GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 1999 🢒 Grand Prix 🢒 Body and Accessories 🢒 Lighting Systems 🢒 Headlights/Daytime Running Lights (DRL) Circuit Description

Headlamps

Battery voltage is applied continuously to the headlamp switch through circuit 2440. When the headlamp switch is in the HEAD position, the following conditions occur:

    • Voltage is applied to the headlamp dimmer switch.
    • Voltage is removed from circuit 306 which disengages the DRL control module.

When the headlamp dimmer switch is in the LOW position, battery voltage is applied to the following components through circuit 12:

    • The underhood accessory wiring junction block
    • The low beam headlamps

When the headlamp dimmer switch is in the HIGH position, battery voltage is applied to the following components through circuit 11:

    • The underhood accessory wiring junction block
    • The high beam headlamps
    • The instrument cluster
    • The instrument cluster head up display for the high beam indicator

Daytime Running Lamps (DRL)

The Daytime Running Lamps (DRL) control module is designed to automatically operate the exterior lights depending on the outside light conditions.

The DRL control module operates in 2 modes:

    • The day mode
        In the day mode, the DRL control module turns ON the low beam headlamps at a reduced intensity when the DRL ambient light sensor detects daylight under the following conditions:
       - The ignition switch is in the RUN position.
       - The headlamps are OFF.
       - The park brake is OFF.
    • The low light mode
        In the low light mode, the DRL ambient light sensor detects darkness, turning ON the low beam headlamps to full intensity through circuit 10.
        Additionally, the DRL ambient light sensor turns ON the following lamps through circuit 9:
       - The park lamps
       - The side marker lamps
       - The license lamps
       - The tail lamps

If the engine stalls with the DRL control module in the low light mode, the DRL control module turns OFF the headlamps while the engine is being cranked. The DRL control module turns ON the following lamps while the engine is being cranked:

    • The side marker
    • The park lamps
    • The license lamps
    • The tail lamps

The following lamps operate in the usual manner:

    • The headlamp
    • The instrument panel lamp dimmer switch

The DRL control module receives voltage from the following fuses:

    • The DRL fuse through circuit 1239
    • The DIC/HVAC fuse through circuit 541

The following lamps may be operated manually at any time by using the headlamp and the instrument panel lamp dimmer switch:

    • The park lamps
    • The headlamps

The DRL module does not operate if the system voltage is less than 9 volts or greater than 18 volts.

Daytime Running Lamps (DRL) Ambient Light Sensor

The DRL ambient light sensor is a light sensitive variable resistor. The DRL ambient light sensor's resistance decreases as the outside light intensity increases. The DRL control module performs the following functions:

    • Measures the voltage drop across the DRL ambient light sensor.
    • Determines whether the DRL ambient light sensor should operate in the day mode or the low light mode.

Park Brake Input

The DRL control module will not operate any headlamps or exterior lamps if the park brake is applied before the ignition switch is turned to the RUN position. This allows the driver to start the vehicle and keep the headlamps OFF, as long as the park brake is applied. The DRL control module activates the headlamps when the park brake is released with the ignition switch in the RUN position. This feature functions only when the park brake is applied before the ignition switch is turned to the RUN position. The headlamps will not turn OFF, if the park brake is applied after the ignition switch is turned to the RUN position.