The exterior lighting system consist of the following lamps:

The headlamp driver module (HDM) is an electronic module that provides electrical power to the vehicle low beam headlamps. The HDM is controlled by a pulse width modulated (PWM) ground signal from the body control module (BCM). The BCM determines the desired lamp intensity using the ambient light sensor, multifunction switch , wiper motor, and the park brake switch as inputs. In general, the HDM will operate in one of three modes:

The normal battery voltage input to the HDM must be between 6 and 16 volts DC to operate. Any deviation to these values could cause improper operation.

Major components of this system are the BCM, HDM, ambient light sensor, multifunction switch, park brake switch and low beam headlamps.

The headlamps may be turned on in three different ways. First, when the driver places the headlamp switch in the HEADLAMP position, normal operation occurs. Second, with the headlamp switch is placed in the AUTO position, automatic lamp control (ALC) occurs. During ALC control, the headlamps will be in daytime running lamp (DRL) operation in daylight conditions, or low beam operation in low light conditions. Third, in wiper activated headlamp (WAH) mode. Only when the wiper system is activated while the vehicle is in gear, the wiper blades have completed at least 3 cycles, and then a 35 second time period expires, will the BCM switch from DRL mode to headlamp mode during daylight conditions. When the gearshift lever is placed in the PARK or NEUTRAL position or if the wipers are turned off, normal manual or ALC operation will resume after approximately 60 seconds.

The HDM and high beam relay receive battery positive voltage directly from the battery. The BCM supplies a 100% duty cycle PWM ground signal to the HDM for automatic headlight control and normal headlight operation. The BCM also supplies ground to the high beam relay coil for high beam headlight operation. When the driver places the headlamp switch in the HEADLAMP position and the dimmer switch is in the low beam position, the HDM switches to full output sending current flow through the left and right low beam headlamp fuses to both low beam headlamps. The dimmer switch sends a ground signal to the BCM in the high beam position and a momentary ground signal in the flash-to-Pass (FTP) position from G102 to deactivate the low beam headlights and activate the high beam headlights. With the headlights in the low beam position, the high beams may be momentarily turned on or flashed by activating the FTP portion of the switch.

The instrument panel cluster (IPC) illuminates the high beam indicator when the body control module (BCM) detects that the high beams are requested. The IPC receives a class 2 message from the BCM requesting illumination.

The Headlamps On at Exit and Perimeter Lighting modes can be adjusted using the steering wheel controls to select the desired fuction as displayed in the driver information center. The Headlamps On at Exit selection will allow the driver to customize the period of time that the headlamps remain illuminated upon exiting the vehicle. This period can be adjusted from 0 seconds to 180 seconds. The Perimeter Lighting feature allows the driver to select if they desire the headlamps and backup lamps to illuminate when the door UNLOCK button on the key fob is activated. This feature can be turned ON or OFF by using the driver information center steering wheel control buttons.

The ambient light sensor is a light sensitive transistor that varies its voltage signal to the body control module (BCM) in response to changes to the outside (ambient) light level. When the BCM receives this signal it will either turn on the daytime running lamps (DRL) throught the HDM using the reduced output or the headlamps at full intensity for auto headlamp operation. Any function or condition that turns on the headlights will cancel the daytime running lamps operation. The daytime running lamps (DRL) are the low beam headlights at a reduced intensity. With the headlight switch in the OFF position, the headlights will either be turned ON or OFF, after an approximate 8 second delay depending on whether daylight or low light conditions are sensed. When the BCM signals the HDM for DRL operation in daylight conditions, the HDM illuminates the low beam headlights at a reduced intensity. The DRL will operate when the ignition switch is in the RUN position, the gear selector is not in the PARK position and the parking brake is released. When these conditions have been met and the ambient light sensor indicates daytime conditions, the DRL will illuminate.

DRL operation in Manual Transmission equipped vehicles will occur when the ignition switch is in the RUN position, and the parking brake is released.

The IGN E fuse in the IP fuse block supplies battery positive voltage to the headlamp leveling switch. When the driver places the headlamp leveling switch in any position, a signal is sent to the headlamp leveling actuators to adjust the headlight aiming to the desired position. The underhood fuse block supplies battery positive voltage and G107 and G103 supplies ground to the headlamp leveling actuators.

With both the ignition switch in the RUN position and the park lamps or low beam head lamps on, the fog lights will illuminate when the driver presses the fog lamp switch. The FOG LAMP fuse in the engine wiring harness junction block supplies battery positive voltage to the fog lamp relay. With the headlamp switch in either the park or headlamp position and the fog lamp switch is pressed, the body control module (BCM) will energize the fog lamp relay control circuit. The current flow is from the fog lamp relay to both front fog lamps and to ground G107 and G103. The state of the fog lamps either ON or OFF will remain the same until the fog lamp switch is pressed again, or the ignition switch is cycled OFF and ON. Fog lamp operation will be cancelled whenever the park lamps are turned OFF or the high beam headlights have been selected.

The rear fog lamp (Export) is located in the left tail lamp assembly. The rear fog lamp has a dedicated relay and both the relay control and current flow operate the same as the front fog lamps. The RR FOG fuse supplies voltage to the lamp and G 401 supplies ground.

The park, tail and marker lamps, including the license lamps, are turned on when the headlamp switch is placed in the PARK or ON position or anytime the headlights are requested. The I/P BATT fuse in the engine wiring harness junction block supplies battery positive voltage to both the park lamp relay switch contacts and the park lamp coil circuit. The body control module (BCM) provides a ground or control circuit to the park lamp relay coil circuit. When the park lamps are turned on, the BCM energizes the park lamp relay. Current flow is from the park lamp relay to the individual park lamp circuit fuses and to their respective park, tail, marker, and license lamps. The front park and marker lamps are grounded at G107 and G103. The tail lamps are grounded at G401 and G402. The license lamps are grounded at G401. If the driver places the headlight switch in the ON position after the ignition switch has been turned OFF, the park, tail and marker, and license lamps will remain on until turned off or the battery runs dead.

The T PARK fuse in the rear fuse-relay block supplies battery positive voltage to a separate park lamp circuit connected to the trailer wiring harness.

The smart flasher (GMSF) is an automotive flasher designed to accept turn, hazard, and brake inputs. The GMSF will provide corresponding outputs to the exterior lamps and instrument panel turn signal indicator lamps. This lighting system has dedicated lamps for the turn/hazard functions and dedicated lamps for the brake function. The system incorporates the use of flash rates for diagnostics. The flash rate is a calculation of the number of flashes that occur within a 60 second period. This system uses a percentage of on-time to off-time called duty cycle. For the bulb outage condition, the indicator of the vehicle side on which the outage occurs shall assume bulb outage flash rate mode. The opposite side indicator will assume the normal flash rate mode for an input from the non bulb outage side of the vehicle. Hazard warning lamp outage will maintain a normal flash rate as long as two or more exterior signaling lamps are functional. During all modes of operation, the IPC will receive the same flash rates and duty cycles that the exterior lamps receive. The IPC will then control the indicators in the cluster. The vehicle operator will be provided with an audible and visual indication of the flasher operation. Below is a table with flash rates and duty cycles:

The IGN E fuse in the engine wiring harness junction block supplies Ignition positive voltage to the turn signal switch assembly. With the ignition switch in the RUN or START position, ignition positive voltage is connected through the turn signal and hazard switch assembly to the GMSF which is powered by the FLASHER fuse also in the engine wiring harness junction block and is grounded at G201. When the turn signal switch is placed in either the left or right position, the circuit is complete from the GMSF to the turn signal lamps and to ground. The GMSF then sends an on-off voltage signal to either the left or right turn signals and their instrument panel cluster (IPC) indicator. When the hazard switch is pressed, all turn signal lamps will flash including both IPC turn indicators. The front turn signals are grounded at G103 and G107. The rear turn signals are grounded at G401 and G402.

The IPC illuminates the TURN SIGNAL ON indicator in the driver information center when the IPC determines that the turn signal is active for more than 1.2 km (0.75 mile). The IPC also sends a class  2 message to the radio in order to activate the audible warning.

On vehicles equipped with the outside rear view mirror turn signal lamps, the turn signal voltage is also sent to the respective front door module. The door modules then distibute power and ground to the activated lamp.

For trailer wiring, separate turn signal circuits are connected through the GMSF to the trailer wiring harness.

The turn signal switch sends battery positive voltage to the cornering lamps which will constantly remain illuminated until turn signal operation is cancelled. The cornering lamps are grounded at G103 and G107.

The STOP fuse in the engine wiring harness junction block supplies battery positive voltage to the normally open stop lamp switch. When the driver presses the brake pedal, the switch contacts close and battery positive voltage is supplied to both the VEHICLE STOP fuse and the VEHICLE CHMSL fuse. The current flow is now to the stop lamps which are grounded at G401 and G402. The center high mounted stop lamp (CHMSL) is grounded at G401.

For trailer wiring, a separate stop lamp circuit is connected through the GMSF to the trailer wiring harness.

On a vehicle equipped with an automatic transmission, the backup lamps request signal is sent from the powertrain control module (PCM) to the body control module (BCM) via class 2. This signal is based on the state of the park neutral position (PNP) switch. On a manual transmission equipped vehicle, the B/U fuse in the engine wiring harness junction block supplies battery positive voltage to the normally open backup lamp switch. When the driver places the gear selector lever is in the REVERSE position, the backup lamp switch closes and the current flow is from the backup lamp switch to the BCM signaling for backup lamp illumination.

The BCM directly supplies voltage for backup lamp operation on both automatic and manual transmission vehicles. The backup lamps are grounded at G401 and G402 (G305 on vehicles that have the backup lamps located in the lower bumper facia).

For trailer wiring, a separate backup lamp circuit is connected through the TRLR B/U fuse to the trailer wiring harness.