Compressor HVAC: Description and Operation

A/C COMPRESSOR CLUTCH

With the engine running, battery voltage from fuse 8 is applied to the heater-A/C control panel. With the blower switch on 1, 2, 3 or 4 and the A/C switch on, battery voltage is applied to the A/C-on indicator, thermo switch, A/C diode, fast idle solenoid valve, dual pressure switch, and the A/C compressor clutch. The A/C compressor clutch engages and the fast idle solenoid valve increases the engine idle adjustment to keep engine speed constant while the A/C is on. If either the thermo switch or dual pressure switch opens, the A/C compressor disengages.

THERMO SWITCH

The thermo switch senses evaporator temperature. With evaporator temperature above 41° F, the switch is closed. With evaporator temperature below 33.8° ± 1.8° F (1° ± 1° C), the switch opens. In this way, the evaporator temperature stays above 32° F (0° C), and moisture on the evaporator fins does not freeze and block air flow.

DUAL PRESSURE SWITCH

A/C system pressure below 28.4 psi (196.1 kPa) indicates a loss of refrigerant or refrigerant oil. The dual pressure switch opens with system pressure below 28.4 psi to prevent compressor damage. Also with the system pressure above 384 psi (2648 kPa), the dual pressure switch opens to prevent compressor damage.

QUICK CHECKS

1. Check main fuse 1 and fuse 8 if the A/C compressor clutch does not operate.
2. Refer to HEATING AND AIR CONDITIONING for A/C refrigerant pressure, A/C compressor belt tension, and heater-NC control panel cable adjustment.