Multiplexing System Overview

Electrical System 54.00General Information

Introduction to MultiplexingThe term "multiplexing" describes how the BusinessClass M2 electrical system works. Multiplexing is de-fined as sending multiple electronic messages throughthe same signal path at the same timein this case,through the M2 wiring.Multiplexing allows the M2 electrical system to simul-taneously perform tasks and to monitor components.A multiplexed system uses electronic control units(ECUs) to operate the system. The electrical systemcomponents, such as switches and lamps, are con-nected to the ECUs, which collect and control all in-formation about the components by communicating onthe data bus.A less formal description might be that multiplexing ismuch like the interstate highway system. Trucks andcars share the roadway, with each vehicle bound for adifferent destination. Every vehicle travels at differentspeeds, enters and exits at different places, and theoccupants of every vehicle have different objectives.Whether it is a truckdriver hauling goods from a factoryto a store or a saleswoman heading home from work,highway users are like the electronic signals flashingalong the datalink.Multiplexing was introduced in vehicles in the 1980swith the first electronically controlled engines and theinitial use of the J1708/J1587 datalink. The conceptwas taken a step further in the early 1990s whentransmissions were electronically connected to en-gines in order to control engine speed and torque out-put during shifting. Multiplexing has now been appliedto the entire vehicle.

General InformationThe multiplexed electrical system replaces traditionalpower distribution module (PDM) devices, such as re-lays and circuit breakers, with electronic devices thatcommunicate over the vehicle datalinks. These elec-tronic devices control power distribution to the electricloads on the vehicle. This is done by monitoring in-puts (such as sensors and switches) and supplyingpower to outputs (such as lighting, displays, gauges,and indicators). This distributed approach to handlingswitch inputs and controlling electrical load outputssharply reduces the number of wires on a vehicle.Rather than having individual wires transmitting volt-age from switches to relays that then supply power tothe components, the multiplexed system continuously

monitors the status of all switches (input devices) andsends messages over the shared-wire J1939 datalinkto control outputs.The system communicates on two datalinks: the J1939datalink and the J1708/J1587 datalink. J1939 is theprimary datalink and is used for all control messagingand troubleshooting; J1708/J1587 is the secondarydatalink and is used for limited troubleshooting. Faultcodes are displayed on the instrument cluster displayand they may also be viewed on ServiceLink.The multiplexed system uses the following controllers:

Bulkhead Module (BHM) Chassis Module (CHM) optional Expansion Module (EXM)

The most important part of the multiplexed electricalsystem is the BHM. The BHM is the brain of the entiresystem, and controls all of the outputs in response tochanges in any of the inputs. The CHM and EXM areslaves to the BHM and respond to commands fromthe BHM and broadcast the status of the inputs andoutputs connected to them. See Fig. 1.See Fig. 2 for an example of how the headlamp signalinputs and outputs are handled in the multiplexedsystem. When the headlamp switch is turned on, theBHM senses the input. The BHM is programmed toknow which outputs it should activate for each inputsignal and where those outputs are located (suchas on the BHM, CHM, EXM, or other controller). Inthis example, the outputs for the left headlamp lowbeam are located on the BHM and the outputs forthe right headlamp low beam are located on theCHM. The BHM can directly activate the left headlamplow beam. However, because the right headlamp lowbeam outputs are located on the CHM, the BHM mustsend a message over J1939 to the CHM to tell it toactivate those outputs. Once the CHM receives themessage, it activates the correct outputs and sends amessage back to the BHM reporting the new status ofthe outputs. This fail-safe design allows at least oneheadlight to work even if the BHM or CHM should fail.For an example of the flash-to-pass function, seeFig. 3. In this case, the input comes from the mul-tifunction turn signal switch mounted on the steeringcolumn. It goes into the instrumentation control unit, orICU3-M2, for processing. The instrumentation controlunit (ICU) sends a message on J1939 to the BHM in-forming it of the multifunction turn signal switch status.

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54.00 Electrical SystemGeneral Information

The output for the right headlamp high beam is locatedon the BHM and the output for the left headlamp highbeam is located on the CHM. The BHM directly flashesthe right headlamp high beam and sends a messageover J1939 to the CHM to tell it to flash the left head-lamp high beam. Once the CHM receives the mes-sage, it flashes the headlamp high beam and sendsa message back to the BHM reporting the new statusof the output. To complete the loop, the BHM sendsa message over J1939 to the ICU reporting that thecommand was completed. These messages are trans-mitted so quickly that the entire process takes only afraction of a second.The final example is the park brake telltale. See Fig. 4.To avoid driving away with the park brake set, thesystem is designed to warn the driver. When the driverpulls out the park brake switch on the dash to set thepark brake, the CHM senses the air pressure inputfrom the park brake switch in the air management unit(AMU). The CHM sends a J1939 message to the BHM,the BHM sends a J1939 message to the ICU, and theICU turns on the park brake telltale dash light.

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Inputs OutputsBulkheadModule(BHM)

J1939ChassisModule(CHM)

ExpansionModule(EXM)

Inputs Outputs

Inputs OutputsInputs Outputs

Inputs OutputsInputs Outputs

f543943

Fig. 1, Multiplexed System Controllers

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Electrical System 54.00General Information

11/13/2001

BulkheadModule(BHM)

J1939

f543944

ChassisModule(CHM)

H

P

OFF

Headlamps

ParklampsLH Headlamp Low Beam

LH Headlamp High Beam

RH Headlamp Low Beam

RH Headlamp High Beam

Fig. 2, Headlamp Switch Example


54.00 Electrical SystemGeneral Information

03/14/2006

BulkheadModule(BHM)

J1939

f543945

ChassisModule(CHM)

Signal Switch

LH Headlamp Low Beam

LH Headlamp High Beam

RH Headlamp Low Beam

RH Headlamp High Beam

J1939

HI/LO/PASS

Fault Code

ICU

Multifunction Turn

Fig. 3, Flash-To-Pass Example

BulkheadModule(BHM)

J1939

f543946

ChassisModule(CHM)

J1939

Park Brake

ICU

AirManagement

Unit

Park BrakePressure

(AMU)

PARKBRAKESWITCH

Switch

02/17/2006

Fig. 4, Park Brake Telltale Example


Electrical System 54.00Abbreviations and Terms

Abbreviations and TermsUse the following list to determine the meaning of theabbreviations and terms used in Group 54.ABS Antilock Braking SystemActivate To begin operating.Address A unique location code for a device or data.AMU Air Management UnitAPI Application Programming InterfaceATC Automatic Traction ControlBHM Bulkhead ModuleCAN Controller Area NetworkCHM Chassis ModuleConfigure To set up a program or system for aparticular device or set of devices.Databus See datalink.Datalink A collection of wires connecting systemcomponents through which data is transmitted.DRL Daytime Running LightsDTC Diagnostic Trouble CodeECM Engine Control ModuleECU Electronic Control Unit, a device that communi-cates on a datalink.EEPROM Electrically Erasable Programmable Read-Only MemoryEMC Electromagnetic CompatibilityEMI Electromagnetic InterferenceEOL End of LineESD Electrostatic DischargeEXM Expansion ModuleFault Code A limited set of alphanumeric charactersrepresenting a corresponding error message. Faultcodes are limited to a maximum number of charactersby the display output and cross-referenced to a moredescriptive message. On J1939, fault codes are madeup of a SA, SPN, and FMI. On J1708/J1587, faultcodes are made up of an MID, PID/SID, and FMI.

FMEA Failure Mode Effects AnalysisFMI Failure Mode Indicator. The part of a J1708/J1587or J1939 fault code that identifies how a part of or itemon a device failed.FMVSS Federal Motor Vehicle Safety StandardHSD High Side DriverHVAC Heating, Ventilating, and Air ConditioningICU Instrumentation Control UnitInput A device that feeds a signal into the system, orsignal that feeds a message into the system.J1708/1587 An older vehicle communications networkprotocol intended to provide simple information ex-change, including diagnostic data between electroniccontrol devices.J1939 A high speed vehicle communications networkusing the CAN protocol, which permits any deviceto transmit a message on the network when thedatalink is idle. Each message includes an identifierthat defines the message priority, who sent it, and whatdata is contained within it. Collisions are avoided dueto the arbitration process that occurs while the identifieris transmitted, permitting high priority messages to getthrough with minimal delay.LCD Liquid Crystal DisplayLCL Low Coolant LevelLED Light-emitting DiodeLegend The icon, symbol or text on a warning lightcover illuminated by a telltale lamp.LSD Low Side DriverMID Message Identifier. Identifies any device thatcommunicates on J1708/J1587.Multiplexing The process of combining several mes-sages for transmission over the same signal path.Output The signal or message that comes out of asystem component or device.Parameter A predetermined variable in a set, eachof which restricts or defines the specific capabilitiesof the system as a whole. Used to customize theconfiguration of the system.


54.00 Electrical SystemAbbreviations and Terms

Pass-through Inputs and outputs on a device capableof allowing data to be transmitted through it withoutaffecting the message or the device.PCB Printed Circuit BoardPID Parameter Identifier. The part of a J1708/J1587fault code that identifies what part of or item on adevice that failed. PIDs are not MID specific.PLC Power Line CarrierPRD Product Requirements DocumentPWM Pulse Width ModulationSA Source Address. Identifies any device that commu-nicates on J1939.SAE Society of Automotive EngineersSID Subsystem Identifier. The part of a J1708/J1587fault code that identifies what part of or item on adevice that failed. SIDs are MID specific.Smart Switch Configurable input device, called"smart" because it is recognized by the system notby its position or physical characteristics but by itsresistance value.

SPN Suspect Parameter Number. The part of a J1939fault code that identifies what part of or item on adevice that failed.Status Condition, position, or relative position of aninput or output at a specific time.TDS Technical Development SpecificationsTelltale Any of a number of colored warning lights onthe ICU instrument cluster that illuminates an icon,symbol, or text covering it.UL Underwriters LaboratoryVCU Vehicle Control Unit


Electrical System 54.00Changing Features and Options

Reference ParametersReference parameters program the BHM to knowwhich outputs to activate for each input and wherethose outputs are located. The two types of referenceparameters are default and optional. Every vehiclehas one default reference parameter and zero to anynumber of optional reference parameters.The default reference parameter programs the BHMwith features that come standard on each vehicle, suchas headlights. Optional reference parameters programthe BHM for vehicle-specific features, such as heatedmirrors.

Each reference parameter is given a part number justlike any other hardware part on the vehicle. A referenceparameter only programs the parameters of the BHM.Reflashing or reprogramming the software is separatefrom programming the parameters, just as it is in anengine controller.

Changing Features and OptionsFeatures can be changed with ServiceLink from theFeatures screen under the Bulkhead Module (BHM)icon. The Features screen displays the features thatare installed in the BHM by listing the referenceparameter numbers and their descriptions. From thisscreen, the user can reload all the currently installedfeatures or make changes to the vehicle by enteringnew reference parameters.


Electrical System 54.00Adding Features

General InformationWhen adding features to a Business Class M2 ve-hicle, some important issues need to be considered.Read the information in this subject before adding fea-tures to the vehicle.

ServiceLink must be used to add features to theunique multiplexed electrical system in the M2 vehicle.1. To access ServiceLink training, go to

www.AccessFreightliner.com and click on Toolsand Services.

2. Click on The Learning Center and log on.3. Select More from the software training icon.4. From the Web Based Training course list, select

ServiceLink Web Based Training (I.D. numberWBTSLN-1).

5. Once you have started the training, click on Fea-tures to access the training that pertains to addingfeatures.

If ServiceLink is not available, you will need to bypassthe multiplexed electrical system and isolate circuits byconnecting only to authorized vehicle interface points.The location of these interface points is explained in"Circuit Isolation."

Control ModulesThe control modules of the multiplexed electrical sys-tem are the Bulkhead Module (BHM), Chassis Module(CHM), and any optional Expansion Module (EXM).While every vehicle will have a BHM and CHM, Ex-pansion Modules will be added as needed to increasethe capacity of the electrical system. The BHM is themain controller, or brain, of the system and is in con-stant communication with the CHM and any EXM overthe J1939 datalink. Think of the CHM and any EXM asextensions of the BHM. The BHM uses the CHM andEXM as its arms and legs. The BHM controls inputs toand outputs from itself, the CHM, and any EXM basedon the reference parameters that are programmed intoit.

Reference ParametersAs with other electronic control units (ECU) on thevehicle, the BHM is programmed through the use

of parameters. Reference parameters are used toadd multiplexed features to the BHM. There is areference parameter for each multiplexed feature, suchas heated mirrors. It is these reference parametersthat a technician will work with through ServiceLink.Each reference parameter has been given its ownpart number with the prefix 26-. Reference parameterscan be found listed under their part number in bills ofmaterial (BOM) and in PartsPro.

Floating PinsFloating pins means that a pin in a connector isnot necessarily always assigned to the same circuiton every vehicle. For that reason, you must usethe Configuration screen in ServiceLink to verify pinassignment. G06 drawings are general guides and arenot vehicle specific.

Adding a FeatureUse the following instructions to add features to thevehicle.

1. Using the Freightliner Business Class M2 DataBook, select the applicable data code that appliesto the requested add-on feature. For example,Daytime Running Lights, 311-001.

2. Contact Freightliner Parts Technical Support andprovide the representative with the vehicle identifi-cation number (VIN) and the data code requested.The representative will advise of the availability ofthe feature.

NOTE: Reference parameters, such as 26-XXXXX-XXX, are needed to determine circuit availability forthe desired feature.NOTE: The following step should be done at the partscounter to ensure that all parts required for the job,including any EXM, are identified before a quote isgiven to the customer and the work begins. Since theBusiness Class M2 makes use of floating pins, it ispossible that one truck may require an EXM to add afeature, and a seemingly identical vehicle will not.3. Log on to the Freightliner mainframe. From the

SOS/MAX menu, press F11, Additional FeaturesMultiplexing Inquiry.3.1 Key in the vehicle serial number, or the last

six digits of the VIN.


54.00 Electrical SystemAdding Features

3.2 Key in the reference parameter numbersfrom all the bills of material that are beingadded at that time.The screen will indicate if the feature canbe added with the existing control modulesor if an additional EXM is needed. SeeFig. 1 for a view of the response screen.See Table 1 for possible responses andnecessary actions.

4. To add a reference parameter to the vehicle, usethe Features screen in ServiceLink. It is best tohave ServiceLink connected to the host and thevehicle at the same time. If this is not possible,connect ServiceLink to the host and add thereference parameter, then take the ServiceLinkcomputer to the vehicle and update the vehicle.

5. When the reference parameters are applied tothe BHM, ServiceLink will provide any necessarywiring instructions via a table with columns fordevice, connector, pin location, circuit number, andaction. This table will be used to make circuitchanges to the BHM, CHM, or EXM as necessaryto add the features.

Circuit IsolationIf features must be added outside of the multiplexedelectrical system, there are a few options for obtainingauthorized interface points.

Data code 353-XXX provides various optionsfor vehicle wiring interfaces, including backof cab, frontwall, and end-of-frame locations.Data code 148-XXX provides options for en-gine wiring, and data code 34C-XXX providesoptions for transmission wiring. Go to http://www.AccessFreightliner.com/newsinfor-mation/m2bodybuilder/default.asp for moreinformation.

Battery power connections must be made at thebattery through one of the four available MEGAFuses.

Inside the cab, there are splice packs behindthe center dash that provide interface pointsfor ignition voltage, ground, and panel lampillumination.

IMPORTANT: When bypassing the multiplex electricalsystem, the interface points previously mentioned arethe only authorized points. Do not splice in to any otherelectrical wiring.

SOS/MAX Additional Feature Inquiry ResponsesSystem Response Action Required

Features can be added to the existing devices. No other action is necessary to present a complete and accurate quote tothe customer.

Features cannot be added to existing devices. Expansion module required.Feature requires additional engineering work. Contact your District Service Manager (DSM).Reference parameter not defined. From the bill of material supplied, first verify and try reentering the

26-XXXXX-XXX number(s) again. If this fails, contact Freightliner PartsTechnical Support for further assistance.

Table 1, SOS/MAX Additional Feature Inquiry Responses


Electrical System 54.00Adding Features

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Vehicle Spec / Additional Feature Inquiry DSOVRISerial Number :Lead Ser No. : Built ToDate : 1 To Be Built:Customer : Build Location : Fleet Size :

Reference Parameter Description

O1

J81277J81277

TECHNICAL TR MT. HOLLY

Engine Block Heater2601005000Y

08:33:31 06/07/02PF1=Help PF3=Menu PF12=Exit PF20=Specs PF21=BOMs PF23=DBCode

Features can be added to the existing devices

Fig. 1, SOS/MAX Additional Feature Inquiry Response Screen


Electrical System 54.00Troubleshooting

TroubleshootingWith the multiplexed electrical system, traditionalmultimeter-based current, voltage and resistancemeasurements are supplemented, or in some casesreplaced, by software tools that can read and controlthe electronic signals and devices of the system.ServiceLink is the tool that is used to troubleshootthe Business Class M2 electrical system.The modules of the multiplexed electrical system com-municate on both J1939 and J1708/J1587. The pri-mary datalink for the electrical system is J1939, andis used for all control messaging and troubleshooting.J1708/J1587 is the secondary datalink and is used forlimited troubleshooting. Fault codes are displayed onthe instrument cluster, and can also be viewed withServiceLink.Since the modules of the electrical system commu-nicate on both J1939 and J1708/J1587, ServiceLinkshows information for both datalinks. Although eachmodule connected to the multiplexed electrical systemis represented by an icon within ServiceLink, the Bulk-head Module (BHM) icon is the main icon for trou-bleshooting the system. This is because the Bulk-head Module is the main controller of the multiplexedsystem. The other icons are secondary and containgeneric screens.The following screens can be accessed under theBulkhead Module icon:

General InfoDisplays information about theBHM such as make, model, hardware version,and software version.

FaultsDisplays the active and historic faultsfor all of the control modules on the multiplexedelectrical system.

ConfigurationDisplays the pinout for all ofthe control modules on the multiplexed electricalsystem compared to the host.

FeaturesDisplays the features that are in-stalled in the BHM. From this screen the usercan reload all the currently installed features, ormake changes to the vehicle by entering newreference parameters.

FlashingAllows the user to update or reflashthe software of the BHM.

TemplatesGives a directory of Datalink Mon-itor Templates available for troubleshooting themultiplexed electrical system. These templatesallow the user to monitor and manipulate the in-puts and outputs of the electrical system.

The other control module icons, listed below, will haveonly a General Info screen, a Faults screen, and aTemplates screen.

Chassis Module (CHM) Expansion Module (EXM)

The General Info screen displays information about theparticular module such as make, model, hardware ver-sion, and software version. The Faults screen displaysthe active and historic faults for the particular mod-ule on the particular datalink. The Templates screengives a directory of Datalink Monitor Templates avail-able for troubleshooting the particular module. Thesetemplates allow the user to monitor and manipulate theinputs and outputs of the electrical system.NOTE: For more specific information about the Bulk-head Module see Section 54.12. For more specifictroubleshooting information see Section 54.12, Sub-ject 300.


Electrical System 54.00Specifications

Device CommunicationsFor information on cross-referencing a J1587 MessageIdentifier (MID) and a J1939 Source Address (SA), seeTable 1.

Device Communications on J1587 and J1939Device Description J1587 MID* J1939 SA

Engine 128 0Transmission 130 3Antilock Brakes 136 11Instrument Cluster 140 23Vehicle Security Unit (VSU) 163

Data Logging Unit (DLU) 179 251Collision Avoidance System (Headway Controller) 219 42Bulkhead Module 164 33Chassis Module 249 71Expansion Module #1 170 235Expansion Module #2 187 236Expansion Module #3 188 237Expansion Module #4 178 238Expansion Module #5 240 239* Message Identifier Source Address

Table 1, Device Communications on J1587 and J1939


Documents

Multiplexing System Overview