Switch, Headlamp: Operation

The headlight 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) on a headlight switch MUX return circuit. The BCM then reads the switch outputs using an internal pull up on the headlight/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. Refer to SWITCH, DIMMER, OPERATION .

The headlight switch operates as follows:

• Front Fog Lamps Control  - For vehicles so equipped, the rotary knob on the headlight switch is depressed to activate or deactivate the optional front fog lamps. The headlight switch provides the appropriate resistor multiplexed output to the BCM over the headlight/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 Panel Cluster (IPC) over the Controller Area Network (CAN) data bus, which controls the front fog lamp indicator as appropriate.
• Exterior Lighting Control  - The rotary knob on the headlight switch is rotated to a detent position to activate or deactivate the exterior lighting. The headlight switch provides the appropriate resistor multiplexed output to the BCM over the headlight/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 IPC over the CAN data bus, which controls the high beam indicator as appropriate. The BCM also remembers which headlight beams were last selected with the multi-function switch, and energizes those beams by default the next time the headlights are turned ON. If the vehicle is equipped with optional automatic headlights and the AUTO  (Automatic) position is selected, the BCM monitors an electronic ambient light level  message received over the Local Interface Network (LIN) data bus from the Light Rain Sensor Module (LRSM) to turn the exterior lighting ON and OFF automatically while the ignition switch status is ON. If the vehicle is not equipped with a LRSM but is equipped with auto head lamps, the BCM also monitors for an electronic ambient light level  message received over the CAN data bus from the air conditioner - heater control module based upon a hardwired input from the sun sensor. On Canadian vehicles, automatic headlamps are standard equipped. The Canadian headlamp switch does not include a selection for auto headlamps.

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