54.30
Interior Lighting Systems
General Information
Introduction
Interior lighting on a Business Class® M2 vehicle includes backlighting and courtesy lighting. Backlighting
is the illumination of the instrumentation control unit
and most of the switch legends.
The Bulkhead Module (BHM) controls the backlighting
using a pulse-width modulated (PWM) signal. Pulsewidth modulation is a method of controlling the percentage of time that the DC voltage is enabled. For
example, a signal that is 80 percent modulated has
the signal on 80 percent of the time, or 0.8 seconds
for every second.
The panel light increase/decrease switch is a twoposition, momentary switch that controls the intensity
of the backlighting. When the driver pushes the upper
part of the switch, labeled INCR+, a ground circuit to
the BHM is completed, indicating a request to increase
the backlighting. When the driver pushes the lower part
of the switch, labeled DECR–, a different pin at the
BHM is grounded, indicating a request to decrease the
backlighting. See Fig. 1.
H
P
Courtesy lighting, or entrance lighting, is interior lighting that is turned on by opening a door on the vehicle.
Most courtesy lights can also be turned on manually
to provide dome/reading lights; however, there are optional door courtesy lights that only provide entrance
lighting.
5
4
A
BHM
3
J1939
+
J1708
−
Backlighting
The backlighting function illuminates the dash display
and numerous control switches throughout the vehicle
cab. Backlighting power is provided to components that
include, but are not limited to the:
• instrumentation control unit
• HVAC control panel
• headlight switch
• smart switches
• cruise control switches
• power door lock/window/mirror switches
• transmission dash shifter
Instead of controlling the level of backlighting with a
rheostat (the common method for many Freightliner
vehicles), the backlighting functionality in the Business
Class M2 is controlled by a pulse-width modulated
signal from the BHM. Backlighting voltage to the
components can vary between 10 and 90 percent of
battery voltage.
PWM does not adjust the voltage strength, it controls
the percentage of time that the DC voltage is enabled.
A multimeter capable of measuring duty cycle will read
a voltage of 12 volts (nominal); a multimeter set to the
DC range will read a voltage that is less than 12 volts.
Business Class M2 Workshop Manual, Supplement 10, September 2006
1
2
f040545
01/15/2002
A.
1.
2.
3.
4.
5.
Pulse-width Modulated Signal (illumination feed)
Instrumentation Control Unit
Dimmer Switch
Bulkhead Module
Headlight Switch
Marker Light Interrupt Switch
Fig. 1, Backlighting Function
Backlighting is active with the headlight switch in either
the on or park positions, or if the marker interrupt
switch is toggled while the headlight switch is in the
off position.
The BHM monitors the backlighting voltage output
and is capable of detecting a short circuit when the
backlighting output is active. Faults discovered by the
BHM may be reported on the J1939 and/or J1708
datalinks and may be viewed through ServiceLink®.
Courtesy Lighting
Courtesy lights include:
• dome lights
• reading/map lights
050/1
54.30
Interior Lighting Systems
General Information
• door entrance lights
• overhead console lights
The number and location of the courtesy lights varies
depending on cab configuration and vehicle options.
Courtesy lighting variations range from day cabs with
one-switch activation (driver door) of a dome light
assembly, to crew cabs with four-switch activation of
the dome lights, reading lights, and optional door
courtesy lights.
The BHM is capable of detecting shorted circuits in the
courtesy lights wiring. Faults discovered by the BHM
may be reported on the J1939 and/or J1708 datalinks
and may be viewed through ServiceLink.
Dome Lights
Dome lights are installed on all cabs. The basic dome
light has a clear lens and is installed above the rear
window. On cabs with an overhead console, there is
an optional lighting assembly that contains two dome
lights and two reading lights. Additional dome lights
located in the headliner are available on crew cabs
and extended cabs.
A dome light can be turned on by opening a door or
pressing the switch on the dome light. There are two
separate power circuits coming from the BHM to the
dome light assembly. One circuit from pin A of BHM
connector B5 has power at all times and is used to
turn the light on when the driver presses the switch on
the dome light. The other circuit from pin B of BHM
connector B5 is powered when a door is opened. See
Fig. 2.
The number and location of door pin switches that
activate the dome light(s) vary with cab configuration
and vehicle options. On day cabs and extended cabs,
the driver door switch is standard and the passenger
side door switch is optional. On crew cabs, all four door
switches are standard.
Reading/Map Lights
Reading lights are clear lights that are available with:
• The optional lighting assembly on a cab with an
overhead console;
• A crew cab that has optional dome/reading light
assemblies.
In the overhead console lighting assembly, the reading
lights are located on either side of the overhead
console next to the dome lights. The reading lights in
this assembly are nonreplaceable light-emitting diode
(LED) modules.
Map lights are red lights that are available instead of
the clear reading lights in the overhead console.
Door Entrance Lights
Door entrance lights are located on the inner door
panels. These lights are only activated when a door
is opened and can not be turned on manually. The
door entrance lights provide additional interior lighting
when getting in and out of the vehicle.
Most cab configurations utilize the dome light output at
pin B of BHM connector B5 to activate the door courtesy lights; however, there are a few crew cab configurations where the Chassis Module (CHM) is used to
activate the door entrance lights to remove some of
the current load from the BHM output. For these configurations, the BHM sends datalink messages to the
CHM requesting courtesy light activation. The CHM
output at pin C of CHM connector C3 powers the door
entrance lights.
5
6
B
7
4
8
BHM
3
A
J1708
2
J1939
1
f040548
01/15/2002
A.
1.
2.
3.
4.
5.
6.
7.
8.
Passenger Side
B. Driver Side
Instrumentation Control Unit
Rear Door Switch
Front Door Switch
Rear Door Switch
Front Door Switch
Bulkhead Module
Dome Light Switch
Dome Lamp Bulb
Fig. 2, Dome Light Function
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Business Class M2 Workshop Manual, Supplement 10, September 2006
54.30
Interior Lighting Systems
General Information
Current Capacity
For cab configurations where the BHM powers many
courtesy lights, it is necessary to be aware that exceeding the current capacity of a BHM output results
in the BHM shutdown of that output. If adding additional interior lights to a vehicle, do not exceed the
current capacity of control module output. See Table 1
for identification of possible courtesy light outputs and
the current capacity of those outputs.
Courtesy Light Outputs
Function
Current
Capacity
Module
Pin
Connector
BHM
A
B5
Dome lights battery power
6.7A*
BHM
B
B5
Dome lights and optional door courtesy lights switched power
6.7A
CHM
C
C3
Door courtesy lights (unused fog light output)
6.7A†
* Pins B5-A and B7-A12 (smart switch battery power) are fed from the same BHM circuit board trace. The maximum combined current capacity for both pins is
6.7A.
† Pins C3-C and C3-D (optional fog light) are fed from the same CHM circuit board trace. The maximum combined current capacity for both pins is 6.7A.
Table 1, Courtesy Light Outputs
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54.30
Interior Lighting Systems
Interior Lights Replacement
Cab Rear Dome Light Bulb
Replacement
Overhead Console Map/Reading
Light Assembly Bulb Replacement
1. Remove the lens using a flat-blade screwdriver to
release the two tabs on the bottom of the lens.
Then gently pry the lens from the bottom of the
housing.
NOTE: The bulbs are replaceable in the inner larger
lights only. The smaller lights use nonreplaceable
LEDs.
2. Remove the two bulbs from the lock clips by pulling
them straight out. See Fig. 1.
1. Using a flat-blade screwdriver, release the tab by
pushing on the edge of the lens that is indented.
Then gently pry the lens from the housing.
3. Press new bulbs into the lock clips.
4. Insert the two tabs on the lens into the slots in the
top of the housing, then push the bottom of the
lens into place, locking the tabs.
Cab Rear Dome Light Assembly
Replacement
1. Remove the lens using a flat-blade screwdriver to
release the two tabs on the bottom of the lens.
Then gently pry the lens from the bottom of the
housing.
2. Insert the screwdriver into the lower slots in the
housing to release the two spring clips, then
remove the assembly from the headliner.
3. Disconnect the electrical connector.
4. Connect the electrical connector of the new assembly.
5. Align the tabs on the top edge of the new assembly
and press the lower edge of the assembly into
place until the locking tabs click into place.
2. Remove the bulb from the lock clip by pulling it
straight out.
3. Press a new bulb into the lock clip.
4. Insert the tab on the lens into the slots in the
housing, then push the lens into place, locking the
tabs.
Overhead Console Map/Reading
Light Assembly Replacement
1. Remove the three Torx® capscrews that hold the
center overhead panel in place. See Fig. 2.
2. Lower the assembly and disconnect the connector
to the light assembly.
3. Remove the four capscrews that attach the light
assembly to the panel, and remove the light assembly.
4. Using capscrews, install a new light assembly on
the panel.
5. Connect the connector to the light assembly.
6. Using capscrews, install the center overhead
panel.
1
07/17/2003
f544339
1. Bulb
Fig. 1, Cab Rear Dome Light
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54.30
Interior Lighting Systems
Interior Lights Replacement
1
3
2
3
3
07/18/2003
f544338
1. Center Overhead Panel
2. Reading Light With Replaceable Bulb
3. Torx Capscrew
Fig. 2, Overhead Panel With Map/Reading Lights
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Business Class M2 Workshop Manual, Supplement 10, September 2006
54.30
Interior Lighting Systems
Troubleshooting
IMPORTANT: ServiceLink® is the diagnostic tool for
troubleshooting the M2 electrical system. For specific
circuit and pin information on how the vehicle is wired,
go to the Configuration screen in ServiceLink and
select the specific function in which you are interested.
To troubleshoot specific inputs and outputs of this
system, go to the Templates screen in ServiceLink and
select the template for the function in which you are
interested.
the BHM determine whether or not a fault code is
broadcast. Therefore, even if the BHM detects a fault,
a fault code may not be transmitted. If the BHM
is programmed to transmit fault codes, they can be
viewed through ServiceLink. Fault messages may be
transmitted on the J1939 and/or the J1708 datalinks
until both the headlight switch and the marker interrupt
switch are turned off.
Dome Light Fault Conditions
Backlighting
See Table 3 for the dome light conditions that will
create a fault. The reference parameters that program
the BHM determine whether or not a fault code is
broadcast. Therefore, even if the BHM detects a fault,
a fault code may not be transmitted. If the BHM
is programmed to transmit fault codes, they can be
viewed through ServiceLink. Fault messages may be
transmitted on the J1939 and/or the J1708 datalinks
until the ignition switch is turned off.
Input and Output Conditions
See Table 1 for the Bulkhead Module (BHM) responses to the backlighting input/output conditions.
Fault Conditions
See Table 2 for the backlighting conditions that will
create a fault. The reference parameters that program
Backlighting Input/Output Conditions
Inputs to BHM
Outputs from BHM
Headlight
Switch
Marker
Interrupt
Switch
Backlight
Status
Illumination
Percent Battery Voltage Output
On/Park
Off
On
Dependent on dimmer switch position
(range dim to bright).
Dependent on dimmer switch position
(range 10 to 90%).
Off
On
On
Bright
100%
On/Park
On
Off
Off
0%
Off
Off
Off
Off
0%
Table 1, Backlighting Input/Output Conditions
Backlighting Fault Conditions
Failed Component or
Circuit
Description of Fault
Action Taken by BHM
Backlight dimmer switch
BHM sees panel light increase and
panel light decrease simultaneously.
BHM may transmit a J1939 and/or a J1708 fault
message.
Headlight switch
Headlight switch is in error.
BHM will assume the headlight switch is on.
Marker interrupt switch
Marker interrupt switch is in error.
BHM will assume the marker interrupt switch is off and
may transmit a fault message on the J1939 and/or
J1708 datalinks.
Backlighting power output
Backlighting power wiring is shorted.
BHM may transmit a J1939 and/or a J1708 fault
message.
Table 2, Backlighting Fault Conditions
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54.30
Interior Lighting Systems
Troubleshooting
Dome Light Fault Conditions
Description of Fault
Action Taken by BHM
Dome light power (hot at all times) wiring is shorted.
BHM may transmit a J1939 and/or a J1708 fault
message.
Dome light switched (hot with door switch closed) wiring is shorted.
BHM may transmit a J1939 and/or a J1708 fault
message.
Table 3, Dome Light Fault Conditions
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Interior Lighting Systems
54.30
Specifications
Wiring Diagrams
IMPORTANT: The following wiring diagrams show
typical interior lighting configurations available for a
Business Class® M2 vehicle. The circuit details shown
may not correspond to every vehicle. ServiceLink® is
the diagnostic tool for troubleshooting the M2 electrical
system. For specific circuit and pin information on how
the vehicle is wired, go to the Configuration screen in
ServiceLink and select the specific function in which
you are interested. To troubleshoot specific inputs and
outputs of this system, go to the Templates screen in
ServiceLink and select the template for the function in
which you are interested.
• An overhead console, four dome lights, and
optional front door entrance lights. See Fig. 7.
• An overhead console, four dome lights, and
CHM-controlled door entrance lights. See Fig. 8.
Backlighting
See Fig. 1 and Fig. 2 for backlighting wiring diagrams
showing dimmer controls and instrument illumination.
Courtesy Lights
The number and location of courtesy lights are dependent on the vehicle configuration and vehicle options.
Day Cabs and Extended Cabs
See Fig. 3 for a wiring diagram of the courtesy lights
for a cab with a single rear dome light.
See Fig. 4 for a wiring diagram of the courtesy lights
for a cab with the optional lighting assembly in the
overhead console.
Crew Cabs
Courtesy light door switches are standard on a crew
cab. The courtesy lights are activated by opening any
of the four vehicle doors. See Fig. 5 for a wiring
diagram of the door switches.
There are many interior lighting configurations available for crew cab vehicles. The Bulkhead Module
(BHM) outputs that drive the interior lights have a
limited load capacity. As supplied interior lighting increases, sometimes it is necessary to feed the door
courtesy lights from an unused Chassis Module (CHM)
output to prevent BHM output overload.
Typical examples of crew cab courtesy light configurations include:
• Two dome lights and optional door entrance
lights. See Fig. 6.
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54.30
Interior Lighting Systems
Specifications
BR
11
9
BK
Smart Switches
Smart switch backlighting
A1
B7
BR
29A
BR
11
9
BK
BR
11
9
BK
BR
11
9
BK
BR
11
9
BK
BK GNDE
BK
GNDE BK−W
2B
Battery Ground
Backlighting increase
B5
BR
29C
B6
B6
BR
29
H
B5
BR
29C
BR
3
29
BR
1
DIM +
OFF
Backlighting decrease
Backlighting PWM output
DIM −
29A
29A
BR
9
GND
BK
7
Bulkhead Module
BK
Backlight feed
A1
C1
BR
Ground
D3
C2
BK−W GNDE
Left Interior
Frontwall Ground
29A
ILLUM
Dimmer Switch
Battery Ground
29A
BK
BK
8
29A
BR
10
BR
Instrumentation Control Unit
GND
Backlight feed
Ground
A2
BR
29A
B8
BK
GND
BK
Headlight Switch
Left Interior
Frontwall Ground
HVAC Control Head
MBE900
BK
GND
12
10
BR
29A
MBE900
Caterpillar
BK
29A
6
29A
BR
GND
BK
Left Interior
Frontwall Ground
Caterpillar
BR
10
BK
8
Cruise Set/Resume Switch
29A
BR
10
GND
BK
9
Cruise Set/Resume Switch
A
BR
Cruise On/Off Switch
11
29A
GND
Illumination Splice Pack
f544786
05/18/2006
Fig. 1, Backlighting Dimmer Controls
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Business Class M2 Workshop Manual, Supplement 10, September 2006
54.30
Interior Lighting Systems
Specifications
A
BHM pulse−width modulated
backlighting output
F
L
BR
A1 29A
BR
29A
K
G
BR
29A
BR
29A
BR
E
BR
29A
29A
J
BR
29A
H
D
BR
29A
BR
C
BR
29A
29A
B
BR
Not Used
29A
M
Illumination
Splice Pack
Dual Mirror Switch
Door Lock
Switches
29A
10
6
ATC Switch
BK
BK
GND
GND
GND
BK
8
BK
Right Power
Window Switch
GND
Left Power
Window Switch
GND
BK
GND
10
BR
29A
GND
GND
BK
8
10
BR
29A
BR
10
8
Overdrive Switch
BK
Fan Clutch
Switch
BK
BK
GND
6
A
B
BK
Transmission
Dash Shifter
GND
9
Engine Check
Switch
GND
BK 10
10
BR
29A
BK A4
Exhaust Brake
Switch
GND
Smart Shift
Display
Left Interior
Frontwall Ground
05/18/2006
f544787
Fig. 2, Backlighting
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54.30
Interior Lighting Systems
Specifications
108C
A PRP−W
BK
B
108C
B
A PRP−W
GND
B
Left Door Entrance
Light (if equipped)
Battery Ground
BK
Left Interior
Frontwall Ground
F
BK−W
GND
BK
Dome Light
BK
Right Door Entrance
Light (if equipped)
Left Door
Courtesy Switch
GND
A
A
C
A6 PRP−W 109C
B6
Right Door
Courtesy Switch
108C
PRP−W 108D
A7 PRP−W 108C
C
Bulkhead Module
A PRP−W 41
B5
GNDE BK−W
Passenger door open
PRP−W
A
Driver door open
PRP−W 108D
C
Dome battery power
B
GNDE BK−W
Dome switched power
Cab Ground Splice
Pack (SP1)
05/18/2006
f544788
Fig. 3, Single Rear Dome Light
400/4
Business Class M2 Workshop Manual, Supplement 10, September 2006
54.30
Interior Lighting Systems
Specifications
A PRP−W 41
B5
A7 PRP−W 108C
Left Door
Courtesy Switch
GND
BK
C
BK
GND
Right Overhead
Console Lights
Left Interior
Frontwall Ground
J
C
BK
Left Overhead
Console Lights
G
GND
B
BK
GND
F
Battery Ground
Center Dome Light
B
BK
Door Entrance
Lights (if equipped)
GND
GNDE BK−W
Reading Light
LED Module
L
BK
C
B
R
Reading Light
LED Module
BK−W
GNDE BK−W
C
Right Door
Courtesy Switch
A PRP−W
A PRP−W
Dome
108C
PRP−W 108D
A
Dome
108C
PRP−W 41
PRP−W 41
A
B
PRP−W 108D
PRP−W 108C
A
C
PRP−W 109C
A
Bulkhead Module
PRP−W 108D
A6 PRP−W 109C
B6
A
Passenger door open
PRP−W 108C
PRP−W 41
Driver door open
PRP−W 108D
B
Dome battery power
B
PRP−W 108D
Dome switched power
Cab Ground
Splice Pack (SP1)
05/22/2006
f544789
Fig. 4, Overhead Console
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54.30
Interior Lighting Systems
Specifications
109C
PRP−W 108C
1
2
Left Rear Door
Courtesy Switch
GNDE BK−W
1
2
GNDE BK−W
Right Front Door
Courtesy Switch
Right Rear Door
Courtesy Switch
C BK−W
C
Left Front Door
Courtesy Switch
BK−W
GNDE BK−W
C
A
PRP−W 108C
PRP−W 109C
A7 PRP−W 108C
B6
A
Bulkhead Module
PRP−W
GNDE BK−W
Left side door open
A6 PRP−W 109C
PRP−W 108C
Right side door open
Battery Ground
06/02/2006
Dash Ground
Splice Pack (SP6)
f544790
Fig. 5, Crew Cab Door Switches
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54.30
Interior Lighting Systems
Specifications
PRP−W 108D
PRP−W
108C
108C
B
Right Front Door
Entrance Light
GND
BK
A PRP−W
B
Left Front Door
Entrance Light
GND
BK
A PRP−W
108C
108C
A PRP−W
B
PRP−W 108D
C
BK
BK
PRP−W 41
A
B
BK
Right Rear Door
Entrance Light
Rear Center
Dome Light
Left Interior
Frontwall Ground
C
D
BK
GND
Front Center
Dome Light
K
F
GND
BK
B
GND
Left Rear Door
Entrance Light
GND
PRP−W 108D
C
B
PRP−W 41
A
Bulkhead Module
108C
A PRP−W 41
B5
A PRP−W
Dome battery power
B
PRP−W 108D
Dome switched power
Cab Ground
Splice Pack
05/18/2006
f544791
Fig. 6, Two Dome Lights
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54.30
Interior Lighting Systems
Specifications
Dome battery power
B
PRP−W 108D
108C PRP−W A
PRP−W 108C
PRP−W 108D
Dome switched power
A PRP−W 41
B5
Bulkhead Module
B
BK GND
BK
Left Interior
Frontwall Ground
Left Front Door
Entrance Light
(if equipped)
108C PRP−W A
B
BK GND
PRP−W 41
PRP−W 108D
A
PRP−W 108D
A
B
PRP−W 41
B
PRP−W 108D
C
GND
M
BK
C
GND
K
BK
C
GND
L
BK
C
Center Rear Dome/
Reading Light
GND
Left Rear Dome/
Reading Light
J
Right Overhead
Console Lights
GND
BK
C
Read
G
BK
B
BK
Dome
GND
Left Overhead
Console Lights
Read
Dome
Reading Light
LED Module
F
Front Console
Dome Light
A
Dome
Read
Reading Light
LED Module
PRP−W 41
PRP−W 108D
A
Dome
Dome
B
PRP−W 41
PRP−W 108D
A
B
PRP−W 41
PRP−W 41
A
B
PRP−W 108D
C
Right Front Door
Entrance Light
(if equipped)
Right Rear Dome/
Reading Light
Cab Ground
Splice Pack (SP1)
05/22/2006
f544792
Fig. 7, Six Dome Lights with Optional Front Entrance Lights
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Business Class M2 Workshop Manual, Supplement 10, September 2006
54.30
Interior Lighting Systems
Specifications
1939+
1939−
Chassis Module
C PRP−W 108C
C3
G
Y
1939−
Dome switched power
Dome battery power
108C PRP−W A
B
1939+
B
Y
108C PRP−W A
BK GND
Right Front Door
Entrance Light
BK GND
Left Rear Door
Entrance Light
B
108D PRP−W A
1939+
J DKG 1939−
B2
B
B
B
108D PRP−W A
BK GND
Left Front Door
Entrance Light
R DKG 1939−
C4
Datalink
1939+
108D PRP−W
BK GND
Right Rear Door
Entrance Light
BK
Door courtesy lights
108C PRP−W
Left Interior
Frontwall Ground
PRP−W 108D
A PRP−W 41
B5
Read
Read
PRP−W 41
PRP−W 108D
A
PRP−W 108D
A
B
PRP−W 41
PRP−W 108D
B
Dome
Read
Cab Ground
Splice Pack (SP1)
Dome
BK
Right Rear Dome/
Reading Light
C GND
C
BK
Center Rear Dome/
Reading Light
M GND
C
GND
K
BK
C
BK
GND
Left Rear Dome/
Reading Light
L
C
BK
Right Overhead
Console Lights
GND
Left Overhead
Console Lights
J
C
BK
G GND
B
BK
GND
F
05/22/2006
Dome
Reading Light
LED Module
Reading Light
LED Module
Front Center
Dome Light
A
Dome
PRP−W 41
PRP−W 108D
A
Dome
B
PRP−W 41
PRP−W 108D
A
B
PRP−W 41
PRP−W 41
A
B
PRP−W 108D
C
Bulkhead Module
f544793
Fig. 8, Six Dome Lights with Four Entrance Lights
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