Heating and Air Conditioning: Description and Operation

When the ignition switch is "ON", battery voltage is applied to both the A/C switch and to the contacts of the A/C cut relay.

When both the A/C switch and blower switch are turned on, battery voltage and ground are applied to the [1][2]A/C relay, energizing it. This relay then applies voltage from the ADD fuse to both the condenser fan motor, powering it, and to the ECM. Voltage to the ECM passes through the thermostat and the pressure switch, and if they are both closed the ECM will energize the A/C cut relay. This relay will then apply voltage to the compressor's magnetic clutch, energizing it, and thereby energizing the compressor. If the thermostat detects an icing condition, or the pressure switch detects an over-pressure condition, then voltage to the ECM will be removed. When this occurs the ECM will de-energize the A/C cut relay, which will in turn de-energize the compressor clutch.