Automatic Headlamp

The two optional automatic headlight systems can include the headlight switch, the Body Control Module (BCM), the Heating, Ventilation, and Air Conditioning (HVAC) module control head. The auto headlight system has two available sensors that give the BCM feedback for lamp control. The light rain sensor and the twilight sensor on the top of the instrument panel are the two available sensors.

If the vehicle is not equipped with a light rain sensor, the twilight sensor provides the ambient light level input to the HVAC module control head whenever the ignition status is ON. The HVAC module control head responds to this input by sending the appropriate electronic light level  messages to the BCM over the Controller Area Network (CAN) data bus. When the AUTO  position is selected with the headlight switch, the BCM receives a hardwired multiplex input from the headlight switch. The BCM also receives electronic messages over the CAN data bus from the Powertrain Control Module (PCM) indicating the engine is running. The twilight sensor is only used for auto headlight operation when the vehicle is not equipped with an optional light rain sensor. 

If the vehicle is equipped with a light rain sensor, then this rain sensor is what is read by the BCM.

The BCM responds to either input condition by automatically controlling a Pulse-Width Modulated (PWM) voltage output to the selected headlight bulb filaments through the right and left low or high beam driver circuits to illuminate the headlights. The BCM also remembers which beams, low or high, were selected when the headlights were last turned OFF, and energizes those beams and lamps again the next time it turns the headlights ON.