With the ignition switch in the ON position, system voltage is applied to the refrigerant pressure switch through the heater fuse. The refrigerant pressure switch closes when the pressure in the A/C system is between 193 kPa (28 psi) and 3137 kPa (455 psi). Voltage is then provided to the A/C condenser fan relay coil.

The A/C condenser fan relay coil is grounded by the A/C compressor control module, provided that the A/C compressor control module does not receive a low temperature signal from the evaporator temperature sensor.

When the relay is energized through the refrigerant pressure switch, battery voltage is applied to the A/C condenser fan through the A/C fuse and the contacts of the relay. Since the A/C condenser fan is permanently grounded at G108, the coolant fan operates as long as the relay is energized.

The A/C compressor control module monitors the evaporator temperature sensor that is mounted inside the evaporator housing. The evaporator temperature sensor converts the evaporator core temperature into a resistance value. Using this resistance value, the A/C compressor control module can sense when the evaporator temperature drops below 0° C (32° F). When this occurs, the A/C compressor control module removes the A/C condenser fan relay coil ground and the A/C clutch relay coil ground. This prevents evaporator frost and ice-up which reduces air flow and the units's cooling capacity.

Whenever the pressure in the A/C system drops below 193 kPa (28 psi) or rises above 3137 kPa (455 psi), the refrigerant pressure switch opens, the A/C condenser fan relay is de-energized and voltage to the A/C condenser fan is interrupted.