Headlamp Switch: Description and Operation

The headlamp switch (1) is located on the instrument panel, to the left of the steering column. Three different switches are used. The standard switch features a three-position rotary knob (4) for exterior lighting control and a thumbwheel (2) for panel lamps dimming and interior lighting control. An optional switch has a momentary push button (3) added for front fog lamp control. A second optional switch has the same thumbwheel and momentary push button, but has a fourth position added to the rotary knob for selecting the optional automatic headlamps feature.

Each of these switches is constructed of molded plastic. The rotary knob is molded plastic and knurled around its circumference to ease operator control. The thumbwheel is also plastic and knurled. The optional front fog lamp push button is plastic with a smooth finish and an International Control and Display Symbol icon for "Front Fog Light" applied to it. The switch face plate is also labeled with graphics and icons to clearly identify the many functions of the rotary knob and thumbwheel.

Three screws secure the switch to the back of the cluster bezel through integral mounting flanges that are molded to each side of the switch housing. The back of the switch housing has an integral connector receptacle containing terminal pins that connect the switch to the vehicle electrical system through a dedicated take out and connector of the instrument panel wire harness. A panel dimmer controlled incandescent bulb soldered to the circuit board within the switch provides back lighting for visibility at night, but is not serviceable. The headlamp switch cannot be repaired and, if faulty or damaged, it must be replaced.

The headlamp switch uses two resistor multiplexed outputs to control the many functions and features it provides. The switch receives a clean ground from the ElectroMechanical Instrument Cluster (EMIC) (also sometimes referred to as the Cab Control Node/CCN) on a headlamp switch return circuit. It then provides outputs to the EMIC on a headlamp switch signal circuit to control exterior lighting functions, and on a panel lamps dimmer signal circuit to control panel dimmer and interior lighting functions.

The switch illumination circuit receives a path to ground at all times through the left instrument panel ground circuit. The illumination level is controlled by a Pulse-Width Modulated (PWM) output received from the EMIC on a headlamp switch illumination control circuit. The EMIC controls this output based upon the dimmer signal select mux input from the headlamp switch.

The headlamp switch operates as follows:
- Front Fog Lamps Control - For vehicles so equipped, the fog lamp push button on the headlamp switch is depressed to activate or deactivate the optional front fog lamps. The headlamp switch provides an output to the EMIC, and the EMIC responds by sending electronic fog lamp switch status messages to the Front Control Module (FCM) over the Controller Area Network (CAN) data bus. The FCM responds by energizing or de-energizing the front fog lamp relay in the Power Distribution Center (PDC).
- 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 an output to the EMIC, and the EMIC responds by sending electronic exterior lighting switch status messages to the FCM over the CAN data bus. The FCM responds by energizing or de-energizing the park lamp relay in the PDC and the high or low beam headlamp circuits. The FCM remembers which headlamp beams were last selected using 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 position is selected, the EMIC also monitors an input from a sun load sensor on the instrument panel and, based upon the monitored ambient light levels, responds by automatically sending appropriate messages to the FCM to turn the exterior lighting On or Off while the ignition switch is in the On position.
- Interior Lighting Control - The thumbwheel on the headlamp switch is rotated to the dome defeat, dome on, parade/funeral mode, or one of the six panel dimmer detent positions to control the interior courtesy/dome and panel lamps. The headlamp switch provides an output to the EMIC, and the EMIC responds by providing the appropriate interior lighting control outputs through its internal courtesy lamp driver circuits, electronic dimming level messages to other modules over the CAN data bus, and/or the proper PWM outputs to control dimming levels through several panel dimmer illumination control driver circuits.

The headlamp switch can be diagnosed using conventional diagnostic tools and methods. However, proper testing of the multiplexed inputs to and PWM processing of the EMIC requires the use of a diagnostic scan tool.