Standard HVAC
WARNING: This page is about a different car, the 2003 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.
A/C request is made by the HVAC control head supplying voltage to circuit 66 to the BCM. The BCM will then request over the serial data link to the engine controller to operate the compressor. The BCM provides evaporator freeze protection by monitoring engine speed and ambient temperature and evaporator temperature from the A/C sensor.