Wired for Weight Loss Multiplex wiring makes its way into small-car market

It seems every great innovation in automobiles celebrates its coming-out on fancy luxury cars. You name the feature - from power steering and automatic transmissions to power windows and tilt steering wheels - and a Cadillac, Lexus or Mercedes-Benz was probably the first to serve it up for consumer consumption.

But some creature comforts are just too appealing or too practical to be hogged by the rich. After a break-in period on high-end vehicles, many features find their way onto virtually every U.S. vehicle, either as standard or optional equipment.

The latest is multiplex wiring. Chrysler Corp. has announced that its least-expensive vehicle, the Neon, will incorporate multiplexing in the 1999 model year for cost and weight savings.

The average driver knows little about multiplexing, but it is playing a critical role in holding down the price and weight of new vehicles as automakers add more new electronic safety features and conveniences (see WAW - Oct. '97, p.76).

In automotive applications, multiplexing allows a host of separate modules to communicate with one another through one or two wires. Without multiplexing, a bundle of wires is necessary to transmit information from module to module.

For instance, between an engine and transmission as many as 10 wires have been necessary to relay information such as speed, temperature and pressure. But with multiplexing, all information flows through one set of wires, which can be extended to include other modules, such as the instrument panel, to communicate with the driver. Other vital systems also can be included in the loop, such as antilock brakes and traction control.

"Right now we have two to three major customers who have up to 23 different electronic systems on a multiplex bus," says Scott Ballantyne, worldwide marketing manager for Motorola, which supplies chips for multiplexing. "On the market today we have up to 20 vehicles in production with multiplexing."

Silicon makes the whole link possible, as two tiny control chips are located at each module and direct the endless stream of information, guiding each signal - thousands of them per second - to its correct destination instantly and without conflict.

The advantages of multiplexing are enormous. The amount of hard wiring in the average car - about 70 lbs. (31.8 kg) - could double in the next few years as carmakers introduce safety devices such as night vision, "intelligent" air bags and navigation systems. Mr. Ballantyne estimates weight savings of up to 20% over conventional wiring.

Up to 40% of new vehicles in North America have some level of multiplexing, says Joe Fadool, marketing manager for electrical/electronic distribution system at Siemens Automotive. And the cost of doing it is going down dramatically.

"The node-connect cost, the cost to put a module on the multiplex link, is 50% less than it was just a few years ago," Mr. Fadool says. The reason: More and more chip sets are becoming available in high volume.

But multiplexing isn't paid for by eliminating wires and connectors, says Fred Miesterfeld, supervisor of advanced electronic development at Chrysler. "Where you save is the fact that you eliminate redundant sensors and you get devices to work together to eliminate modules and circuits," he says.

Multiplexing has been used in the telephone industry for years, allowing millions of people to carry on conversations at once, over great distances. Automakers and suppliers have been working with multiplexing since the 1960s, but Chrysler was the first to use it in a production vehicle, in 1986.

Experts say the move toward more multiplexing across all vehicle lines is inevitable.

"We will see all cars having it eventually," says Roch Basson, vice president of electrical and electronic products for United Technolgies Automotive, which is multiplexing the new Honda Accord. "From our exposure, every single vehicle is considering multiplexing as a serious option. For small cars, yes they have fewer electronic features, but they also have less space to package those wires."

Chrysler had been using its Chrysler Collision Detection (CCD) multiplex system since 1988, but it switched to the new J1850 system in the 1998 Concorde and Dodge Intrepid. Each vehicle has 10 to15 modules on the multiplex bus, says Jack Yellin, marketing manager for Harris Semiconductor, which supplies Chrysler's new control modules.

J1850 was developed in 1994 by the Big Three as a standard for multiplexing to provide a diagnostic port on new vehicles to check fuel economy and emissions levels, as required by the California Air Resource Board and the U.S. Environmental Protection Agency. Aside from environmental implications, the diagnostic port also will make it easy for mechanics to pinpoint trouble.

The switch to J1850 will allow Chrysler to add more electronic modules to the multiplex bus than were allowed with CCD and will allow those modules to communicate more quickly, all at lower cost, Mr. Miesterfeld says.

Although the future of multiplexing looks bright, there seems to be some debate about which type of multiplexing system will prevail. J1850 appears ideal for body electrical systems, but some automakers, particularly in Europe, prefer the more expensive Controller Area Network (CAN) system, especially for engine-control functions, because it operates at higher data speeds.

"We see a pervasive nature of CAN into the North American market. GM has announced a few nodes will have CAN, probably in the engine," says Mr. Ballantyne of Motorola. "But it looks like there is space for both."