Circuit/System Description
WARNING: This page is about a different car, the 2015 Buick Encore. However, it is still accessible from the selected car via links, so may be relevant.
When the A/C switch is pressed, the HVAC control module sends a serial data message to the engine control module (ECM) indicating the A/C request. In response to this message, the ECM grounds the A/C compressor clutch relay control circuit energizing the A/C compressor clutch relay. With the relay contacts closed, battery voltage is applied through the A/C compressor clutch control circuit activating the A/C compressor clutch.