Headlamp: Description and Operation

HEADLAMPS
The headlamps consist of two interchangeable single filament bulbs or 2 high intensity discharge (HID) arc tubes and ballast on each side of the vehicle which provide high and low beams. The lower bulb in the headlamp is the high beam and the upper bulb is the low beam.

The headlamps may be turned on in three different ways:
- When the headlamp switch is placed in the ON position, for normal operation
- When the headlamp switch is placed in the AUTO position, for automatic lamp control (ALC)
- When the headlamp switch is placed in the AUTO position, with the windshield wipers ON in daylight conditions, after a 6 second delay

With the headlamp switch in the ON position, the headlamp switch signal circuit is grounded through the headlamp switch to G200. In response to this input, the dash integration module (DIM) provides ground to either the LO BEAM relay control circuit or the HI BEAM relay control circuit. The position of the dimmer switch determines which relay coil has ground. The DIM supplies battery positive voltage to the relay coil circuits, if the headlamps are necessary.

The underhood fuse block supplies battery positive voltage to both relay switch circuits. When the low beam relay coil is energized, current flows through the LT LOW BEAM fuse and the RT LOW BEAM fuse to the low beam headlamps. The low beam headlamps have ground at G104. When the HI BEAM relay coil is energized, current flows through the LT HI BEAM fuse and the RT HI BEAM fuse to the high beam headlamps. The high beam headlamps also have ground at G104. When the headlamp dimmer switch lever is pulled toward the driver, the Flash-To-Pass switch closes. This grounds the DIM Flash-To-Pass input. In response to this input, the DIM energizes the high beam relay. 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 low beams remain on. The headlights may be switched to high beam with the opposite movement of the dimmer switch lever. All CTSs exported to Norway, Sweden, Finland, or Israel will operate so that the low beam headlamps will also remain illuminated when the dimmer switch is in the high beam position. With the dimmer switch in the high beam position the DIM sends a class 2 message to the instrument panel cluster (IPC) requesting the IPC to illuminate the high beam indicator. If the headlamp switch is left in the ON position, the inadvertent power control feature will turn off the headlights 10 minutes after the ignition switch is turned to the OFF position. If the driver places the headlight 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.

During automatic lamp control (ALC) the headlamps will be in low beam operation during 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 night time conditions are sensed. The auto position of the headlamp switch is supplied a 5 volt reference from the dash integration module (DIM). With the headlamp switch in the auto position, the 5 volt reference is pulled low and enables the auto lamp feature. When the ignition is on the DIM will automatically turn the headlamps on or off according to the day/night status of the ambient light sensor.

The headlamps on with wipers feature operates when the following conditions are true:
- The ignition is in the RUN position
- The headlamps are currently OFF
- The headlamp switch is in the AUTO position
- The ambient light sensor indicates DAYTIME conditions

When the headlamps on with wipers feature is active the DIC will display the HEADLAMPS SUGGESTED message.

HEADLAMPS - HIGH INTENSITY DISCHARGE (HID)
The HID system is made up of the ballast and arc tube. The headlamp ballast receives the headlamp low beam supply voltage, which would normally be used to directly supply the low beam headlamp. Each ballast has a separate ground. The HID headlamp operation consist of three states. The startup or strike state, run up state, and steady state. When the headlamp switch is turn to the ON position the ballast may draw up to 20 amps to being the strike state. The ballast will provide multiple high voltage pulses to the arc tube in order to start the bulb. The voltage provided by the ballast during the startup state ranges from -600 volts to +600 volts. Once an arc is established, the ballast will provide a higher then steady state voltage to the arc tube in order to keep the bulb illuminated. This is called the run up state, on a cold bulb this state could last as long as 41 seconds. On a hot bulb the run up state may last as long as 15 seconds. Steady state operation begins when the bulb reaches the 35 watt power level. The ballast provides the bulb with a minimum of 55 volts during steady state operation.

For proper operation of the HID system ensure the battery and the harness are capable of supplying up to 20 amps of current per ballast, with less than 2 volts of system loss or voltage drop. Each ballast requires up to 20 amps in order to ensure normal startup and run up of the lamp.

The range of white light that is acceptable is broad when compared to halogens. Therefore, some variation in headlight coloring between the right and left headlamp will be normal. Replace the arc tube only if the arc tube is determined to be at the bulb failure stage.

HID HEADLAMPS LOW VOLTAGE OPERATION
The DIM monitors the vehicle's system voltage while in the RUN power mode. When the system voltage drops below 8.9 volts, the following will occur:
STAGE 1: If the low beam headlamps are on, whether manually or automatically the DIM will turn ON the high beam headlamps by activating Flash to Pass output control and turn OFF the low beam headlamps

When the system voltage drops below 8.5 volts, the following will occur:
- STAGE 2: If the low beam headlamps are currently on, whether manually or automatically the DIM will turn OFF the low beam headlamps by deactivating the low beam relay output.
- If the headlamps are turned on manually, the manual switch redundancy will cause the low beam headlamps to remain on.
- If the headlamp dimmer switch is in the HIGH BEAM position, the low beam headlamps will remain on. The DIM will activate the flash to pass output control.

If during the same ignition cycle, the DIM enters either STAGE 1 or STAGE 2 operations noted above, then determines that the system voltage has risen, the following will occur:
- If the DIM has entered STAGE 2 operation, it will not return to STAGE 1 operation until the system voltage rises above 9.4 volts.
- If the DIM has entered STAGE 1 operation, it will not return to normal operation until the system voltage rises above 9.8 volts.

AUTOMATIC HEADLAMP LEVELING - EXPORT ONLY
The Automatic Headlamp Leveling systems consist of the following components:
- The right headlamp leveling controller
- The left headlamp leveling controller
- The right headlamp leveling actuator
- The left headlamp leveling actuator
- The right ballast module
- The left ballast module
- The front chassis position sensor
- The rear chassis position sensor

The automatic headlamp leveling system automatically maintains the vertical alignment of the headlamps when the vehicle load and driving conditions change. The right and left headlamp leveling controllers share the inputs from the front and rear axle position sensors to determine vehicle pitch. The axle sensors send an output to the headlamp leveling controllers as the vehicle suspension compresses and rebounds. The controllers calculate the difference in vehicle pitch and send a command to the HID ballast. The ballast then drive the headlamp leveling actuators to the position commanded by the controllers.

The headlamp leveling system also monitors the performance of the HID bulb and ballast. When the headlamp switch is placed in the ON position the headlamps will go down, and then back up to the center position.