Lighting System: Overview GMC Cutaway G3500

Circuit/System Description

The body control module (BCM) supplies a voltage reference through the instrument panel (I/P) dimming voltage reference circuit to the interior lamp dimmer switch, which is part of the headlamp switch. When the dimmer switch is placed in a desired brightness position, reference voltage is applied through the dimmer switch rheostat and the I/P lamps dimmer switch signal circuit to the BCM. The BCM interprets this voltage signal, then applies a pulse width modulated (PWM) voltage through the I/P lamps supply voltage circuit and the LED dimming supply circuit to all related interior lamps, illuminating them to the desired level of brightness.

Battery voltage is applied at all times to the body control module (BCM) from the BCM 2 fuse located in the underhood fuse block. The BCM then applies this voltage to the following lamps

1. Underhood lamp
2. Front reading lamps
3. Middle reading lamps
4. Rear reading lamp
5. Vanity mirror lamps

In the event that any of these lamps were to remain illuminated for more the 20 minutes with the ignition switch in the OFF position, the BCM will deactivate the inadvertent power courtesy lamps circuit to prevent total battery discharge.

When the transmission range selector is placed in the REVERSE position, a signal is sent to the body control module (BCM). The BCM then applies voltage through the backup lamps relay control circuit energizing the backup lamps PCB relay. With the relay energized, battery voltage is applied through the left and right backup lamp fuses and the supply voltage circuits to the backup lamps. The engine may need to be running for the backup lamps to function.

The CTSY/RF TRN fuse supplies battery voltage to the body control module (BCM). When the BCM commands the courtesy lamps ON, it applies battery voltage through the courtesy lamp relay control circuit to the coil side of the OVRHD LAMPS Relay, energizing the relay. Battery voltage from the inadvertent power courtesy lamp circuit is applied through the switch side of the relay and the courtesy lamp supply voltage circuit to the overhead console lamp, illuminating the courtesy lamp. The BCM also applies battery voltage through the courtesy lamp supply voltage circuit to the left and right roof rail courtesy lamps and the front, middle, and rear door courtesy lamps. When any door is opened, the door open switch contacts close and the BCM receives a door-open input and it will illuminate the courtesy lamps. If the BCM receives a door lock or unlock input or a rear hatch open input, it will illuminate the courtesy lamps.

The low beam headlamps are controlled by two different inputs to the body control module (BCM), the headlamp switch, or when in the Auto mode, the ambient light sensor. When the BCM receives a low beam headlamps ON command, it grounds the headlamp low beam relay control circuit in order to energize the low beam PCB relay. With the relay energized, battery voltage is applied through switch side of the relay, both LO BEAM fuses, and the low beam supply voltage circuits to the low beam headlamps.

The body control module (BCM) supplies a B+ reference voltage through each of the headlamp switch signal circuits to the headlamp switch. When the headlamp switch is in the AUTO position, all headlamp switch signal circuits to the BCM are open and the BCM operates in the automatic mode. When the headlamp switch is turned to the PARK position, only the park lamp switch signal circuit is pulled low signaling the park lamps ON request. When the headlamp switch is turned to the HEAD position, the park lamp and headlamp switch signal circuits are both pulled low, signaling the headlamps ON request. When the headlamp switch is turned to the OFF position, only the headlamp off switch signal circuit is pulled low, signaling the lights OFF request.

The high beam headlamps can be controlled by two different inputs to the body control module (BCM), the dimmer switch and the flash to pass switch. When the BCM receives a high beam headlamps ON command, it grounds the headlamp high beam relay control circuit in order to energize the high beam PCB relay. With the relay energized, battery voltage is applied through switch side of the relay, both HI BEAM fuses, and the high beam supply voltage circuits to the high beam headlamps.

Battery positive voltage is applied at all times to both the coil and switched sides of the PRK LAMP relay. The park lamps are controlled by the body control module (BCM) by inputs from park lamp switch. When the BCM receives a park lamps ON command, it grounds the park lamps relay control circuit in order to energize the PRK LAMP relay. With the relay energized, battery voltage is applied through switch side of the relay, both PARK fuses, and the park lamp supply voltage circuits to all park, tail, and marker lamps.

The body control module (BCM) supplies a voltage reference through the instrument panel (I/P) dimming voltage reference circuit to the interior lamp dimmer switch, which is part of the headlamp switch. When the dimmer switch is placed in a desired brightness position, reference voltage is applied through the dimmer switch rheostat and the I/P lamps dimmer switch signal circuit to the BCM. The BCM interprets this voltage signal, then applies a pulse width modulated (PWM) voltage through the LED dimming control circuit illuminating the components listed below

1. Radio
2. Inflatable Restraint I/P Module Disable Switch
3. Driver Information Center (DIC) Switch
4. Steering Wheel Control Switches

Battery voltage for the courtesy lamps is supplied at all times, from the BCM 3 fuse located in the underhood fuse block, to the body control module (BCM). When the BCM receives a signal to command the courtesy lamps ON, it applies voltage through the courtesy lamps supply voltage circuits to the courtesy lamps listed below.

1. The left and right I/P courtesy lamps
2. Dome lamp

Battery voltage for the courtesy lamps is supplied at all times, from the BCM 3 Fuse located in the underhood fuse block, to the body control module (BCM). When the BCM receives a signal to command the courtesy lamps ON, it applies voltage through the courtesy lamps relay control circuit in order to energize the relay. With the relay energized, battery voltage is applied through switch side of the relay and the courtesy lamp supply voltage circuits to the courtesy lamps listed below.

1. Front dome lamp
2. Rear dome lamp

The body control module (BCM) supplies a voltage reference through the instrument panel (I/P) dimming voltage reference circuit to the interior lamp dimmer switch, which is part of the headlamp switch. When the dimmer switch is placed in a desired brightness position, reference voltage is applied through the dimmer switch rheostat and the I/P lamps dimmer switch signal circuit to the BCM. The BCM interprets this voltage signal then applies a pulse width modulated (PWM) voltage through the LED dimming supply circuit illuminating the HVAC control module to the requested level.

The ambient light sensor is used to monitor outside lighting conditions. The body control module (BCM) provides a 5 V reference signal to the ambient light sensor. The sensor is a variable resistor, it's resistance changes as outside lighting conditions change. With the headlamp switch in the AUTO position, the BCM monitors the sensor signal circuit to determine if the outside lighting conditions are correct for either daytime running lamps (DRL) or headlamp low beam operation. In daylight conditions, the BCM will command the DRLs ON. In low light conditions, the BCM will command the low beam headlamps ON.

The body control module (BCM) supplies a voltage reference through the instrument panel (I/P) dimming voltage reference circuit to the interior lamp dimmer switch, which is part of the headlamp switch. When the dimmer switch is placed in a desired position, reference voltage is applied through the dimmer switch rheostat and the I/P lamps dimmer switch signal circuit to the BCM. The BCM interprets this voltage signal, then applies a pulse width modulated (PWM) voltage through the instrument panel lamp supply voltage circuit illuminating the components listed below

1. Headlamp Dimmer Switch
2. Tow/Haul Switch
3. Traction Control Switch
4. HVAC Control Module
5. Driver Window Switch
6. Driver Door Lock Switch
7. Auxiliary Blower Motor Switch
8. Passenger Window Switch
9. Passenger Door Lock Switch
10. Auxiliary Front HVAC Control Assembly
11. Auxiliary Rear HVAC Control Assembly

Battery voltage is applied at all times to the body control module (BCM) from the BACKUP/STOP fuse located in the underhood fuse block. When the brake pedal is applied and the BCM receives a brake applied signal from the stop lamp switch. It then applies battery voltage through the CHMSL supply voltage circuit illuminating the CHMSL.

The instrument panel (I/P) dimmer switch is used to increase and decrease the brightness of the interior backlighting components. The body control module (BCM) supplies a voltage reference through the instrument panel (I/P) dimming voltage reference circuit to the interior lamp dimmer switch, which is part of the headlamp switch. When the dimmer switch is placed in a desired brightness position, reference voltage is applied through the dimmer switch rheostat and the I/P lamps dimmer switch signal circuit to the BCM. The BCM interprets this voltage signal, then applies a pulse width modulated (PWM) voltage through the I/P lamps supply voltage circuit and the LED dimming supply circuit to all related interior lamps, illuminating them to the desired level of brightness.

Battery voltage is applied at all times to the body control module (BCM) from the BCM 5 Fuse located in the underhood fuse block. When the brake pedal is applied, battery voltage is applied through the stop lamp switch and the stop lamp signal circuit to the BCM. The BCM then applies battery voltage through the stop lamp relay coil supply voltage circuit energizing the STOP LAMPS PCB Relay. With the relay energized, battery voltage is applied through the switch side of the relay and the control circuit to the auxiliary 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 V 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 voltage to the right and left stop lamp control circuits, transmission control module (TCM), engine control module (ECM), and center high mounted stop lamp (CHMSL) control circuit.

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 V 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 voltage to the right and left stop lamp control circuits, transmission control module (TCM), engine control module (ECM), and center high mounted stop lamp (CHMSL) control circuit.

Battery voltage is applied at all times to the backup lamp PCB relay which is an internal part of the body fuse block. When the PRNDL is placed in the REVERSE position, a signal is sent from the transmission control module (TCM) to the body control module (BCM). The BCM then applies battery voltage to the backup lamp relay control circuit energizing the relay. With the relay energized, battery voltage is applied through the switch side of the relay, the T/LAMP BCK/UP fuse, and the backup lamp supply voltage circuit to the left and right backup lamps. Or, the AUX/TRLR BCK/UP fuse and the supply voltage circuit to the backup alarm and the trailer connector. The engine may need to be running for the backup lamps to function.

The BCM 3 fuse, located in the underhood fuse block, supplies battery voltage to the body control module (BCM). The BCM applies this voltage to the courtesy lamps through the courtesy lamp supply voltage circuit. When any door is opened, the door open switch contacts close and the BCM receives a door-open input. If the BCM receives a door lock or unlock input, the BCM will activate the courtesy lamp supply voltage circuit, providing battery voltage to the dome lamp and instrument panel (I/P) courtesy lamps. The courtesy lamps can be manually activated by placing the interior lamp switch in the DOME position.

Inadvertent power courtesy lamp feature provides a similar function for the interior lamps as a retained accessory power feature. The BCM will energize the courtesy lamps supply voltage circuit when the BCM is in the active state (awake). When the inadvertent power courtesy lamps supply voltage circuit is energized, it supplies voltage from the BCM 2 fuse located in the underhood fuse block, to all the interior lights listed below. The BCM will continue to supply voltage to the interior lamps after the ignition is turned OFF so that the interior lamps may be operated.

1. Front Dome/Reading Lamp
2. Middle Dome/Reading Lamp
3. Rear Dome/Reading Lamp
4. Sunshade Mirror Lamps
5. Underhood Lamp

In the event that any of these lamps were to remain illuminated for a period of more than 20 minutes with the ignition switch in the OFF position and no BCM input activity, the BCM will deactivate the inadvertent power supply voltage circuit to prevent a total battery discharge condition.

The ambient light sensor is a light sensitive transistor that varies the voltage signal to the body control module (BCM) based on outside lighting conditions. When the ambient light sensor indicates daytime conditions with the transmission in gear, the body control module (BCM) grounds the DRL relay control circuit, energizing the DRL relay. With the DRL relay energized, voltage is applied through the relay switch contacts, the DRL 1 fuse, and the DRL 2 fuse to the left and right low beam headlamps. Any function or condition that turns the headlamps ON will cancel DRL operation.

The body control module (BCM) controls the hazard lamps based on inputs from the hazard switch. With the hazard switch closed, a ground path is provided on the hazards switch signal circuit, indicating a hazard lamp request. When this request is seen, the BCM will pulse the turn signal lamp supply voltage circuits, illuminating the turn signal lamps.

The vehicle headlamps are controlled by the body control module (BCM) based on inputs from the headlamp switch and the turn signal/multifunction switch. The headlamp switch has four positions: Off, Auto, Park, and Headlamps. The default headlamp switch position is the Auto position, which the BCM uses the ambient light sensor to determine when headlamps are needed. The Off position of the headlamp switch is a momentary switch that overrides the automatic headlamps and turns off the vehicle headlamps. When the headlamp switch is in the Park position, the park lamp switch signal circuit is pulled low signaling the BCM to illuminate the vehicle parking lamps. With the headlamp switch in the Headlamps ON position, the headlamp switch ON signal circuit and the park lamp switch signal are both pulled low signaling the BCM to illuminate the headlamps and the tail/park lamps.

The headlamp high beams are controlled through the headlamp dimmer switch and the flash to pass switch, both located in the turn signal/multifunction switch. The flash to pass switch is a momentary type switch, designed to illuminate the high beams as long as the switch is held. With the switch closed, the flash to pass switch signal circuit is grounded, prompting the BCM to turn on the high beam lamps. The headlamp dimmer switch allows the operator to select between full time high or low beam operation. Unlike the flash to pass switch, it is not a momentary switch. When the headlamp dimmer switch is in the high beam position, the headlamp dimmer switch high beam signal circuit is grounded. This signals the BCM to turn on the high beam headlamps.

The BCM controls the headlamps based on the inputs explained above. When a low beam request is received, the BCM grounds the headlamp low beam relay control circuit energizing the LOW BEAM PCB Relay. With the low beam relay energized, B+ is applied through the switch side of the relay, both LO BEAM fuses, and the low beam supply voltage circuits illuminating the low beam headlamps. When a high beam request is received, the BCM grounds the headlamp high beam relay control circuit energizing the HIGH BEAM PCB Relay. With the high beam relay energized, B+ is applied through the switch side of the relay, both HI BEAM fuses, and the high beam supply voltage circuits illuminating the high beam headlamps.

The body control module (BCM) supplies a voltage reference through the instrument panel (I/P) dimming voltage reference circuit to the interior lamp dimmer switch, which is part of the headlamp switch. When the dimmer switch is placed in a desired brightness position, reference voltage is applied through the dimmer switch rheostat and the I/P lamps dimmer switch signal circuit to the BCM. The BCM interprets this voltage signal, then applies a pulse width modulated (PWM) voltage through the instrument panel lamp dimming control circuit and the LED backlighting dimming control circuit illuminating all backlighting lamps.

The vehicle park, license, and tail lamps are controlled by the body control module (BCM) based on an input from the headlamp switch. The headlamp switch has four positions: Off, Auto, Park, and Headlamps. The default headlamp switch position is Auto, which the BCM uses the ambient light sensor to determine when exterior lamps are needed. The Off portion of the headlamp switch is a momentary switch which overrides the automatic lamps and will turn off the vehicle lamps. The Park position of the headlamp switch will only illuminate the vehicle parking lamps. The Headlamp position of the headlamp switch will illuminate both the parking lamps, as well has the headlamps. With the headlamp switch in the Park position, the park lamp switch signal circuit is grounded, prompting the BCM to turn on the exterior lamps, regardless of other factors such as ambient light.

When a park lamp request is received, the BCM grounds the park lamp relay control circuit. This energizes the PRK LAMP Relay and battery voltage is applied through the switch side of the relay, the exterior lamp fuses, and the supply voltage circuits illuminating the front park, license, tail, and marker lamps.

The body control module (BCM) controls the stop lamps based on the input from the stop lamp switch. When the BCM detects the brake pedal is depressed, B+ is applied to the stop lamp relay control circuit energizing the Stop Lamp PCB Relay. With the relay energized, B+ is applied to the stop/turn lamp supply voltage circuits illuminating both stop lamps. At the same time the BCM applies B+ to the center high mounted stop lamp (CHMSL), transmission control module (TCM), and engine control module (ECM).

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 left or right turn position, ground is applied through the respective left or right turn signal switch signal circuit to the body control module (BCM). The BCM then applies a pulsating voltage to the front and rear turn signal lamps through there respective voltage supply circuits. At the same time, the BCM sends a serial data message to the instrument panel cluster (IPC) requesting the respective turn signal indicator be pulsed.

Low Beam Headlamp Operation

The headlamps may be turned ON in 3 different ways

1. When the headlamp switch in the HEAD position for normal operations
2. When the headlamp switch is in the AUTO position, for automatic lamp control (ALC)
3. When the headlamp switch is placed in the AUTO position, with the windshield wipers ON in daylight conditions, after a 6 second delay

Battery voltage is applied at all times to the coil and switch sides of the LOW BEAM PCB Relay located in the underhood fuse block. With the headlamp switch in the headlamps ON position, ground is applied through the headlamps ON switch signal circuit to the body control module (BCM) signaling the headlamps ON request. In response to this signal, the BCM applies ground through the low beam relay control circuit energizing the LOW BEAM PCB Relay. With the relay energized, battery voltage is applied through the switch side of the relay, the LT and RT LOW BEAM fuses, and the low beam supply voltage circuits illuminating the low beam headlamps.

High Beam Headlamp Operation

Battery voltage is applied at all times to the coil and switch sides of the HIGH BEAM PCB Relay located in the underhood fuse block. When the headlamp dimmer switch is placed in the high beam position, the headlamp dimmer switch signal circuit to the BCM is pulled low signaling the headlamp high beam request. In response to this signal, the BCM applies ground through the high beam relay control circuit energizing the high beam relay. With the relay energized, battery voltage is applied through the switch side of the relay, the LT and RT HIGH BEAM fuses, and the high beam supply voltage circuits illuminating the high beam headlamps. At the same time the BCM sends a serial data message to the instrument panel cluster (IPC) requesting the IPC to illuminate the high beam indicator.