Steering Wheel Switches
WARNING: This page is about a different car, the 2014 Ford F-150. However, it is still accessible from the selected car via links, so may be relevant.
The RH steering wheel switches contain series of resistors. Each steering wheel switch function corresponds with a specific resistance value within the switch.
The SCCM sends a 5-volt reference voltage to the RH steering wheel switch on the input circuits. When a switch is pressed, the voltage is routed through the corresponding resistor(s), and then to ground through the SCCM . The SCCM monitors the resultant voltage drop to determine which switch was pressed. The voltage drop varies depending upon the resistance of the specific switch pressed.
The SCCM sends a network message to the APIM for the SYNC® switches and to the ACM for the redundant audio controls.