Daytime Running Lamps (DRL)

The LO Beam Headlights are automatically controlled by the Daytime Running Lights (DRL) Module. With sufficient ambient light (daylight), the DRL Module will activate the LO Beam Headlights at a reduced intensity. With low light conditions, the DRL Module will activate the Park and Tail Lights, and LO Beam Headlights at full intensity. If the DRL System is inoperative, the Headlight Switch overrides the DRL System to provide manual control of the Headlights. If the Park Brake is set when the Ignition Switch is turned to "RUN," the DRL Module will not activate the LO Beam Headlights until the Park Brake is released.
Battery voltage is applied at all times to the DRL Module through Fuse 14 to power the Park and Tail Lights. Voltage is applied to the DRL Module through Fuse 6 to power the control circuitry of the DRL Module. Battery voltage is applied to the DRL Module through CKT 274 with the Headlight Switch "OFF," to power the LO Beam Headlights at reduced or full intensity.
As the intensity of light reaching the Photocell Light Sensor decreases, the resistance of the Photocell Light Sensor increases. When the DRL Module senses a high resistance in the Photocell Light Sensor, the DRL Module de-energizes the internal DRL Relay and energizes the internal Head Relay. Battery voltage is applied through the closed contacts of the Head Relay to CKT 10. With the Headlight Dimmer Switch in "LO," the LO Beam Headlights are "ON" at full intensity. With the Headlight Dimmer Switch in "HI," the HI Beam Headlights and Indicator are "ON."
As the intensity of light reaching the Photocell Light Sensor increases, the resistance of the Photocell Light Sensor decreases. When the DRL Module senses a low resistance in the Photocell Light Sensor, the DRL Module de-energizes the internal Head Relay and energizes the internal DRL Relay. Battery voltage is applied through the closed contacts of the DRL Relay to the DRL Dropping Diode. Due to the voltage drop across the DRL Dropping Diode, the LO Beam Headlights are "ON" at a reduced intensity.
When the Park Brake Switch is closed (Park Brake set), CKT 1492 is grounded. If the Ignition Switch is turned to "RUN" when CKT 1492 is grounded, the DRL Module senses a voltage drop at the Park Brake Input and will not energize the Head Relay or the DRL Relay until the ground through CKT 1492 is removed (Park Brake Switch open).