Principles of Operation
WARNING: This page is about a different car, the 2009 Mercury Milan, 2009 Lincoln MKZ, and 2009 Ford Fusion. However, it is still accessible from the selected car via links, so may be relevant.
NOTE:
The smart junction box (SJB) is also known as the generic electronic module (GEM).
For vehicles equipped with an automatic transaxle, the transmission control module (TCM) sends gear status information (through the instrument cluster [IC] gateway) to the SJB over the communication network. When the ignition switch is in the RUN position and a message is sent over the communication network stating the transaxle is in REVERSE (R), the SJB energizes an internal relay which supplies voltage to the reversing lamps.
For vehicles equipped with a manual transaxle, the SJB supplies voltage to the reversing lamp switch when the ignition switch is in the RUN position. When the transaxle is placed in REVERSE (R), the switch routes the voltage to the reversing lamps.