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 condenser fan 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 dual A/ C pressure cutout switch, the A/C thermostat, the A/ C switch, and blower switch to ground. The relay contacts close and the fan goes on.

A/C Compressor Clutch
The A/C compressor clutch is controlled by the clutch relay. The A/C delay relay controls the 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 dual A/C pressure cutout switch, the A/C thermostat, the A/C switch and blower switch to ground. The coil operated contacts in the delay relay close and the idle boost solenoid operates. This solenoid increases the engine idle speed 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, the A/C temperature switch (DX, Automatic) 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 operates.

Dual A/C Pressure Cutout Switch
A/C system pressure below 28 psi indicates a loss of refrigerant or refrigerant oil. The dual A/C pressure cutout switch opens with system pressure below 28 psi to prevent compressor damage from loss of refrigerant. The dual A/C pressure cutout switch also opens with system pressure above 384 psi to prevent damage from high pressure.

A/C Thermostat
The A/C thermostat senses evaporator temperature. With evaporator temperature above 41 degrees F, the switch closes to activate 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 operating when the radiator fan temperature switch closes, and the radiator fan goes on.