Principles of Operation
WARNING: This page is about a different car, the 2005 Mercury Montego, 2005 Ford Freestyle, and 2005 Ford Five Hundred. However, it is still accessible from the selected car via links, so may be relevant.
The smart junction box (SJB) monitors the headlamp switch position by sending multiple voltage reference signals to the headlamp switch. When the headlamp switch is in any given position, that input signal is routed to ground.
The SJB also monitors the light sensor with a voltage reference signal. The light sensor input to the SJB varies with the ambient light conditions.
When the SJB receives an input from the headlamp switch indicating a request for the autolamps, the SJB monitors the light sensor for the ambient light condition. If the SJB determines the ambient light level is dark, the SJB supplies voltage to the exterior lamps.