SWITCH-HEADLAMP
DESCRIPTION
The headlamp switch (1) is located on the instrument
panel, to the left of the steering column. Three differ-
ent switches are used. The standard switch features a
three detent position rotary knob (4) for exterior light-
ing control, a thumbwheel (2) for panel lamps dimming
and interior lighting control, and a momentary push
button (3) for cargo lamp control. An optional switch
has a momentary “Push” function added to the rotary
knob for front fog lamp control. A second optional
switch has the same thumbwheel and momentary
push button, but has a fourth detent position added to
the rotary knob for selecting the optional automatic
headlamps feature.
Each of these switches is constructed of molded plas-
tic. The rotary knob is molded plastic and knurled
around its circumference to ease operator control. On
models with optional fog lamps the rotary knob also
has the text “PUSH” and an International Control and
Display Symbol icon for “Front Fog Light” applied to it.
The thumbwheel is also plastic and knurled. The cargo
lamp push button is plastic with a smooth finish and the text “CARGO” 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.
OPERATION
The headlamp switch uses two resistor multiplexed outputs and one conventional output to control the many func-
tions 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, on a panel
lamps dimmer signal circuit to control panel dimmer and interior lighting functions, and on a cargo lamp return circuit
to control the cargo lamps.
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 head-
lamp 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:
•
Cargo Lamps Control - The cargo lamp push button on the headlamp switch is depressed to activate or
deactivate the cargo lamps. The headlamp switch provides an output to the EMIC, and the EMIC responds by
providing the appropriate cargo lighting control outputs through its internal cargo lamp driver circuit.
•
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 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
8L - 102
LAMPS/LIGHTING - EXTERIOR - SERVICE INFORMATION
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