Circuit/System Description

WARNING: This page is about a different car, the 2022 Cadillac CT4. However, it is still accessible from the selected car via links, so may be relevant.

Low Beam Headlamps

The K9 Body Control Module (BCM) monitors three signal circuits from the S78 Turn Signal Switch. When the Turn Signal Switch is in the AUTO position, all three signal circuits are open. When placed in the AUTO position, the BCM monitors inputs from the B10B Sun Load Temperature and Ambient Light and Security Indicator Sensor to determine if headlamps are required or if daytime running lamps will be activated based on outside lighting conditions. When the Turn Signal Switch is placed in the OFF position, the Turn Signal Switch headlamps OFF signal circuit is grounded momentarily, indicating to the BCM that the exterior lamps should be turned OFF. With the Turn Signal Switch in the PARK position, the Turn Signal Switch park lamps ON signal circuit is grounded, indicating that the park lamps have been requested. When the Turn Signal Switch is placed in the HEADLAMP position, both the Turn Signal Switch park lamps ON signal circuit and the Turn Signal Switch headlamps ON signal circuit are grounded. The BCM responds to the inputs by sending a serial data message to the K219 Lighting Control Module, the Lighting Control Module responds by sending a pulse width modulated voltage signal to the headlamp assembly. The headlamp assembly responds by illuminating the respective LED(s). If serial data communication is lost between the Lighting Control Module and BCM, all Park, Tail, Low Beams and License Plate Lamps will default on with the vehicle in service mode.

High Beams

When the low beam headlamps are ON and the S78 Turn Signal Switch is placed in the high beam position, ground is applied to the K9 Body Control Module (BCM) through the high beam signal circuit. The BCM responds to the input by sending a serial data message to the K219 Lighting Control Module. The Lighting Control Module responds by sending a pulse width modulated voltage signal to the headlamp assembly. The headlamp assembly responds by illuminating the respective high beam LEDs.

High Beam Indicator

The status of the high beam lamps is shown by a blue indicator located on the instrument cluster. When high beams are commanded, the indicator will be illuminated continuously. If the driver turns off the high beams, the indicator will turn off.

Ambient Light Sensor

The B10B Sun Load Temperature and Ambient Light and Security Indicator Sensor is used to monitor outside lighting conditions. The ambient light sensor provides a voltage signal that will vary between 0.2 V and 4.9 V depending on outside lighting conditions. The K9 Body Control Module (BCM) provides a 5 V reference signal to the ambient light sensor and a low reference ground. The BCM monitors the ambient light sensor signal circuit to determine if outside lighting conditions are correct for either daytime running lamps or automatic lamp control when the Turn Signal Switch is in the AUTO position. In daylight conditions the BCM will send a serial data message to the K219 Lighting Control Module to command the daytime running lamps ON. During low light conditions the Lighting Control Module will command the low beam headlamps ON. When equipped with RPO T87, the daytime running lamps and front park lamps use the same serial data input from the BCM to the Lighting Control Module, additionally the front park and daytime running lamps share the same control circuit from the Lighting Control Module.