Principles of Operation

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

The vehicle 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. 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.

Trailer Tow
The trailer lighting is controlled by the Trailer Tow Module (TTM) (if equipped). The architecture of the vehicle lighting system requires the TTM to be used for correct trailer lighting operation. The TTM controls the park, turn, and stoplamps of the trailer lighting when connected. Trailer lighting is powered through the trailer tow relay and Trailer Tow Auxiliary Junction Box (TTAJB). The TTM, which is hardwired to the REM, senses the vehicle's rear exterior park, turn, and stoplamps. For turn lamp operation, when either turn lamp is activated, the TTM will apply power to the appropriate TTM. For stoplamps operation, the TTM senses the vehicle right rear stoplamps only, and when activated, the TTM will apply power to both trailer stoplamps. For tail lamp operation, the TTM senses the vehicle right rear lamp only, and when activated, the TTM will apply power to the trailer tail lamps.

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. 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, go to the SSP Index Component Tests and General Diagnostics
for diagnosis and testing of SSP related concerns. Refer to Headlamps. Headlamp