Principles of Operation

NOTE: The Front Electronic Module (FEM), the Rear Electronic Module (REM) and the instrument cluster must 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 heavily on the Standard Corporate Protocol (SCP) communication network in order to transmit and receive signals. 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 FEM and REM. The front exterior lights are controlled by the FEM and the rear exterior lights are controlled by the REM. The headlamp and multifunction switches are hardwired to the instrument cluster. When the lamps are commanded ON or OFF, the instrument cluster sends the command for the exterior lamps through the standard corporate protocol (SCP) network to the FEM and REM. To provide complete input and output functionality, each exterior lamp feature will be described individually. All exterior lighting is powered by the Switched System Power (SSP) feature (refer to Switched System Power). A failure of all or any of the SSP features will cause inoperative exterior lighting. When diagnosing exterior lighting, it is essential to determine if all related symptoms and DTCs are controlled by the SSP feature.

Headlamps
The headlamps are controlled by the FEM and the instrument cluster. The headlamp multifunction switch is hardwired to the instrument cluster. When the headlamp switch is in any position, the instrument cluster will send a command to the FEM via 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 low and high beams and flash-to-pass positions. Daytime Running Lamps (DRL) is an additional feature which the FEM may be configured to include. Left and right low, as well as high beams, will provide a lamp outage indication. Fault management of the headlamps will provide some headlamp functionality. In the event of all multifunction switch, all headlamp switch or ignition switch invalid or missing data failures, the low beams will be illuminated. There is also a headlamp battery saver feature. This feature turns off the headlamps in 10 minutes after the ignition is turned off.

Low Beam Headlamps
The FEM receives commands from the instrument cluster to control the low beam headlamps. The FEM regulates the voltage to the headlamps using pulse-width modulation.

High Beam Headlamps
The REM receives commands from the instrument cluster to control the high beam headlamps. The REM regulates the voltage to the headlamps using pulse-width modulation.

Switched System Power (SSP)
The SSP is invoked by both the FEM and REM. This function removes power from relays that provide power to the exterior lamps, interior lamps, and power door locks. This is only accomplished when both the FEM and the REM are in sleep mode. The sleep function of the FEM and REM places the modules in sleep mode when the ignition switch is in the OFF position, and no wake up (input) signals occur for 30 minutes. If a state change does occur, the 30 minute timer will reset and begin again. The module will not sleep if the parking lamps or the hazard lamps are active. The following relays are controlled by the SSP function: SSP1, SSP2, SSP3, and SSP4. When either the FEM or REM are not in sleep mode, all SSP relays will be energized. When energized, each relay will supply power to multiple features/functions. For additional information, refer to the SSP Index for diagnosis and testing of SSP related issues. When diagnosing multiple symptoms and/or DTCs it is essential to determine if all are related to a single SSP or multiple SSP relay powered functions.