Exterior Lighting Systems

Exterior Lighting Systems Description and Operation

Exterior Lamps

The exterior lighting consist of the following lamps:

* Headlamps
* Daytime running lamps (DRL)
* Fog lamps
* Park, tail, license and marker lamps
* Turn signal lamps
* Hazard warning lamps
* Repeater lamps (export)
* Stop lamps
* Backup lamps

High Intensity Discharge (HID) Low Beam Headlamps

Warning

The high intensity discharge system produces high voltage and current. To reduce the risk of severe shocks and burns:

* Never open the high intensity discharge system ballast or the arc tube assembly starter.
* Never probe between the high intensity discharge system ballast output connector and the arc tube assembly.

The headlamps may be turned ON in 2 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)

Battery voltage is applied at all times to the coil and switched side of the low beam relay. Ground is applied at all times to the turn signal/multifunction switch. When the turn signal/multifunction switch is placed in the HEAD position, ground is applied from the turn signal/multifunction switch through the headlamps ON signal circuit to the body control module (BCM). The BCM applies ground to the headlamp low beam relay control circuit. This energizes the low beam relay, closing the switched side and applies battery voltage to the LEFT and RIGHT LOW BEAM fuses. Battery voltage is then applied from the fuses, through the low beam voltage supply circuits to the ballast in the headlamp assembly. When battery voltage is applied to the headlamp ballast through the low beam voltage supply circuits, the ballast charge the starter to start the lamp. High intensity discharge (HID) headlamps do not have filaments like traditional bulbs, instead the starter uses a high voltage transformer to convert the input voltage into a higher voltage. This increased voltage is used in order to create an arc between the electrodes in the bulb.

High Beam Headlamps

The high beam headlamps receive voltage from the headlamp high relay located in the underhood fuse block. The BCM controls the relay on the headlamp high beam relay control circuit. High beam control is determined by a signal on the headlamp dimmer switch signal circuit from the multifunction switch. When the headlamp dimmer switch signal circuit is grounded, the BCM grounds the headlamp high beam relay control circuit, illuminating the high beam headlamps. The right and left high beam circuits are independent and are fused independently in the underhood fuse block.

Automatic Lamp Control/Twilight Sentinel

Place the turn signal/multifunction switch in the OFF position for automatic lamp control. During automatic lamp control the headlamps will be off during daylight conditions but will turn on when the ambient light sensor detects low outside light level. The ambient light sensor is a light sensitive transistor that varies the voltage signal to the HVAC control module. The HVAC control module sends a signal to the body control module (BCM) via class 2 serial data commanding the BCM to apply ground to the headlamp low beam relay control circuit. This energizes the low beam relay, closing the switched side and applies battery voltage to the LEFT and RIGHT LOW BEAM fuses. Battery voltage is applied from the low beam fuses, through the low beam voltage supply circuits to the ballast in the headlamp assembly. When battery voltage is applied to the headlamp ballast through the low beam voltage supply circuits, the ballast will then charge the starter to start the lamp. High intensity discharge (HID) headlamps do not have filaments like traditional bulbs, instead the starter uses a high voltage transformer to convert the input voltage into a higher voltage. This increased voltage is used in order to create an arc between the electrodes in the bulb.

Flash to Pass (FTP)

When the low beam headlamps are on and the turn signal/multifunction switch is momentarily placed in the flash to pass (FTP) position, ground is applied to the turn signal/multifunction switch. The turn signal/multifunction switch applies ground to the body control module (BCM) through the FTP switch signal circuit. The BCM then applies ground to the high beam relay control circuit. This energizes the high beam relay, closing the switched side of the high beam relay, applying battery voltage to the LEFT and RIGHT HIGH BEAM fuses. Battery voltage is applied from the High Beam fuses through the high beam solenoid switched voltage supply circuit to the high beam solenoid assembly inside of the headlamp. This causes the solenoid to retract a plate partially covering the low beam headlamps and allowing the lamps to illuminate at full brightness momentarily.

Daytime Running Lamps (DRL)

The daytime running lamps (DRL) will illuminate the right and left front turn signal bulbs continuously. The DRLs will operate when the following conditions are met:

1. The ignition is in the RUN or CRANK position
2. The shift lever is out of the PARK position for vehicles equipped with automatic transmissions or the parking brake is released for vehicles with manual transmissions.
3. The high and low beam headlamps are OFF.

The ambient light sensor is used to monitor outside lighting conditions. The ambient light sensor provides a voltage signal that will vary between 0.2 and 4.9 volts depending on outside lighting conditions. The HVAC control module provides a low reference ground and a 5-volt reference signal to the ambient light sensor. The HVAC control module monitors the ambient light sensor signal circuit to determine if outside lighting conditions are correct for either daytime running lights (DRL) or automatic lamp control (ALC) when the headlamp switch is in the AUTO position. The HVAC control module sends a serial data signal to the body control module (BCM) commanding the BCM to turn the front turn lamps on steady (non-flashing). Any function or condition that turns on the headlamps will cancel the DRL operation. With the turn signal/multifunction switch in the OFF position, the turn signal lamps will either be turned ON or OFF after a 30 second delay, depending on whether daylight or low light conditions are sensed by the ambient light sensor.

Front Fog Lamps

Ground is applied at all times to the front fog lamp switch inside of the turn signal/multifunction switch. When the front fog lamp switch is placed in the ON position, ground is applied through the front fog lamp switch signal circuit to the body control module (BCM). The BCM applies battery voltage through the front fog lamp relay control circuit to the coil side of the fog lamp relay. Battery voltage is applied at all times from the FRT FOG LP fuse to the switch side of the FRT FOG LP relay. When the relay is energized, the relay switch contacts close and battery voltage is applied from the FRT FOG LP fuse, through the relay switch contacts, to the front fog lamps.

The BCM will send a message via class 2 serial data to the instrument panel cluster (IPC) to enable the front fog lamp indicator. The front fog lamps will deactivate when either the flash-to-pass or the high beam headlamps are turned ON.

Rear Fog Lamps

Ground is applied at all times to the rear fog lamp switch inside of the turn signal/multifunction switch. When the rear fog lamp switch is placed in the ON position, ground is applied through the rear fog lamp switch signal circuit to the body control module (BCM). Battery voltage is applied at all times from the REAR FOG/ALDL TOP SW fuse to the BCM. The BCM applies ground through the rear fog lamp relay control circuit to the coil side of the REAR/FOG Relay causing the relay to energize. When the relay is energized, the switch contacts close and battery voltage is applied from the RR FOG/ALDL TOP SW fuse, through the relay, to the rear fog lamps.

The BCM will send a message via class 2 serial data to the instrument panel cluster (IPC) to enable the rear fog lamp indicator.

Park, Tail, License and Marker Lamps

Battery voltage is applied at all times from the PK LAMPS fuse to both coil and switched sides of the PARK LAMP Relay located in the underhood fuse block. Ground is applied to the turn signal/multifunction switch at all times. When the headlamp switch is place in either the HEAD or PARK position, ground is applied to the park lamp relay control circuit. This energizes the PARK LAMP Relay and applies battery voltage to all park, tail, license, and marker lamps.

Turn Signal Lamps

Ground is applied at all times to the turn signal/multifunction switch. The turn signal lamps may only be activated with the ignition switch in the ON or START position. When the turn signal/multifunction switch is placed in either the TURN RIGHT or TURN LEFT position, ground is applied to the body control module (BCM) through either the right turn or left turn signal switch circuit. Battery voltage is applied at all times from the BATT MAIN 3 fuse to the BCM. The BCM then applies a pulsating voltage to the front and rear turn signal lamps.

Hazard Flasher Lamps

The hazard flashers may be activated in any power mode. The hazard switch is permanently grounded. When the hazard switch is placed in the ON position, ground is applied through the hazard switch signal circuit to the body control module (BCM). The BCM supplies battery voltage to all four turn signal lamps in an ON and OFF duty cycle. When the hazard switch is placed in the ON position, the instrument panel cluster (IPC) receives a signal from the hazard switch and applies voltage through the hazard flash indicator circuit to the hazard switch to illuminate the hazard indicator.

Stop Lamps

The brake pedal position sensor is used to sense the action of the driver application of the brake pedal. The brake pedal position sensor provides an analog voltage signal that will increase as the brake pedal is applied. The body control module (BCM) provides a low reference signal and a 5-volt reference voltage to the brake pedal position sensor. When the variable signal reaches a voltage threshold indicating the brakes have been applied, the BCM will apply battery positive voltage to the stop lamps, transmission control module (TCM), engine control module (ECM), and stop lamp relay coil side. The BCM also supplies the stop lamp relay coil with ground. When the stop lamp relay receives battery voltage and ground from the BCM, the relay coil is energized and the stop lamp relay switch contacts close applying battery voltage through the STOP/B/U/LPS fuse to illuminate the center high mounted stop lamp (CHMSL). The stop lamps on this vehicle will not illuminate unless the ignition is in the accessory, run, or crank positions. When the ignition is in the OFF position the stop lamps will not illuminate when the brake pedal is applied.

Repeater Lamps (export)

The repeater lamps are located on the side of the front fender. The repeater lamps are used as additional turn signal lamps, and operate as described in the Turn Signal Lamps description. The repeater lamps are hard wired to their respective front turn signals and receive ground.

Backup Lamps

Automatic Transmission

The underhood fuse block supplies battery voltage through the STOP/B/U/ LPS fuse to the coil and switch terminals of the BCK/UP LAMP Relay located in the body control module (BCM). When the transmission is placed in the REVERSE position, the transmission control module (TCM) sends a class 2 message to the BCM. The message indicates that the gear selector is in the REVERSE position. The BCM grounds the back up lamp relay control circuit of the reverse lamp relay. This energizes the reverse lamp relay, closing the switch side contacts and provides battery voltage to the REVERSE LAMPS fuse through to the backup lamps. The backup lamps are permanently grounded. With both power and ground the backup lamps illuminate. Once the driver moves the gear selector out of the REVERSE position, a message is sent by the TCM via class 2 serial data requesting the BCM to remove ground from the REVERSE LAMP Relay control circuit. This opens the REVERSE LAMP Relay switch contacts turning OFF the backup lamps.

Manual Transmission

The underhood fuse block supplies battery voltage through the STOP/B/U/ LPS fuse to the coil and switch terminals of the BCK/UP LAMP Relay located in the body control module (BCM). The BCM provides the normally open reverse switch with voltage. When the transmission is placed in the reverse position the reverse switch closes providing a ground path through the reverse lamp relay control circuit to the BCM. The BCM then grounds the back up lamp relay control circuit of the reverse lamp relay. This energizes the reverse lamp relay, closing the switch side contacts and provides battery voltage to the REVERSE LAMPS fuse through to the backup lamps. The backup lamps are permanently grounded. Once the driver moves the gear selector out of the REVERSE position, the reverse switch opens and the BCM removes ground from the REVERSE LAMP Relay control circuit. This opens the REVERSE LAMP Relay switch contacts turning OFF the backup lamps.