Principles Of Operation

Parking, Rear and License Plate Lamps

Principles of Operation

The Body Control Module (BCM) monitors the headlamp switch position by sending voltage signals on multiple circuits to the headlamp switch. There is one circuit for each headlamp switch position. At any given time, one of the signal circuits is switched to ground to indicate the headlamp switch position.

If, when the ignition is in RUN, the BCM does not detect any active inputs from the headlamp switch or if the BCM detects multiple headlamp switch input circuits short to ground, the BCM turns the parking lamps and headlamps on and keeps them on until the ignition is transitioned OFF.

If either situation occurs, the BCM cannot be ruled immediately as being at fault. This is normal behavior of the BCM design as a fault has been detected with the inputs from the headlamp switch.

When the BCM receives an input from the headlamp switch indicating a request for the parking lamps, the supplies voltage to the exterior lamps.

Field-Effect Transistor (FET) Protection

A Field-Effect Transistor (FET) is a type of transistor that, when used with module software, monitors and controls current flow on module outputs. The FET protection strategy prevents module damage in the event of excessive current flow.

The BCM utilizes a FET protective circuit strategy for many of its outputs (such as a turn lamp output circuit). Output loads (current level) are monitored for excessive current (typically short circuits) and are shut down when a fault event is detected. A short circuit DTC sets when the fault event occurs.

The circuit remains off until the fault is corrected and the ignition is cycled off and then back on, the module resets the FET protection and allows the circuit to function.

The FET protected circuits for the parking lamp system are the LH rear (and license plate lamps), the RH rear, LH front, RH front and front marker lamp voltage supply circuits.