Medium-Duty: Plug & play

June 1, 2004
At its most basic, multiplexing refers to the process of sending multiple signals over one wire, as opposed to the old method, where each signal requires its own wire. Easy concept, no? The basic benefits of multiplexing are fairly obvious: being able to transmit multiple signals on one wire means far fewer wires are needed to carry far greater amounts of information. In theory, that means fewer wire

At its most basic, multiplexing refers to the process of sending multiple signals over one wire, as opposed to the old method, where each signal requires its own wire. Easy concept, no?

The basic benefits of multiplexing are fairly obvious: being able to transmit multiple signals on one wire means far fewer wires are needed to carry far greater amounts of information. In theory, that means fewer wire bundles are needed for a truck's electrical system.

“The whole idea is to simplify a truck's electrical structure and create a single source for diagnostic information,” says Tony Molla, vp-industry affairs for the National Institute of Automotive Service Excellence (ASE).

Multiplexed systems also have the potential to provide some very tempting fruit: better and faster integration between truck chassis and body; electrical systems that can be quickly upgraded to handle new functions without adding more wires; and prognostic capability.

“Anything that improves maintenance capability is of huge benefit to trucks,” Molla says. “The trucking industry lives and dies by preventive maintenance, so the ability to get information about a vehicle before it breaks down — so the repair can occur before a component fails — is a major benefit. If multiplexed electrical systems allow fleets to do that, I think you'll see wide adoption of it.”

The reality, however, can be very different. First, multiplexed electrical architecture is far more expensive than the “one wire, one signal” systems used in today's trucks. And the simplicity of multiplexing can be misleading: Signals can get crossed, causing all sorts of unexpected malfunctions that can be hard to correct.


“Think of multiplexing like a network of computers all hooked together,” explains Dan Farmer, assistant chief engineer-product development for Kenworth Truck Co. “If one computer on the network has a problem, you can crash the whole system. That's one of the risks involved with multiplexing and why we're approaching it carefully.”

Then there's the learning curve service technicians must overcome in order to successfully maintain and repair trucks using multiplexed electrical systems.

“Multiplexing represents a whole new knowledge base and skill set for technicians,” says ASE's Molla. “Once they learn how to work with multiplexing, then yes, their lives probably get easier. But first they have to acquire that knowledge and put it into practice.”

One of the misconceptions about multiplexing is that it's an all-or-nothing system, that is, you either use it on a truck or you don't.

Not so, says Jim Crowcroft, product-marketing manager for Sterling Truck Corp. “Almost all trucks use some level of multiplexed electronics,” he explains. “For example, we use it as a way to vastly improve electrical packaging, such as for in-dash gauges and readouts.”

Expanding the use of multiplexing throughout the truck, however, makes it a whole new ball game, largely because it gives customers and outside parties, such as truck body installers, access to the system.

“That's when you have to be careful,” Crowcroft points out. “You… better know what you're doing when you tap into [a multiplexed system.]”

International Truck & Engine Corp. is one OEM that decided there were more advantages than disadvantages to expanding the use of multiplexing. International introduced its multiplexing design, known as “Diamond Logic,” in several stages, starting in 2001 primarily for its medium-duty truck line.

First, International designed a basic multiplexed electrical system for solid state power switches, self-calibrating gauges, low-current switches and a liquid crystal display (LCD) to provide real-time updates on performance indicators such as mileage, advanced diagnostics, and fuel economy.

With the multiplexed system designed as a series of modules instead of long wire bundles, each module could perform self-diagnosis to identify problems in the operation of the truck or its attached equipment. According to Jeff Bannister, director of truck electronics, this reduced overall repair troubleshooting time by as much as 80%.

In 2002, International upgraded the system to enable the end user or truck equipment manufacturer to order factory-installed electrical features designed to control specific body equipment.

The “Diamond Logic Application Solution” includes features such as power take-off (PTO) controls, multiple modes of remote engine speed control, switches with interlocks, audible and visual alarm systems, and high-amperage output features.

In addition, this technology upgrade allows equipment parameters to be pre-programmed, automating and customizing the truck's performance, says Bannister. “For example, by electronically controlling operating limits, the potential for equipment damage — and its associated costs — is greatly reduced,” he points out.

“For end users, that also translates into less ‘hacking and whacking’ by an up-fitter into our truck's wiring harness, which ensures better reliability and performance for the vehicle,” says Bob Donnenberg, International's chief engineer for truck electronics.

In 2003 International took multiplexing a step further with the introduction of “Diamond Logic Builder,” which allows International dealers, body builders and fleets to write custom software to control body equipment in a manner that is unique to the customer's business.

“This is the advancement that significantly simplifies truck chassis and body equipment integration — and performance — for body builders and end users,” adds Donnenberg. “It provides the flexibility to re-map circuits and switches using software rather than by adding or splicing wiring harnesses.”

The types of customers that benefit most are those using complicated body equipment such as utility, fire/emergency, and waste collection bodies. “We use multiplexing to modify each vehicle's electrical system to its precise application,” says Bannister. “This will not only help fleets customize specific operating features on their trucks, it will allow the body installers to hook up necessary harness connections without drilling holes in the truck cab and splicing wires.”

The biggest warranty issue International faces in the medium-duty market, says Bannister, concerns the electrical system on its trucks, largely because wire splicing can expose it to corrosion and other forms of damage. By using factory-installed interface modules in the multiplexed electrical system on its new trucks, bodies can now be easily “plugged” into the vehicle.

The Diamond Logic package goes even further, allowing fleets to program specific truck and body component functions into the vehicle. And Bannister says the materials and labor costs involved are lower than those associated with more traditional drilling and splicing techniques.

Donnenberg summarizes the advantages this way: “Advanced diagnostics reduce downtime with self-protected intelligent modules that help drivers and technicians pinpoint electrical problems. Remote power modules centralize electrical connections outside the cab, eliminating the need to route and splice wires.”

For many of the same reasons, Freightliner LLC decided to incorporate a completely multiplexed wiring architecture for its Business Class M2 line in 2002. “The use of multiplexing simplifies the entire electrical system, improving reliability and making electrical diagnostics and repair much easier,” says Michael von Mayenburg, senior vp-engineering & technology.

“In the multiplexed system, multiple electrical signals are carried along a simplified set of wires, eliminating large wiring bundles,” he says. “With significantly fewer wires overall, there's less chance of damage, shorts and other problems.” The multiplexed system also features intelligent controls, allowing technicians to quickly pinpoint any problems, he adds.


To reap the benefits of multiplexing, however, it's important to approach it with open eyes, weighing the risks as well as the rewards, says Sterling's Crowcroft.

“One concern is that most fleets and body builders have based their truck specifications and designs on conventional wiring systems,” he explains. “If you move to a multiplexed architecture, you have to start over on both fronts, rebuilding your spec sheet and designs with the multiplexed system at its heart.”

In addition, Crowcroft says, “It's more costly, can be more complex, and requires a big learning curve on the part of the [fleet] customer and body builder. You have to subscribe to the new electronic format to make your truck design work. That can be good or bad, depending on your comfort level with multiplexing.”

Bannister makes the point, however, that those potential complications on the front end can be big advantages on the back end, especially in terms of integrating the truck chassis and body.

“In addition to offering a set of pre-programmed features, [multiplexing] enables body builders to develop a custom logic solution with very little effort, eliminating the use of third-party body control modules or relay-logic designs,” he says, referring to the installation of extra packages of switches and controls in the truck's cab to operate exterior equipment, e.g., utility booms, snowplows, etc.

Multiplexing makes it possible to share functions with existing switches already mounted on the dasboard via software programming, not wire splicing. “We provide hardware modules that are factory installed and pre-wired; users then just create a custom software solution that satisfies body control requirements,” says Bannister.


One of the most exciting benefits of multiplexing may be generated when it's used in conjunction with telematics.

This July, International plans to offer a telematics system, developed with the help of IBM, that offers remote vehicle monitoring, performance diagnostics and security elements — all integrated and controlled through a multiplexed electrical system, explains Bannister.

“Fleet owners, managers and technicians lose control over some of their most important assets when their trucks leave the lot and go to work each day,” he says. “In most cases, our customers want to know everything about the performance and productivity of these vehicles. Telematics offers a way for our customers to track location, monitor performance, diagnose maintenance issues, and ensure driver and vehicle security by taking advantage of wireless telemetry.”

Bannister says it's like forming an electronic umbilical cord between a fleet's vehicle and home base, using an in-vehicle device to transmit telemetry, location, and other information via cellular wireless technology that is then organized and displayed on a secure Web site.

“Diamond Logic gives us the platform for launching a telematics system that can be upgraded in order to collect and transmit a wide variety of information,” says Cortney Gluzas, International's business line manager for truck electronics.

“Whatever data we want to collect in the future, including fuel purchases for fuel-tax reporting, and engine and transmission operating conditions for prognostic maintenance, the multiplexed architecture allows us to develop modules to collect it through simple programming — not by adding wiring harnesses,” she explains. “The same holds true for security needs. Multiplexing allows telematics to be more flexible. That's why we view multiplexing as the platform upon which we can solve customer needs in the future — needs they may not even know they have yet.”

Survey Says …

To find out just how much the fleet community understands what ”multiplexing“' really means and who benefits most from this kind of electrical system design, International Truck & Engine Corp. conducted an online survey with about 30 customers and found they still have a long way to go to get everyone on the same page. Here's what the survey found:

36.8% of respondents are very familiar with multiplexing, while 41.7% are not too familiar or have no familiarity at all with the term.

22.3% believe multiplexed electrical systems will provide the greatest benefit to emergency, fire and rescue fleet applications; 16.5% believe utility fleets stand to gain the most; and 9.7% felt government fleets will benefit most from this technology. However, 41.7% were unsure what kind of fleet application would benefit most from multiplexing.

22.3% of respondents felt truck equipment manufacturers — those making and installing the ambulance, utility, and other truck bodies for medium-duty trucks — stood to benefit most from multiplexing, followed closely by mechanics (21.3%) and drivers and fleet managers (18.4% each).

The biggest benefit fleets said they expected to get from multiplexing ran a wide gamut, reported International: 28.1% felt the biggest gain would be faster service and maintenance, while 21.3% cited improved durability and reliability, 19.4% tapped lower operating costs, and 12.6% pointed to easier body installation.

About the Author

Sean Kilcarr | Editor in Chief

Sean reports and comments on trends affecting the many different strata of the trucking industry -- light and medium duty fleets up through over-the-road truckload, less-than-truckload, and private fleet operations Also be sure to visit Sean's blog Trucks at Work where he offers analysis on a variety of different topics inside the trucking industry.

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