With Electronic A/C

CIRCUIT OPERATION
The Compressor for the Air conditioning system is belt-driven by the engine through the A/C Compressor Clutch. The Clutch allows the Compressor to be disengaged when Air Conditioning is not required or when the Air Conditioning load should be removed from the engine, such as during full throttle.

Whether or not the A/C Compressor should operate depends on both the particular mode selected at the Heater and A/C Control Assembly and the Control Assembly temperature setting. When the A/C Compressor is not running, the A/C Clutch Control Signal at Terminal C3 of the Heater and A/C Control Assembly is at 12 volts. When the Clutch is to be engaged, the voltage is at approximately 5 volts.

The A/C Clutch control signal is applied to Terminal C on Connector C2 of the Blower Control module. The Blower Control Module contains the circuits for operating the A/C Compressor Clutch. Voltage is applied to the Clutch through the A/C Fuse and ground is provided by the circuits in the Blower Control Module.

The Compressor Clutch Diode is connected across the A/C Compressor Clutch. Whenever the Clutch is de-energized, a voltage is induced in the Clutch coil by the collapsing magnetic field. The Diode provides a path for this voltage. If the Diode is not connected, the induced voltage can damage the A/C Compressor Clutch Control Circuit in the Blower Control Module.

The A/C High Pressure Cut-Out Switch and Pressure Cycling Switch are both closed under normal conditions and maintain Terminal C1 on the Heater and A/C Control Assembly at ground. Should either switch open, Terminal C1 will rise to the 5 volt level through the pull-up resistor. Whenever this happens, the circuits in the Heater and A/C Control Assembly will apply 12 volts to Terminal C3 which signals the Blower Control Module to open the circuit at Terminal A to disengage the Clutch.

The pressure cycling switch opens when refrigerant pressure drops below about 172 kPa (25 psi). it closes again when refrigerant pressure rises above about 324 kPa (47 psi). When the pressure cycling switch is open, the heater and A/C control assembly will disengage the compressor. Under control of the switch, the A/C compressor will be cycled on and off to prevent the refrigerant from cooling the evaporator to a point where icing will occur.

When the A/C compressor is turned on by the heater and A/C control assembly, a 12 volt level is present at Terminal C6 and is applied to the Electronic Control Module (ECM). This signals the ECM to increase the idle speed to compensate for the additional load of the compressor.