Compressor Control

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 Head and the Control Head temperature setting. When an A/C or Defrost mode is selected, ground is applied to the Engine Control Module (ECM) A/C Request Signal input. The ECM then applies ground to one end of the A/C Clutch Relay coil energizing the Relay.

When energized, voltage is applied to one end of the A/C Compressor Clutch coil by way of the A/C Clutch Relay contacts. With this voltage applied, the clutch is engaged to the Engine.

The voltage that is applied to the A/C Compressor Clutch coil is also applied to the A/C "ON" input of the ECM. The ECM uses this signal to adjust idle speed when the Compressor Clutch is engaged.

The Compressor Clutch Diode is connected across the A/C Compressor Clutch. Whenever the Clutch is de-energized, an unwanted voltage is induced in the Clutch coil by the collapsing magnetic field. The Diode insures this voltage will remain at .7 volt, thus protecting the other electrical controls on the vehicle.

The Pressure Cycling Switch signals the ECM to turn the Compressor off when the evaporator temperature is low enough to cause icing. It opens when the refrigerant pressure is less than 170 kPa (25 psi). This switch also closes to signal the ECM to cycle the Compressor on again when the evaporator temperature and pressure rise enough to require more cooling.

The A/C Pressure Sensor will signal the ECM to disengage the A/C Compressor Clutch if the pressure is above 2965 kPa. This prevents the Engine from overheating.