With HVAC System - Electronic

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/Vent/Air Conditioning (HVAC) Control Head and the Control Head temperature setting.

The HVAC Control Head sends signals to the HVAC Programmer by way of the two Data Lines whenever a particular Heating or A/C mode is selected. Each mode has a unique signal which is decoded by the HVAC Programmer.

When an A/C or Defrost mode is selected, ground is applied to the Engine/Powertrain Control Module (ECM/PCM) A/C Request Signal input from the HVAC Programmer through the A/C High Pressure Cut-Out Switch (VIN J only) and the Pressure Cycling Switch. The ECM/PCM 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/PCM. The ECM/PCM uses this signal to adjust idle speed when the Compressor Clutch is engaged and also disengages the Compressor Clutch during full throttle operation.

The Compressor Clutch Diode is connected across the A/C Compressor Clutch. Whenever the Clutch is de-energized, an undesirable voltage is induced in the Clutch coil by the collapsing magnetic field. The Diode insures this voltage will be no more than 0.7 volt.

The Pressure Cycling Switch turns 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 opens the circuit to the A/C Request Input at the ECM/PCM. The ECM/PCM will then de-energize the A/C Clutch Relay, thus de-energizing the A/C Compressor Clutch. This switch also closes to cycle the Compressor on again when the evaporator temperature and pressure rise enough to require more cooling.

The A/C High Pressure Cut-Out Switch (VIN J only) will open if the pressure is above 2965 kPa (430 psi). This disengages the A/C Compressor Clutch and prevents the Engine from overheating.