Compressor HVAC: Description and Operation

With the A/C-Heater Select Control in any mode except VENT or HEAT current flows from the HTR A/C fuse through the A/C Low Pressure Cutout to the A/C Compressor Clutch and the A/C Compressor operates. Also a signal is sent at the same time to the Transmission Control Module (TCM) or Powertrain Control Module (PCM). This A/C Request signal increases engine idle speed.

If the A/C system pressure drops below 25 psi, the A/C Low Pressure Cutout Switch opens stopping current flow to the A/C Compressor. When the A/C system pressure rises to 45 psi, the switch closes and current again flows to the A/C Compressor.

The High Pressure Cutout Switch located in the back of the A/C Compressor senses compressor outlet pressure. When the pressure exceeds 2,827-3,103 kPa (410-450 psi), the compressor is shut off until the pressure drops below 1,448-1,724 kPa (210-250 psi).

The 7.4L V8 gasoline engine is equipped with an additional compressor cut-out switch. If the engine temperature gets too high (usually during idle after high loading conditions), this switch shuts off the A/C Compressor, reducing the load on the engine and preventing engine overheating.

The Engine Coolant Temperature Cutout Switch, is located in the thermostat housing. When the coolant in the top tank reaches 124°C (255°F), the switch shuts off the compressor. When the coolant temperature drops below 116°C (240°F), the switch allows the compressor to engage.

The A/C Compressor Clutch Diode is connected across the terminals of the A/C Compressor Clutch. Whenever the clutch is de-energized, the magnetic field around it collapses, generating an induced voltage in the clutch coil. The Diode provides a path for the current resulting from the induced voltage so that other circuit components are not damaged.