Automatic Headlamp

The two optional automatic headlamp systems can include the headlamp switch, the Body Control Module (BCM), and the Heating, Ventilation and Air Conditioning (HVAC) module. The auto headlamp system has two available sensors that give the BCM feedback for lamp control. The light rain sensor on the windshield and the sun 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 sun sensor provides the ambient light level input to the HVAC module whenever the ignition switch status is ON. The HVAC module 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  (Automatic) position is selected with the headlamp switch, the BCM receives a hardwired multiplex input from the headlamp switch. The BCM also receives electronic messages over the CAN data bus from the Powertrain Control Module (PCM) (also known as the Engine Control Module/ECM) indicating the engine is running. The sun sensor is only used for auto headlamp 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 headlamp bulb filaments through the right and left low or high beam driver circuits to illuminate the headlamps. The BCM also remembers which beams (low or high) were selected when the headlamps were last turned OFF, and energizes those beams and lamps again the next time it turns the headlamps ON.