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.

Autolamps
The autolamps are controlled by the headlamp switch, autolamp sensor (also referred to as the photocell and amplifier), instrument cluster and the FEM. The headlamps may be in the high or low beam position depending on the multifunction switch position. When the headlamp switch is in the autolamp position, the instrument cluster will send an ON or OFF command to the FEM, depending on the signal from the auto lamp 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.

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. 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.