Explorer 2WD V8-4.6L (2008)
Trailer Lamps: Initial Inspection and Diagnostic Overview
Principles of Operation
PRINCIPLES OF OPERATION
NOTE: The smart junction box (SJB) is also known as the generic electronic module (GEM).
The SJB uses existing inputs to determine if voltage needs to be supplied to the individual trailer stop/turn lamps. The SJB has separate drivers that
provide voltage for the trailer turn lamps. The trailer parking and reversing lamps share the vehicle lighting outputs.
The vehicle is supplied with wiring to support an aftermarket trailer brake control (TBC) module.
The trailer tow battery charging is controlled by a relay. When the ignition switch is in the RUN or ACC position, the trailer tow battery charge relay is
energized, which routes voltage to the trailer tow connector.
Field-Effect Transistor (FET) Protection
The SJB utilizes a FET protective circuit strategy for many of its outputs (for example, the headlamp output circuit). Output loads (current level) are
monitored for excessive current (typically short circuits) and are shut down (turns off the voltage or ground provided by the module) when a fault is
detected. A continuous DTC is stored at the time of the fault. The circuit then resets after a customer demand of the function (switching the component
on, battery saver being energized). When an excessive circuit load occurs several times, the module shuts down the output until a repair procedure is
carried out. At the same time, the continuous DTC that was stored on the first failure cannot be cleared by a command to clear the continuous DTCs.
The module does not allow this code to be cleared or the circuit restored to normal until a successful on-demand self-test proves that the fault has been
repaired. After the on-demand self-test has successfully completed (no on-demand DTCs present), the continuous DTC clears and the circuit function
returns.
Each circuit has 3 predefined levels of short circuit tolerance established in the module based on each circuits' capability. When the first or second
level is reached, the continuous DTC associated with the circuit sets along with DTC B106E. These DTCs may be cleared using the Clear DTC
operation on the scan tool as long as the fault itself has been corrected. If any of the circuits are shorted past the third level, then B106F and B1342 set
along with the associated continuous DTC. These DTCs cannot be cleared and the module must be replaced.
The SJB FET protected output circuits for the trailer lamp system are the LH and RH trailer stop/turn output circuits.
Inspection and Verification
INSPECTION AND VERIFICATION
1. Verify the customer concern.
2. Verify the exterior lighting system of the vehicle is operating correctly. If not, refer to the appropriate pinpoint test.
3. Visually inspect for obvious signs of electrical damage.
VISUAL INSPECTION CHART
Electrical
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Battery junction box (BJB) fuse(s):
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13 (30A) (trailer tow battery charge relay - switched)
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17 (30A) (trailer brake control (TBC) module)
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Smart junction box (SJB) fuse(s):
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7 (15A) (trailer stop/turn lamps)
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12 (5A) (trailer tow battery charge relay coil)
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16 (10A) (trailer reversing lamps)
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25 (15A) (trailer parking lamps)
-
Circuity
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Trailer
-
SJB
4. If an obvious cause for an observed or reported concern is found, correct the cause (if possible) before proceeding to the next step.
NOTE:
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Make sure the headlamp switch is in the OFF position.
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Make sure the multifunction switch is in the LOW BEAM position.
5. NOTE: Make sure to use the latest scan tool release software.
If the cause is not visually evident, connect the scan tool to the data link connector (DLC).
