With the ignition switch on, voltage is supplied through the HVAC fuse and circuit 41 (BRN) to the heater-A/C control. The voltage from circuit 1199 (DK BLU) is varied from B+ to approximately 1.5V by the temperature control potentiometer. The heater-A/C control receives ground from G 200 through circuit 1450 (BLK).

Control of air through the system is regulated by vacuum. At various positions on the heater-A/C control, mode valves mix and direct cooled, heated and outside air through the air ducts.