Equipment Type

Multiplexed Wiring Aids Truck-Equipment Integration

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.

March 01, 2008
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 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 buses so all systems work together properly.

ECMs analyze data sent by sensors at various points on the chassis to make operating decisions. An anti-lock braking system, for example, considers wheel speeds and, sensing one is slower than the others, reduces braking power at that wheel. It 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 tries 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 reduce the amount of NOx produced. Meanwhile, it 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). On EPA-'07 diesels, the ECM tells after-treatment devices to heat up so accumulated soot particles are burned off.

Not all ECMs in a truck operate so elaborately, but each one is vital to the system it regulates. And they must be integrated so the truck and all 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 usually reliable, but wiring and connectors — the simple mechanical parts of the system — are troublesome.

A way around much of that is the multiplexed electrical system, now offered on some Freightliner and International trucks. 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 for the DSL for your computer. The signals are digital, arranged in codes of 1s and 0s, so each signal does only what it's meant to do. Other items on that same wire are controlled by other coded signals from other switches. As with a hard-wired truck, many signals course simultaneously through the electrical system to keep the truck's many systems operating.

Multiplexing integrates systems to protect against improper operation. For example, a dump truck's multiplexed system can be programmed to limit engine and road speed — 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 return 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.

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 example, a technician can pinpoint wiring and connector problems, not just problems on a 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.

Multiplexing can cut the number of wires and leads almost in half, 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 on the truck.

International's Diamond Logic multiplexed wiring is standard on DuraStar 4000 and WorkStar 7000 trucks and on 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. Both manufacturers expect to increase the number of models that are multiplexed.

"International's Diamond Logic system makes our life so much easier because we can utilize pre-wired systems," says Ron Wright of J&J Truck Bodies and Trailers, which builds and installs equipment on many makes of vehicles. "It eliminates the need to go into the dash to pick up electrical current for auxiliary lighting and/or other functions."

Pre-installed switches are programmed to run a truck's various functions, so 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 dump, which can require 100 to 150 man-hours to assemble.

"Wiring becomes part of the diagnostics, so it saves time in troubleshooting," says Wright. "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's Diamond Logic has. They had maintenance people in mind when they came up with the multiplexing."

J&J's certified technicians program the systems using Diamond Logic software as they install and hook up bodies and equipment. The software allows technicians to totally integrate the truck's electronics with the new body and equipment. Freightliner's multiplexed system doesn't allow upfitters to program the truck; dealer technicians must do it.

It is vital that a truck be ordered with the proper electrical system options so the switching and plug-in capabilities can be utilized. 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 the 210-page manual.

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.

"There is no need for cutting and splicing into our electrical system," says Lepage.

International supplies all the wiring, connectors and switches needed for hooking up an almost endless list of equipment. Lepage uses a municipal truck 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 that the equipment upfitter can tie in to," he says. "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 in to using Quality Connect fittings. You cut your air line, and the 1/4-inch control line, and push it into the fittings."

For at least 10 years, truck builders have been including junction boxes or modules at the rear of cabs and along frames, and there is no reason to cut or splice any modern electrical system, whether hard-wired or multiplexed.

Cutting into a system can create unforeseen 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 use them. Even a truck that's a decade old may have plug-in points indicated on wiring diagrams in the truck's manuals. If not, a manufacturer's website should be able to provide the information.

Continuity tester

According to Bob Johnson, director of fleet relations at the National Truck Equipment Association, another danger 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 a nick in the wire and wick through much of its length, attacking the copper and eventually ruining the wire and rendering the circuit it supports useless. Also, the tester's lamp draws 1.5 amps and some sensitive circuits won't support that much, which can blow a distribution module.

Use proper tools to test circuits in today's trucks. That includes using a digital volt-ohm ("vohm") meter at metal connectors in junction boxes. Preliminary analysis can be done with a scan tool 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.

Except for the SAE J1939 data bus 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 having technicians thoroughly trained on systems.

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