Headlamp: Description and Operation

HEADLAMP POWER
The headlamp switch is a direct input to the instrument cluster Instrument Cluster / Carrier which is sometimes known as the Cab Control Node (CCN). The instrument cluster sends a Programmable Communications Interface (PCI) bus Information Bus message to the FCM Body Control Module informing it of a headlamp switch status change.

The FCM then turns power ON or OFF (depending on the bus message) to the headlamps through four "fuseless" circuits. These circuits are electronically controlled and continuously monitored for malfunctions.

Power is supplied to each filament in a separate circuit. For vehicles equipped with Daytime Running Lamps (DRL), the FCM electronically steps down the headlamp voltage to provide the desired illumination levels.