Multiplexed Wiring Aids Truck-Equipment Integration

By Tom Berg, Truck Editor | September 28, 2010

Fewer wires in a multiplexed system means a relatively simple and well-organized package inside the dash.

Municipal dump truck chassis being upfitted by J&J came with factory installed switches and hookups for the blade's joystick and controls for the box and spreader. The truck's wiring system must be properly spec'd to take full advantage of its capabilities.

Trucks Are Smarter

Electronic control modules, or ECMs, are actually small but increasingly powerful computers that run engines, transmissions, brakes, instruments and other on-board systems. Since 1992, when electronically controlled diesels first went into trucks, Freightliner has added almost one more ECM each year, says Paul Menig, chief engineer for electronics. There are now 10 ECMs in a typical heavy truck, and sometimes that many on a medium-duty truck. Most of those "talk" to each other via industry-standard data busses so that all systems work properly together.

ECMs analyze data sent by sensors at various points on the chassis to make operating decisions. In an anti-lock braking system, for instance, consider wheel speeds and, sensing one that's slower than the others, reduces braking power at that wheel, and does it several times per second, at as many wheels as necessary, until the speeds equalize and the skid has been controlled.

If the truck or trailer has electronic stability control, the same ECM also considers data from accelerometers on the chassis, as well as steering input (as the driver turns the wheel) and engine operation; if the ECM senses an impending rollover, it tried to prevent it by cutting engine power and applying the brakes, pulsing them if the uneven wheel speeds indicate a slippery road surface.

The engine's ECM constantly looks at ambient temperatures, road speed, temperatures and pressures inside air and oil passages, demands for power (as the driver mashes or lets up on the accelerator) and other conditions and decides how much fuel to send to the cylinders and how to inject it for the cleanest possible combustion.

If the engine uses exhaust-gas recirculation, the ECM decides how much gas to put into the cylinders to crowd out oxygen, cut burn temperature and therefore reduce the amount of NOx produced. It meanwhile regulates operation of the turbocharger to get the right amount of pressurized air to the cylinders (some variable-geometry turbos now have their own ECMs, which talk continuously to engine ECMs), and tells aftertreatment devices to heat up so accumulated soot particles are burned off.

Not all ECMs in a truck operate so elaborately, but each one's function is vital to the system it regulates. And all must be tied together — integrated — so the truck and all of its equipment function properly to get the job done cleanly and safely.

Electronic systems pervade today's trucks. Where trucks of yesteryear had hundreds of electrical circuits, trucks now have thousands. Thus, cabs and chassis now have hundreds of feet of wires, and scores of connectors, plugs, sensors and electronic control modules; and the more there are, the more chances there are of failure. Electronic parts — ECMs and sensors — are rather reliable, fleet managers say, but wiring and connectors — the simple mechanical parts of the electrical system — are troublesome. But then wiring and lights have always topped managers' lists of maintenance woes.

A way around much of this is the multiplexed electrical system, now offered on certain Freightliner and International trucks. Those builders explain that multiplexing is the sending of multiple signals over a single line. It's not new, as the phone line into your house might now carry signals for not just the phone but also the DSL for your computer and maybe TV programs.

The signals are digital, arranged in codes of 1s and 0s, so each signal does only what it's meant to in the truck; it can turn a specific light or piece of equipment on or off. Other items on that same wire are controlled by other coded signals from other switches. As with a hard-wired truck, many signals simultaneously course through the electrical system, and must for the truck's many systems to operate normally.

Multiplexing integrates all affected systems to protect some against improper operation by another, according to engineers and equipment upfitters. For example, a dump truck's multiplexed system can be programmed so that engine and road speed are limited — say, to 900 rpm and 20 mph — when the body is raised. Those speeds would be higher with a municipal dump that spreads salt at near-highway speeds during snowfalls; the system might also turn off the tarp mechanism at 15 mph and the dump hoist at 50. On a bulk-hauling tractor, engine revs can go to a preset speed when the PTO is engaged to operate a pump.

These things are accomplished on hard-wired trucks by adding wiring, relays and interlocks. But with multiplexing, it's all done through programming and the wiring system stays as it is. A new "stock" truck sitting on a dealer's lot might be usable in a given application if its wiring system can be reasonably accessed and its power- and drive trains have the right components.

Diagnostics are enhanced with multiplexing. When something goes wrong, multiplexing aids in finding the faults because some switches and gauges have built-in intelligence. These complement the diagnostic capabilities of ECMs on the various components. For instance, the technician can pinpoint wiring and connector problems, not just problems on a certain circuit.

Ivan Neblett, Freightliner's vocational product manager, says this happens if the system encounters an unexpected sequence when power is sent down a circuit. If power doesn't arrive at the intended destination, the system notes where the blockage is and stores a fault code. If the power is being sent to a headlight and gets no results there, it warns the driver the headlight is out, before it becomes a safety problem or legal issue.

Multiplexing allows use of fewer wires and leads, like from 600 to 350, and the jumble of wires in the dashboard is reduced. This makes the harness simpler and easier to understand for the technician. It also reduces the potential for trouble, speeds manufacturing, and saves weight and cost.

International's Diamond Logic multiplexed wiring is standard on DuraStar 4000 and WorkStar 7000 trucks and the ProStar 9000 highway tractor. Freightliner's Business Class M2 and the new Cascadia highway tractor are multiplexed, but the wiring system has no special name. Other models will be multiplexed later, those builders say. All other truck makers' chassis are still hard-wired, but a few are beginning to work on multiplexing.

Ron Wright at J&J Truck Bodies and Trailers in Somerset, Pa., which builds and installs equipment on many makes of vehicles, says International's Diamond Logic "makes our life so much easier because we can utilize pre-wired systems. It eliminates the need to go into the dash to pick up electrical current for auxilary lighting and/or other functions." Pre-installed switches are programmed to run a truck's various functions, so the installers can just plug into junction boxes under the dash, behind the cab and at the rear of the chassis. This can save five to six hours on a complicated municipal dumper, which can take a crew 100 to 150 man-hours to assemble.

"Wiring becomes part of the diagnostics, so it saves time in troubleshooting," Wright says. "Plus it's a much more professional setup because the wiring's already done for us. No other chassis has given us the amount of leeway that International has. It's well thought out; they had maintenance people in mind when they came up with the multiplexing."

J&J was the first upfitter to get technicians completely certified on Diamond Logic, he says. This included training at J&J's home base and at International's facilities near Chicago. This allows J&J's technicians to program the system using Diamond Logic software as they install and hook up bodies and equipment. Special "Program" software, purchased from dealers, lets technicians totally integrate the truck's electronics with the new body and equipment.

Within 72 hours of completing a truck, they must go online to International's computer network and register the truck; if they don't, J&J's resident software locks up. This keeps International informed of what exactly is being done to its trucks so technicians at its dealers can work on them when the time comes. Freightliner's multiplexed system does not allow upfitters to program the truck, Neblett says. Dealer technicians must do it.

It is vital that a truck be ordered with the proper electrical system options so its switching and plug-in capabilities can be utilized, Wright emphasizes. Diamond Logic can be spec'd with switch packs in the dash as well as light connectors, remote power modules and pneumatic control boxes at various places on the chassis. It is a highly flexible system and its capabilities are outlined in detail in a 210-

page manual. Dealer sales people should know how to spec the system, and many do; but inexperienced salespeople are not yet aware of the system's intricacies, and this creates headaches.

About 65 percent of upfitters now use Diamond Logic correctly, says Josh Lepage, sales manager for product integration at International Truck and Engine. Those who don't can mess up installations, and typically trucks then go to dealer shops for fixing. While dealer technicians can undo and fix ill-conceived cuts and splices, they are not equipment installers and therefore probably can't know what each upfitter-installed item on the truck is supposed to do and how it operates. So it takes them longer, and this adds to the repair bill.

"There is no need for cutting and splicing into our electrical system," Lepage declares, and doing that is the cause of most problems with Diamond Logic. International supplies all the wiring, connectors and switches needed for hooking up an almost endless list of equipment. He uses a municipal snow- and ice-control truck, whose suppliers have taken multiplexing the furthest, as an example.

"We provide connectors for body lighting, for the plow lights and fog lights, and for the plow and spreader equipment, with a remote power module outside the cab which the equipment upfitter can tie into. It's near the rear of the cab, usually on the left side in an 18-inch-square area. They crimp terminal ends onto their wires and plug them into the module. Air solenoids are placed in the same area and they can be plugged into, using Quality Connect fittings; you cut your air line, the 1/4-inch control line, and push it into the fittings."

There is no good reason for cutting into or splicing on any modern electrical system, whether hard-wired or multiplexed, electrical experts have been saying for 10 or more years. For at least that long, truck builders have been including junction boxes or modules at the rear of cabs and along frames, and cringe when they hear about body and equipment installers who don't use them.

Cutting into a system can create unforseen problems with existing components because most things today are electronically integrated. Any upfitter or owner who wants to wire something into the chassis, from simple body clearance lights to complex power-driven pumps and controls, should find out where those boxes or modules are and learn how to plug into them, the experts say. Even a truck that's a decade old might have plug-in points, and these should be pointed out in wiring diagrams in the truck's manuals should indicate locations, and if not, a nearby dealer or manufacturer's website should be able to provide information.

Another forbidden practice, notes Bob Johnson, director of fleet relations at the National Truck Equipment Association, is sticking a continuity tester through a wire's insulation. The tester's sharp point breaches the wire's insulation and, if the wire is anywhere outside the cab, opens a spot where road salts can enter. Calcium chloride and other aggressive salts will invade any nick in a wire and wick through much of its length, attacking the copper and sooner or later ruining the wire and rendering useless the circuit it supports, and maybe others.

Also, the tester's lamp draws 1.5 amps and some sensitive circuits won't support that much; this can blow a distribution module or something else that circuit leads to. It's easy for a mechanic wielding a tester to do this as he probes for the wire that's supposed to be sending current to that unlit light, or whatever. He can hit the wrong wire, and it might be one carrying a sensitive circuit.

Learn the proper way and use the proper tools to test circuits in today's trucks, Johnson advises. That includes using a digital volt-ohm ("vohm") meter that's applied at metal connectors in junction boxes. Preliminary analysis can be done with a scan tool that's plugged into the J1939 connector under the dash.

Multiplexed systems offer many benefits for upfitters and truck owners, Johnson says, but there's confusion because the International and Freightliner systems are different and require different knowledge to work with. The confusion will grow as more manufacturers introduce their own systems. Except for the SAE J1939 data buss that all wiring systems now utilize to access electronic controls on the engine and elsewhere, there are no industry standards on truck multiplexing. The answer for now is educating upfitters and users and thorough training of their technicians.

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