Headlamp: Description and Operation

The headlamps consist of two interchangeable single filament bulbs which provide high and low beams. The inner bulb in the headlamp assembly is the high beam headlamp and the outer bulb is the low beam headlamp.

The headlamps may be turned on in three different ways:
^ When the headlamp switch is placed in the HEADLAMP position, for normal operation.
^ When the twilight delay switch is moved out of the OFF position, for Automatic Lamp Control (ALC) when low light conditions are present.
^ When the twilight delay switch is moved out of the OFF position, with the windshield wipers ON in daylight conditions, after a twenty second delay.

During ALC control the headlamps will be in Daytime Running Lamp (DRL) operation in daylight conditions, or low beam operation in low light conditions. A HEADLAMPS SUGGESTED message will appear on the Driver Information Center (DIC) when the headlamps and park lamps are off due to all exterior lighting controls being inactive when nighttime conditions are sensed. The message will also appear if the wiper activated headlamps feature is off due to the twilight sentinel being off, but the windshield wipers are on. The Instrument Cluster (IPC) provides battery positive voltage to the headlamp switch signal circuit. The signal circuits include the headlamp switch signal circuit and the headlamp dimmer switch high beam signal circuit.

The headlamps are controlled by the Body Control Module (BCM). The underhood fuse block provides battery positive voltage to the HEADLAMP relay switch through the HDLPS fuse. The rear compartment fuse block provides battery positive voltage to the HIGH/LOW BEAM HEADLAMP relay coil.

With the headlamp switch in the HEADLAMP position, the headlamp switch signal circuit is grounded through the headlamp switch to G205. In response to this input, the IPC sends a class 2 message to the BCM in order to activate the low beam headlamps. When the ignition switch is placed in the ON position The BCM provides battery positive voltage to the HEADLAMP relay coil. When the HEADLAMP relay coil is energized, current flows through the normally open contacts of the HIGH/LOW BEAM HEADLAMP relay switch to L HDLP LO fuse and the R HDLP LO fuse to the low beam headlamps. The low beam headlamps have ground at G101. When the ignition switch is in the RUN position, the twilight enable switch is on, and the IPC receives an input through the windshield wiper motor relay coil supply voltage circuit that the windshield wipers are on for approximately 20 seconds, the IPC will send a class 2 message to the BCM to activate the low beam headlamps. The wiper activated headlamp feature can be disabled by either turning the windshield wiper switch off, the twilight enable switch off, or by placing the ignition switch in the OFF position. The wiper activated headlamp feature will override the Daytime Running Lamps (DRL). When the HIGH/LOW BEAM relay coil is energized, current flows through the L HDLP HI fuse and the R HDLP HI fuse to the high beam headlamps. The high beam headlamp ground circuit is divided into a left and right side. The current from the right high beam headlamp flows to ground at G101. The current from the right high beam headlamp flows through the DRL relay switch to ground at G101. The underhood fuse block provides battery positive voltage to the headlamp dimmer switch. When the turn signal lever is pulled toward the driver, the headlamp dimmer switch closes and the current flows through the headlamp dimmer switch high beam signal circuit to the IPC. In response to this input, the IPC sends a class 2 message to the BCM in order to energize the HIGH/LOW BEAM relay. The IPC also illuminates the high beam indicator when the high beam headlamps are requested. When the HIGH/LOW BEAM relay is energized, battery positive voltage is removed from the low beam headlamps causing the high beam headlamps to activate instead of the low beams. Both high beams remain on until the driver releases the switch handle. If the low beam headlamps were on during flash-to-pass operation, the headlamps will be switched to high beams. If the headlamp switch is left in the ON position, the inadvertent power control feature will turn off the headlamps 10 minutes after the ignition switch is turned to the OFF position. If the driver places the headlamp switch in the ON position after the ignition switch has been turned OFF, or if the ignition switch is in the ACCY position, the headlights will remain on until turned off or the battery runs dead.