Lamps/Lighting - Exterior (Service Information): Operation: Headlamps
The headlamp system includes the Steering Column Control Module (SCCM) microcontroller, the Electro-mechanical Instrument Cluster (EMIC) and the Body Control Module (BCM), the headlamp switch on the instrument panel and the multi-function switch on the steering column. On vehicles with the optional High Intensity Discharge (HID) headlamps, the HID electronic ballast module of each front lamp unit is also part of the headlamp system. Each halogen headlamp bulb and the HID electronic ballast module has a path to ground at all times through their connection to the FEM wire harness. The FEM wire harness has takeouts with eyelet terminals that are secured by ground screws to the front end sheet metal within the engine compartment. The BCM will store a Diagnostic Trouble Code (DTC) for any shorts or opens in the headlamp circuits.
The EMIC monitors a hardwired multiplex input from the headlamp switch to determine the status of the headlamp switch. The SCCM microcontroller monitors a hardwired multiplex input to determine the status of the multi-function switch and whether the headlamp high or low beams are selected. The SCCM microcontroller then sends the appropriate electronic headlamp beam select switch status messages to the BCM over the CAN data bus. The EMIC then sends the appropriate electronic headlamp request messages to the BCM over the CAN data bus. The BCM responds to these messages by providing a Pulse Width Modulated (PWM) voltage output to the proper halogen headlamp bulbs or a control voltage to the HID electronic ballast modules through high side drivers on the right and left low and high beam feed circuits to illuminate the selected headlamp beams. The BCM also sends the appropriate electronic messages to the EMIC to control the illumination of the high beam indicator.
On vehicles with the optional HID headlamps, the HID electronic ballast module uses a high voltage Alternating Current (AC) output to activate the HID igniter integral to the lighting element to provide a high voltage surge, which creates a light arc between the lighting element electrodes. Once the electronic ballast module detects a suitably stable light arc, it switches over to a power-limiting mode to sustain the light arc. Also an electric motor drives a shutter within each HID lamp unit and the position of the shutter controls whether the low beams (shutter closed) or high beams (shutter open) are illuminated.
When the optical horn feature is selected, the low beams will shut OFF about 200 milliseconds after the high beams are activated. The BCM also remembers which beams (LOW or HIGH) were selected when the headlamps were last turned OFF, and energizes those beams again the next time the headlamps are turned ON.
The BCM provides a battery saver (load shedding) feature for the headlamps, which will turn these lamps OFF if they are left ON for more than about eight minutes after the electronic ignition switch status messages from the WIN transition to LOCK. The EMIC and the BCM each provide a fail-safe feature for the headlamps, which will cause the BCM to turn the low beam headlamps ON automatically if there is no input available from the headlamp switch. The BCM also provides a fail-safe feature for the headlamps that will turn the headlamps ON automatically whenever a loss of CAN bus communication is detected while the electronic ignition switch status messages from the WIN indicate the ignition ON position is selected.
Each headlamp includes an integral reflector adjustment screw to be used for static aiming of the headlamps.