For 1990-2009 cars

The audio steering wheel control switches duplicate the function of the primary controls of the associated component through a network of multiple momentary contact switches and a series of resistors. The radio supplies voltage to the switches and monitors the return signal for specific voltage drop across the resistors. The radio identifies the switch selected and activates the feature. Otherwise the radio sends a serial data message to the component controlled by the switch activating the feature.

This section is intended to diagnose the circuits between the radio and the audio steering wheel control switches. If the primary control for the device is inoperative, refer to the appropriate section for the component.