Chevrolet Silverado / GMC Sierra. Service manual — part 2508

whether daylight or low light conditions are sensed. When the BCM energizes the DRL relay, the
DRL receives voltage on the DRL supply voltage circuit.

Fog Lamps

With both the ignition switch in the RUN position, and the park lamps ON, the fog lights will
illuminate when the driver depresses the fog lamp switch. The BCM receives the park lamp ON
signal from the interior park lamps supply voltage signal circuit. With the headlamp switch in
either the park or headlamp position, and the fog lamp switch is in the ON position, the BCM
will ground the fog lamp relay control circuit causing the relay to energize. The current flows
from the fog lamp relay to both front fog lamps. The state of the fog lamps will remain the same
until the fog lamp switch position is changed, 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.

Park, Tail and Marker Lamps

The park, tail and marker lamps, including the license lamps, are turned on when the headlamp
switch is placed in the PARK or HEAD position or anytime the headlights are requested. The
underhood fuse block supplies battery positive voltage to both the park lamp relay switch
contacts and the park lamp coil circuit. The 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. If the headlight switch is left in the ON position, the inadvertent power control feature will
turn OFF the park, tail and marker lamps approximately 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 park,
tail and marker lamps will remain on until turned OFF or the battery runs dead.

Exterior Courtesy Lamps

The BCM, driver door switch, and the passenger door switch communicate through the GMLAN
serial data system. The driver door switch and passenger door switch supply power and ground to
the left and right exterior lamps respectively. When the BCM commands illuminated entry, a
command is also sent to the door switches for illumination of the exterior courtesy lamps. If the
door locks are activated to the LOCK position, or if the ignition switch is turned to either the ON
or RUN position the exterior courtesy lamps will deactivate immediately.

Turn Signal/Hazard Lamps

When the turn signal switch is placed in either the left or right position, a ground signal is
completed from the turn signal switch to the BCM. When the hazard flasher switch is activated, a
ground signal is completed from the turn signal/hazard switch to the BCM. The BCM then sends

2008 Chevrolet Silverado 1500

2008 ACCESSORIES & EQUIPMENT Lighting - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

an ON-OFF voltage signal through the fuses to the appropriate turn signal and side repeater
(export only) lamps or to all of the turn signal fuses for hazard operation. The driver door switch
and passenger door switch also receive voltage from the BCM turn signal supply voltage circuits
for the mirror turn signals. The door switches have no function or control of the mirror turn
signals other than a pass through connection for the circuits. The audio chime is also activated
when the turn signals are ON. The instrument panel cluster (IPC) receives the signals to activate
the turn signal indicators over the GMLAN serial data system. When the hazard switch is pressed,
all turn signal lamps will flash including both IPC turn indicators.

Stop Lamps

When the driver presses the brake pedal, the switch contacts open and battery positive voltage is
interrupted to the BCM. The BCM then supplies voltage to both the turn/stop lamp supply
voltage circuit and trailer rear turn/stop lamp supply voltage circuits. The BCM also supplies a
control voltage to the CHMSL relay energizing the relay and illuminating the CHMSL.

Backup Lamps

The backup lamp request signal is sent from the PCM to the BCM on the GMLAN serial data
system. This signal is based on the park neutral position (PNP) switch signal. The BCM supplies
voltage to the backup relay and to the automatic day/night mirror when the vehicle is in reverse.
The backup relay supplies a voltage on the backup lamp supply voltage circuit to the backup
lamps and trailer backup circuit.

Cargo Lamps

The cargo lamp is controlled by the courtesy lamp supply voltage circuit. The BCM supplies
voltage to the cargo lamp anytime courtesy lamp activation is requested.

Clearance Lamps

The clearance lamps, are turned ON when the headlamp switch is placed in the PARK or HEAD
position or anytime the headlamps are requested. The underhood fuse block supplies battery
positive voltage to both the park lamp relay switch contacts and the park lamp coil circuit. The
BCM provides a ground or control circuit to the park lamps relay coil circuit. When the park
lamps are turned ON, the BCM energizes the park lamp relay. If the headlamp switch is left in the
ON position, the inadvertent power control feature will turn OFF the park, tail and clearance
lamps approximately 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 park, tail and marker lamps will
remain ON until turned OFF or the battery runs dead.

2008 Chevrolet Silverado 1500

2008 ACCESSORIES & EQUIPMENT Lighting - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

INTERIOR LIGHTING SYSTEMS DESCRIPTION AND OPERATION

Interior Lamps

The interior lighting consists of 2 groups. The first group includes lamps that may not be dimmed.

Courtesy/illuminated entry lamps

Dome lamps

Map lamps

Reading lamps

Vanity mirror lamps

Courtesy/Illuminated Entry Lamps

The courtesy/illuminated entry lamps, may be manually turned ON or OFF by placing the interior
lamp switch in the ON or OFF position. If an outside door handle is lifted and a door is opened,
the courtesy/illuminated entry lamps illuminate. After all the doors have been closed the
courtesy/illuminated entry lamps will remain illuminated approximately 15 seconds. The body
control module (BCM) will then turn them OFF through the theater dimming feature. The
courtesy/illuminated entry lamps will also turn ON prior to any door being opened, and remain
illuminated approximately 40 seconds when the driver removes the ignition key. When the driver
places the dome lamp defeat switch in the OFF position, the courtesy/illuminated entry lamps will
be disabled.

The BCM and the door modules communicate through the GMLAN serial data circuit for the
lamp illumination commands of the door switch signals. The left and right rear door latch
switches are both on one circuit that is also an input to the BCM.

The left and right front door courtesy lamps (if equipped) receive voltage and ground from the
driver door switch and the passenger door switch respectively. The remaining courtesy lamps are
controlled and receive voltage from the BCM. Voltage for the courtesy lamp supply voltage
circuit is from the INT LTS fuse in the instrument panel fuse block.

Keyless Entry Interior Illumination

When the driver uses the door key in order to unlock the doors, the body control module (BCM)
receives a door unlock signal. The BCM must have inputs or serial data messages that indicate
that the ignition switch is OFF, the courtesy lamp switch is OFF and all the doors are closed. The
BCM will then activate the interior lamps. The courtesy lamps will remain ON, approximately 15
seconds after the door is closed. The BCM will then turn them OFF through the theater dimming
feature. If the door locks are activated to the LOCK position, or if the ignition switch is turned to

2008 Chevrolet Silverado 1500

2008 ACCESSORIES & EQUIPMENT Lighting - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

either the RUN or CRANK position the courtesy lamps will turn OFF immediately.

When the driver uses the remote function actuator transmitter to unlock the doors, trunk or
presses the panic button, the BCM will keep the courtesy lamps ON approximately 40 seconds.
When the driver uses the remote function actuator transmitter to lock the doors, the BCM will
keep the courtesy lamps on approximately 2 seconds. If the ignition switch is turned to either the
RUN or CRANK position the courtesy lamps will turn OFF immediately.

Vanity Lamps

The inadvertent power supply voltage circuit of the body control module (BCM) provides battery
positive voltage to each vanity mirror lamp. When the vanity mirror cover on the sunshade is
opened, a switch closes to provide a ground circuit and the vanity lamp illuminates.

Reading Lamps

The inadvertent power supply voltage circuit of the body control module (BCM) provides battery
positive voltage to the reading lamps. The reading lamps and switch are integral to the
courtesy/reading lamp assembly. When the switch is closed, voltage is supplied and the lamp
illuminates.

Battery Rundown Protection/Inadvertent Power

The body control module (BCM) inadvertent power supply voltage circuit provides battery
positive voltage to all of the interior reading lamps, visor vanity mirror lamps, and cargo lamps if
equipped. In the event that any of these lamps were to remain illuminated for a period of more
than 10 minutes with the ignition switch in the OFF position, the BCM will deactivate the
inadvertent power supply voltage circuit to prevent a total battery discharge condition.

Interior Lamps Dimming

The second interior lighting group includes lamps which may be dimmed. This group may use a
combination of vacuum fluorescent (VF) illumination, LED illumination and incandescent lamps.

When the ignition switch is turned to the RUN position, the radio VF display turns ON at
maximum brightness. When the park lamps are ON, all incandescent and LED back lighting turn
ON at the dimming level indicated by the instrument panel (I/P) dimmer switch. At the same time
all VF displays dim to match the indicated dimming level. The panel dimmer switch
potentiometer is an input to the body control module (BCM). When the driver selects a dimming
setting by moving the I/P dimming switch potentiometer, all incandescent back lighting lamps are
provided with a specific voltage. When the I/P dimmer switch is moved from MIN to MAX, all
VF displays, as well as all incandescent back lighting respond from minimum intensity to

2008 Chevrolet Silverado 1500

2008 ACCESSORIES & EQUIPMENT Lighting - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado