Compressor HVAC: Description and Operation

A/C COMPRESSOR CLUTCH
With the ignition switch in START and the inhibitor switch in PARK or NEUTRAL (automatic transaxle), or the clutch pedal depressed (manual transaxle), voltage is applied to the restart relay and the relay energizes. When the ignition switch is turned to RUN, the relay deenergizes and voltage is applied from the alternator to the heater and A/C relay, through the normally closed contacts of the restart relay. The relay energizes, allowing voltage to be applied from the main fuse to the A/C switch, the dual pressure switch, and the A/C compressor relay through fuse 2. When the A/C switch is switched on, voltage is applied to the coil of the thermo switch relay. Ground is provided to the coil of this relay through the electro thermo unit and the blower switch (XS) (provided the blower switch is not in the OFF position) or temperature lever and fan control unit (RS).

When ground is provided, the relay energizes and voltage is applied through the dual pressure switch to the A/C compressor relay and the electronic control module. Ground is provided to the A/C compressor relay by the electronic control module and the relay energizes. Power is provided to the A/C compressor clutch and the clutch engages.


A/C COMPRESSOR TEMPERATURE SWITCH
If the A/C compressor's temperature rises above 302 ± 20° F (150 ± 7° C) the A/C compressor temperature switch will open, disengaging the A/C compressor clutch. The A/C compressor temperature switch will close when the temperature falls below 275 ± 20° F (135 ± 7° C). Power is reapplied to the A/C compressor clutch and the clutch engages.


DUAL PRESSURE SWITCH
A/C system pressure below 28 psi (196 kPa) indicates a loss of refrigerant or refrigerant oil. The dual pressure switch opens with system pressure below 28 psi to prevent compressor damage. Also, with system pressure above 386 psi (2667 kPa), the dual pressure switch opens to prevent compressor damage.

ELECTRO THERMO UNIT
The electro thermo unit senses the temperature of the cool air from the evaporator and the evaporator fins by means of the evaporator temperature sensor. This information is input by the electro thermo unit, which determines whether ground is to be provided to the thermo switch relay. When the temperature becomes too low, ground is not provided.