Headlamp Switch - Operation

OPERATION

The headlamp switch uses a single resistor multiplexed output to control the many functions and features it provides. The switch receives a clean ground from the Body Control Module (BCM) (also known as the Common Body Controller/CBC) on a headlamp switch MUX return circuit. The BCM then reads the switch outputs using an internal pull up on the headlamp/fog lamp switch signal circuit to control the exterior lighting functions.

The switch illumination circuit receives a path to ground at all times through an instrument panel ground circuit. The illumination level is controlled by a Pulse-Width Modulated (PWM) output received from the BCM on a panel lamps driver circuit. The BCM controls this output based upon the panel lamps dimmer signal circuit input from the interior lighting thumbwheel switch in the dimmer module. Dimmer Switch - Operation.

The headlamp switch operates as follows:

- Front Fog Lamps Control - For vehicles so equipped, the rotary knob on the headlamp switch is depressed to activate or deactivate the optional front fog lamps. The headlamp switch provides the appropriate resistor multiplexed output to the BCM over the headlamp/fog lamp switch signal circuit, and the BCM reads and responds to this input by energizing or de-energizing the right and left front and rear fog lamp control circuits through internal High Side Drivers (HSD) and by sending an electronic confirmation message to the Instrument Cluster (IC) (also known as the Common Instrument Cluster/CIC) over the Controller Area Network (CAN) data bus, which controls the front and rear fog lamp indicators as appropriate.
- Exterior Lighting Control - The rotary knob on the headlamp switch is rotated to a detent position to activate or deactivate the exterior lighting. The headlamp switch provides the appropriate resistor multiplexed output to the BCM over the headlamp/fog lamp switch signal circuit, and the BCM responds by energizing or de-energizing the right and left park lamp feed circuits and the right and left high or low beam driver circuits through internal HSDs and by sending an electronic confirmation message to the IC over the CAN data bus, which controls the high beam indicator as appropriate. The BCM also remembers which headlamp beams were last selected with the multi-function switch, and energizes those beams by default the next time the headlamps are turned ON. If the vehicle is equipped with optional automatic headlamps and the AUTO (Automatic) position is selected, the BCM also monitors an electronic ambient light level message received over the CAN data bus from the air conditioner - heater control module based upon a hard wired input from the sun sensor to turn the exterior lighting ON and OFF automatically while the ignition switch is in the ON position.

The headlamp switch as well as the hard wired inputs and outputs of the switch may be diagnosed using conventional diagnostic tools and procedures. Refer to the appropriate wiring information.