Air Conditioning System Circuits

How The Circuit Works
Two electrical circuits in the air conditioner system control the devices that cool conditioned air. One circuit controls the A/C condenser fan motor and the other controls the A/C compressor clutch.


A/C Condenser Fan Motor
The A/C relay controls the A/C condenser fan motor. With the ignition switch in "Run" and the A/C switch on the heater control panel closed, the relay coil is energized. Current flows through the relay coil, the A/C diode box, the A/C pressure cutout switch, the A/C thermostat and the A/C switch to ground: The relay contacts close and the fan motor operates.

A/C Compressor Clutch
The A/C compressor clutch is controlled by the A/C compressor clutch relay. The A/C delay relay controls the compressor clutch relay. The delay relay has two contacts: One is coil controlled and the other is solid- state controlled.
With the ignition switch in "Run" and the A/C switch on the heater control panel closed, the coil in the A/C delay relay is energized. Current flows through the relay coil, the A/C diode box, the A/C pressure cutout switch, the A/C thermostat, and the A/C switch to ground. The coil-operated contacts in the delay relay close and the idle boost solenoid operates: The engine idle speed increases to compensate for the extra load of the A/C compressor.
The solid-state controlled contacts close .7 of a second after the delay relay coil is energized. Current flows through the A/C compressor relay coil and the solid-state controlled contacts of the delay relay to ground G102. The contacts in the compressor clutch relay close and the compressor clutch engages.

A/C Pressure Cutout Switch
A/C system pressure below 28 psi indicates a loss of refrigerant or refrigerant oil. The A/C pressure cutout switch opens with system pressure below 28 psi to prevent compressor damage from loss of refrigerant.

A/C Thermostat
The A/C thermostat senses evaporator temperature. With evaporator temperature above 41 degrees F, the switch closes to engage the A/C compressor clutch. With evaporator temperature below 33 degrees F, the A/C thermostat switch opens. In this way, the evaporator temperature stays above 32 degrees F and moisture on the evaporator fins does not freeze and block air flow.

A/C Diode Box
The diodes in the A/C diode box keep the compressor from running when the radiator fan temperature switch closes, and the radiator fan goes on.