Circuit System Description
WARNING: This page is about a different car, the 2019 GMC Savana 3500, 2019 GMC Savana 2500, 2019 Chevrolet Express 3500, and 2019 Chevrolet Express 2500. However, it is still accessible from the selected car via links, so may be relevant.
When the A/C is in the MAX A/C, A/C, Bi- Level, Defog, or Defrost position, the HVAC control assembly applies voltage to the A/C request signal circuit. This input will request the body control module (BCM) send a serial data message to the engine control module (ECM) to engage the A/C compressor. The ECM will then ground the A/C compressor clutch relay control circuit, which will switch the A/C CMPRSR CLUTCH relay on. With the relay contacts closed, battery voltage is supplied to the A/C compressor clutch assembly.