In-cab RNU assemblyWIRED FOR WORK

Truck Australia September 1996


There’s a Kenworth currently running around with fewer electrical wires than its predecessors . Over the next 12 months it will be field testing what is known as ’multiplexing technology’ – a system that substantially reduces the number of wires and looms normally found in a modern heavy duty truck.

Scott Pianalto, who handles international marketing for the Deutsch Company, in the US, recently launched the field test of the prototype multiplexed truck.

In explaining the reason behind the move to implement such a system, he says the current approach to electrical harnessing on heavy duty trucks is much the same as it was at the beginnings of truck manufacturing.

An electrical signal needs to travel from one location to another. Electrical power is needed in a location and wires are harnessed on the vehicle to achieve this.

The introduction of connectors into the electrical system was driven largely by modern assembly technology and the need for manufacturing cost reduction. If fact, many connectors on a truck are intended to be mated or unmated only during initial assembly or subsequent repair.

As the number of sophisticated electronic systems continues to increase, the current harness technology is beginning to have its limitations, he claims .

Space limitations and bundle size are quickly becoming an issue. Today a heavy truck has over 100 wires going into the cab, more than 550 metres of wire, over 300 connectors, and a variety of buss bars and relays. Clearly, a new approach was needed to reduce complexity.

The Deutsch system

Multiplexing – a technology offering a means where two or more messages can be sent simultaneously in either or both directions over the same wire – was seen by Deutsch Industrial Products Division, in California, as offering the solution.

However, it had to use standard components where possible; use existing vehicle harnesses, offer diagnostic capabilities; use existing truck assembly line technology; be easily expanded and maintain or reduce overall costs.

The company developed a prototype system for a Class 8 truck which ran in a test fleet for over 600,000km across the US for a year.

Deutsch then decided to develop a system for mass production at the time that Rick Carthew of Ararat Manufacturing Enterprises – a systems manufacturer for the Australian truck industry based in Western Victoria – sought the assistance of Kenworth Trucks Australia to field test the system in this country under our demanding conditions.

In a nutshell, the multiplexing sy tem tackles conventional wiring systems by reducing the number of wires through the cab from 100 to six. It standardises on the trunk cable and keeps the longest wire to around one metre. In multiplexing, all relays and busses are contained in one of four nodes.

Scott Pianalto believes that a multiplexed vehicle will be more reliable than a conventionally-harnessed one as it will use fewer connectors, wires and relays, thus reducing the number of potential failure points.

There are three on-board electronic units with cable linking them togethe, The electronics consist of the Main Control Unit (MCU), the Remote Node Units (RNUs) and the Display Interface Unit). These are the backbone of the system, Pianalto states.

He says the technology in the MCU and the RNUs is identical to that used on electronic engines and has been in the field for over 10 years. The MCU and RNU’s share many of the same functions: scanning inputs, controlling communication between network modules, and controlling outputs.

Each node contains a number of relays which have automatically rese ting circuit breakers as well as electronic fusing to protect the electrical system in the event of electrical shorts, electrical spikes, or overheating of the nodes .

Main Control Unit

The MCU has a number of additional functions. First, the MCU monitors all input/output for the system supplying an up-to-date status on the DIU. Second, the MCU acts as the main distribution point for power distribution from the battery to the other systems.

RNU close to rear axleRemote Node Unit

The RNU is a smaller, less complicated unit with only four interfaces: twc interfaces for input/output functions and two interfaces for bringing the trunk cable into and out of the nodes.

Display Interface Unit

Mounted on the dash board is the Display Interface Unit. This display unit is a four-line, menu driven screen. The DIU allows the truck operator to scroll though a list of monitored systems evaluating the status. If the operator prefers, the display unit can be set to constantly display the status of identified systems. If any parameters are out of specified range, the system will emit an audible beep and identify the fault. In addition, the system is programmed for ’key less’ security. The operator enters a password via the key pad into the DIU to start up the truck.

Trailer Node Unit

Attached to the Sleeper RNU is the Trailer Node Unit (TNU). This unit is the interface between the multiplexing system and the trailer. The link between the tractor and the trailer is not multiplexed. The TNU contains relays that are activated by the output from the sleeper node which drive the trailer lighting systems.

Some time in the future, the trailers will be multiplexed to handle ABS and EBS (Electronic Braking System) requirements as well as other electronic systems.


The jacketed cable used in the system comprises two 13mm2 cables providing power to lights and other electrical systems; a shielded twisted pair for the data transfer and a single 1.0mm2 wire for clean power to the nodes. The jacketed cable removes the need to use convoluted tubing with the harness to meet AlR requirements.

The software

The software for the system utilises the SAE J-1939 protoco1.0

The system has been programmed to include some special features that add value to the end user. In addition to the ’key less’ security system discussed above, there is a ’soft start’ which heats bulb filaments at a controlled rate to extenci bulh life. Another feature associated with the lightinq systems is amperage-v-expcted level. If lower than expected, the driver is advised via the Display Interface Unit, to check the light bulbs.

The multiplexing system which has hecn installed on the Kenworth is capable of c

The Deutsch System’s approach is to multiplex major suh-assemblies. For example, the cah would have all wires that pass through the fire wall and the sleeper, rnultiplexed. Circuits such as the dome light would not he multiplexed. This provides the henefits without unnecessary costs.

Remote Node Unit (RNU) attached to chassis at front of the truckSystem activation

When the ignition system is turned off, the RNUs automatically turn off. The MCU places itself in a standhy mode. This minimises any draw frorn the battery.

The system powers up if either of the following occurs: (1) The ignition system is turned on. This will cause the MCU and each of the RNUs to power-up. (2) For safety purposes, the system has been designed so that if circuits covering low beam(marker lights, hazard lights, tool box lights or brake lights are switched on, the MCU will power-up and activate the multiplexing system.

For instance, in switching on the low beam lights, the following sequence occurs as the truck is being driven down the road: (1) The light switch is turned ’ON’. This completes the electrical circuit connecting the appropriate input locations. (2) The MCU (or RNU if applicable) creates a message sequence which (3) travels along the buss line to the RNUs. (4) The RNU processes the data, identifies the appropriate output location, and turns the low beam and marker lights on. Depending on the type of signal, the above process is completed either on demand (i.e. turning on the light) or at a predetermined rate (i.e. tachometers or speedometers).

System reliability

Scott Pianalto believes that while a great degree of reliability exists, the possibility of system failure is still present. In the remote case that multiplexing system failure occurs, systems are in place to assure continual operation of the vehicle.

The key areas of the vehicle, the engine and transmission, the ABS, and the light systems have built in default modes as a safety feature.

Diagram of the CAN networkEngine and transmission

In the event of system failure, the signals to both the engine and transmission would be read as a fault. This would cause both systems to move to their default settings, allowing for continued operation but at a lower efficiency. Gauge and sensory input would be lost, but the truck would run until the operator could find an appropriate place for servicing.

In the event of system failure, the anti-lock function of the ABS system would be lost, but the brakes would continue to function without the anti lock capability. He concedes that lights present the greatest difficulty. If the running lights are ’ON’ and the system fails, the lights continue to operate since there is no signal to open the relay.

Future development areas for multiplexing include the doors and steering column. This has been requested by Kenworth Australia.

Increasing the MCU capability could provide system diagnostics. Multiplexing is said to also offer easy installation of optional and after market components. It also allows for additional system enhancement without significant extra harness costs not cost competitive in the past. Scott Pianalto is adamant that in production volumes the system will have a lower installed cost than conventional wiring, be easier to install, have lower service costs, and add value to the end customer.



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