Compressor HVAC: Description and Operation

With the A/C-Heater Select Control in any mode except VENT or HEAT, current flows form the HTR A/C Fuse through the Evaporator Pressure Control Switch to the A/C Compressor Clutch and the A/C Compressor operates. For TBI engines, a signal is sent at the same time to the Electronic Control Module (ECM). This A/C request signal increases engine idle speed.

If the A/C System pressure drops below 25 psi, the Evaporator Pressure Control 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 7.4L V8 gasoline engine is equipped with two additional compressor cut-out switches. If the engine temperature gets too high (usually during idle after high loading conditions), these switches shut off the A/C Compressor, reducing the load on the engine and preventing engine overheating.

One switch, a 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 closes allowing the compressor to reengage.

The second switch, the High Pressure Cutout Switch is located in the back of the A/C Compressor. This switch senses copressor outlet pressure. When the pressure exceed 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 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.