Circuit/System Description
WARNING: This page is about a different car, the 2022 Cadillac CT5. However, it is still accessible from the selected car via links, so may be relevant.
When compressor operation is needed for the HVAC system, the body control module sends a request message to the engine control module (ECM) via serial data. The ECM then grounds the air conditioning compressor clutch relay coil control circuit, closing the relay contacts. The closed relay contacts provide voltage to the compressor clutch causing it to engage.