Principles Of Operation

Headlamps

Principles of Operation

Exterior Lighting

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.

Flash-to-Pass and High Beam

The BCM monitors the multifunction switch for a flash-to-pass or high beam request. With the ignition in RUN and the multifunction switch in the FLASH-TO-PASS position, the BCM energizes the high beam relay until the multifunction switch is released.

If the low beams are on when the BCM detects the multifunction switch is in the HIGH BEAM position, the BCM energizes the high beam relay to provide voltage to the high beams (high beam mode). The BCM defaults to low beam mode if the headlamps are turned off using the headlamp switch or the ignition transitions out of RUN.

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 headlamp system are the low and high beam relay coil ground circuits.