Radio Circuit Operation

The main radio power is supplied by the 10-amp RADIO fuse located in the body control module (BCM).
The radio does not use a discrete ignition feed circuit for power moding. The power mode master (PMM) provides the system power mode to the radio via GMLAN signals. The PMM determines the system power mode by processing power mode information from ignition switch inputs. GMLAN vehicle power modes supported by the radio are OFF, ACCESSORY, RUN, and CRANK REQUEST.

The main radio ground provides a ground for the radio circuits. The radio is also case grounded.

The radio communicates with other modules via the GMLAN data bus which is a CAN based protocol. This vehicle uses 2 variants of the GMLAN data bus. The variants are a low speed and high speed vehicle bus. The radio connects to the low speed vehicle bus which runs between 12.5-40 kbps.

At minimum volume, the plus (+) and minus (-) speaker outputs are approximately 5-6 volts, measured to vehicle ground. If a plus or minus for any speaker output is shorted to ground or voltage, the radio circuitry will turn OFF the affected speaker output for component protection and set a diagnostic trouble code (DTC). As the volume increases, the plus and minus change to create a voltage difference between each other driving the voice coil of the speaker.

Dimming and backlighting levels are determined by the GMLAN signal data the radio receives. The radio sets the backlight and VF display dimming to the value indicated by the interior dimming level and interior dimming display level signals in the GMLAN dimming information frame.
The radio provides faceplate and control/graphics backlighting in the OFF (RAP inactive) when the exterior lighting VN is activated. The radio display is consistent then with the surrounding devices that use analog PWM dimming to backlight their control/graphics when the power mode is OFF and the park lights are ON.