GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2000 🢒 Century (China) 🢒 Body and Accessories 🢒 Lighting Systems 🢒 Headlights Circuit Description

Headlamps

Battery voltage is applied continuously to the headlamp switch through circuit 3240. When the headlamp switch is in the HEAD position, voltage is applied to the headlamp dimmer switch.

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

Automatic Headlamps

The automatic headlamps control module is designed to automatically operate the exterior lights depending on the outside light conditions.

The automatic headlamp control module operates when low light is detected,

    • The ignition switch is in the RUN position.
    • The headlamps are OFF.
    • The park brake is OFF.

In low light the ambient light sensor detects darkness, turning on the low beam headlamps to full intensity through circuit 10. Additionally the auto headlamp control module turns on the following lamps through circuit 74:

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

If the engine stalls with the automatic head lamp control module in the low light mode, the automatic headlamp control module turns OFF the headlamps while the engine is being cranked. The automatic headlamp 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 automatic headlamp control module receives voltage from the following fuses:

    • Fuse D6 -- D7 through circuit 1239
    • Fuse B3 -- B4 through circuit 41

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

Ambient Light Sensor

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

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

Park Brake Input

The automatic headlamp 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 automatic headlamp 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.