Headlamp: Description and Operation

Principles of Operation (How Does It Work?)

NOTE: The front electronic module (FEM), rear electronic module (REM), and the instrument cluster must all be reconfigured before installing a new module. Refer to Module Communications Network (Information Bus).

The vehicle's electronic functions are divided into zones. The FEM controls the front portion of the vehicle and the REM controls the rear portion of the vehicle. These systems rely on the standard corporate protocol (SCP) communication network in order to transmit and receive signals. As a technician it is very important to understand:
- where the input (command) originates from.
- all information (messages) necessary in order for a feature to operate.
- which module(s) receive(s) the input or command message.
- does the module which received the input (message), control the output of the feature, or does it output a message over the SCP communication network to another module?
- which module controls the output of the feature.

Exterior Lighting
The exterior lighting outputs are zone-controlled by the front electronic module (FEM) and rear electronic module (REM) respectively. An additional function of the lighting system is a lamp outage function which provides indication to the driver if certain exterior bulbs are not functioning. Rear exterior lighting is powered by the switched system power feature. When diagnosing exterior lighting, it is essential to determine if all related symptoms and DTCs are controlled by the switched system power feature.

Headlamps
The headlamps are controlled by the FEM and the instrument cluster module. The headlamp and multifunction switches are hardwired to the instrument cluster. When the headlamp switch is in any position, the instrument cluster module will send a command to the FEM through the SCP communication network. The FEM will process this information and output the appropriate command to the headlamps, which are hardwired to the FEM. This is the same for the high beams and flash-to-pass functions, which are controlled by the multifunction switch.

The FEM may be configured to include autolamps and daytime running lamps (DRL) if equipped.

Left and right either, low or high beams will provide a lamp outage indication. Fault management provides some headlamp functionality. In the event of invalid or missing data, the low beams will be illuminated.


Autolamps
The autolamps are controlled by the headlamp switch, autolamp sensor, instrument cluster module and the FEM. Depending on the multifunction switch position, the headlamps are in either the high or low position. When the headlamp switch is in the autolamp position, the instrument cluster module will send an ON or OFF command to the FEM, depending on the signal from the autolamp sensor. The FEM will process this information and output the appropriate command to the headlamps, which are hardwired to the FEM. The autolamps will remain ON for approximately 20 seconds after the ignition switch is changed from the RUN position to the ACC or OFF position.

The FEM may be configured to include autolamps and daytime running lamps if equipped.

Left and right, low, or high beams will provide a lamp outage indication. Fault management provides some headlamp functionality. In the event of invalid or missing data failures, the low beams will be illuminated.


Switched System Power
The switched system power is invoked by the REM. This function removes power from relays that provide power to the exterior lamps and interior lamps. This is only accomplished when the REM is in sleep mode. The Sleep function of REM places the module in sleep mode when the ignition switch is in the OFF position, and no wake up (inputs) signals occur for 30 minutes. The module will not sleep if the parking lamps or the hazard lamps are active. When the REM is not in sleep mode, both switched system power relays will be energized. When energized, each relay will supply power to multiple features/functions. For additional information, go to the Switched System Power Index for diagnosis and testing of switched system power index related issues.