing the pressures and temperatures in the A/C system are in the proper ranges. The PCM looks at the engine coolant temperature and the HVAC control reads the evaporator temperature sensor.

Circuit C13 connects from the relay coil, ground side, to the PCM. Power for the coil side is supplied on circuit F87. This circuit is HOT in the START and RUN position only, and protected by a 9 amp Positive Temperature Coefficient (PTC) located in PTC location 1.

Power for the PTC is supplied on circuit A21. This circuit connects from the ignition switch to the PTC. Power for the A21 circuit is supplied on circuit A1. This circuit originates at the Power Distribution Center (PDC) and is protected by a 40 amp fuse located in cavity 5.

When the relay is energized, contacts in the relay CLOSE connecting circuit #10 and circuit C3. The #10 circuit is protected by a 15 amp fuse located in cavity 18 of the PDC. Circuit C3 connects from the relay to the A/C compressor clutch.

The A/C compressor clutch receives this current and creates a magnetic field, energizing the clutch. The ground path for the A/C compressor clutch is on circuit Z1.

The A/C compressor clutch has a diode located in it. This diode is used to control the induced Voltage resulting from the magnetic field collapsing when the clutch is disengaged.

The A/C compressor clutch relay is also used by the PCM to disengage the compressor in a Wide-Open Throttle (W.O.T.) condition.

To monitor the pressure in the system the PCM looks at the A/C pressure transducer. Circuit K6 is the 5 Volt feed from the PCM to the sensor, and circuit K4 is the ground back to the PCM. These circuits are spliced with many of the components used in the fuel/ignition system.

Circuit C18 is used as the sensor input to the PCM. This circuit connects to cavity 42 of the PCM connector.

When the system is in operation, an indicator is illuminated. This indicator is a Light Emitting Diode (LED). Power for the LED is supplied from the HVAC control, and ground is supplied on circuit Z2. This ground terminates at the instrument panel ground.