Circuit/System Description

WARNING: This page is about a different car, the 2019 GMC Savana 3500, 2019 GMC Savana 2500, 2019 Chevrolet Express 3500, and 2019 Chevrolet Express 2500. However, it is still accessible from the selected car via links, so may be relevant.

The vehicle headlamps are controlled by the body control module (BCM) based on inputs from the headlamp switch and the turn signal/multifunction switch. The headlamp switch has four positions: Off, Auto, Park, and Headlamps. The default headlamp switch position is the Auto position, which the BCM uses the ambient light sensor to determine when headlamps are needed. The Off position of the headlamp switch is a momentary switch that overrides the automatic headlamps and turns off the vehicle headlamps. When the headlamp switch is in the Park position, the park lamp switch signal circuit is pulled low signaling the BCM to illuminate the vehicle parking lamps. With the headlamp switch in the Headlamps ON position, the headlamp switch ON signal circuit and the park lamp switch signal are both pulled low signaling the BCM to illuminate the headlamps and the tail/park lamps.

The headlamp high beams are controlled through the headlamp dimmer switch and the flash to pass switch, both located in the turn signal/multifunction switch. The flash to pass switch is a momentary type switch, designed to illuminate the high beams as long as the switch is held. With the switch closed, the flash to pass switch signal circuit is grounded, prompting the BCM to turn on the high beam lamps. The headlamp dimmer switch allows the operator to select between full time high or low beam operation. Unlike the flash to pass switch, it is not a momentary switch. When the headlamp dimmer switch is in the high beam position, the headlamp dimmer switch high beam signal circuit is grounded. This signals the BCM to turn on the high beam headlamps.

The BCM controls the headlamps based on the inputs explained above. When a low beam request is received, the BCM grounds the headlamp low beam relay control circuit energizing the LOW BEAM PCB Relay. With the low beam relay energized, B+ is applied through the switch side of the relay, both LO BEAM fuses, and the low beam supply voltage circuits illuminating the low beam headlamps. When a high beam request is received, the BCM grounds the headlamp high beam relay control circuit energizing the HIGH BEAM PCB Relay. With the high beam relay energized, B+ is applied through the switch side of the relay, both HI BEAM fuses, and the high beam supply voltage circuits illuminating the high beam headlamps.