Multi-Question Exam

Understanding school bus multiplex wiring issues

By Bill Siuru, PhD, PE

The average 1948 car had 55 wires with 150 feet of wiring weighing 10 pounds. By 1994, this mushroomed to 1,500 wires weighing 165 pounds with a mile of wiring. Besides adding weight and consuming space, complicated wiring harnesses brought assembly and maintenance headaches. The solution was multiplexing.

Electronically controlled engines with OBD (On Board Diagnostics) and anti-lock braking resulted in vehicles with multiplexing beginning in 1999. Multiplexing using the SAE J1939 standard for heavy trucks has been installed in school buses for at least four years by Blue Bird, IC Corporation and Thomas Built Buses.

Digitally-based multiplexing replaces individual wires between switches and devices in analog systems, with a “data bus” sharing command signals, feedback signals and data between devices. Input and output commands in one part of the vehicle are combined in a multiplex module containing a microprocessor. Multiplexing is used for “switching” circuits, that is on/off commands, as well as for vehicle operation, such as engine RPM, road speed, fuel level and other data combined on the “data bus.”

Typically, a single-power wire is used. Because an excellent ground is vital to preclude ground differentials between data link modules or nodes that result in unreadable messages or the appearance of a loss of functionality, a common ground, usually not the vehicle, is used.

Data bus communications consist of individual packets of binary-coded (0’s or 1’s) information transmitted in microseconds, like information sent over the Internet. In most multiplex systems, information is received by all the modules, but ignored by all except the specific module to which it is addressed.

Multiplexing is great in the family car, but some wonder if it is necessary in school buses without power windows, locks, seats and other creature comforts. However, school buses do have complex electrical features such as safety lights, fans, doors and air conditioning.

“Multiplexing wiring in school buses is just like when we switched form gas to diesel,” said Chuck Stevenson, the assistant director of vehicle maintenance at Brevard County Schools in Florida. “There’s a learning curve.”

Some have been hesitant to embrace the technology. Retired ASE Master Technician Richard Stafford, who now works on school buses part-time, questions the need for multiplexing in school buses.

“A commercial vehicle’s basic electrical system must be straight-forward (simple) and rugged,” Stafford says. “Electrical problems on school buses are common and a well thought-out electrical design is critical for reliability and trouble shooting.”

Greasing the Wheel with Technology
Decades of bus manufacturing have shown multiplexing is not required to turn on lights, heater motors, fans, turn indicators, stop signs and so on. But it can simplify operations for the driver. And for the shop, there are also many benefits of multiplexing versus analog systems. Multiplexed systems, if designed and implemented properly, can be simpler, more trouble-free and easier to repair. Diagnosing problems can be easier than locating faults with a troublelight or multimeter.

Multiplexing not only eliminates complicated wiring harnesses, but also many terminals, connections and splices, often the source of faults. Multiplexing in Thomas Built’s Saf-T-Liner C2, for example, reduced the number of wires from 650 to 350, meaning 300 fewer possible failure points. Because wires can run only from a “receiver” V-Mux to a controlled device, wires can be short and don’t have to run the length of the bus. Thus, it is easier to locate problems, and only short wires need to be replaced to correct a problem.

Two-way communication on the data bus means receiving devices can communicate back to the sending module to inform that the message was received. This feedback capability allows self-diagnosing of faults. A “none” response to a command can trigger a diagnostic trouble code that can be read later by a technician. Feedback also makes school buses safer. For example, the driver can be immediately alerted of unsafe conditions, such as a burned-out brake light.

Multiplexing brings a big change from familiar analog devices, so technicians need to understand system components and their role in each step of the multiplex process. Like any new technology, training is a must to keep buses in service.

“Most mechanics seem to be intimidated at first by the unknown,” notes Bud Fletcher of the Florida Department of Education. “However, when they start understanding how multiplexing works, they become interested in learning more.”

Though differing with each individual and their demonstrated willingness to learn, Fletcher adds “a good technician could gain the basics in one day of training by a good trainer. Then, there should be two or three follow-up, in-depth classes along with everyday on-the-job application in order for a technician to become really proficient.”
However, many of the “old-timers” may never take an interest in some of the newer technology.

Besides trained technicians, quality measuring equipment for troubleshooting the complex multiplex circuits is needed. Typically, a laptop computer with the correct software is required, usually the same laptop and software already used for engine and transmission diagnostics. It is rare a technician that isn’t already using a computer, even in a school bus garage.

Myth vs. Reality?
Multiplexing for school buses is just emerging, thus some of those charged with maintaining school buses wonder about such things as how they survive in the harsh school bus environment. Real-world concerns remain, such as, can the new breed of diagnosis and repairs really be performed in a school district’s own shop?

“School buses are held to strict safety standards, specifically SAE 1455 for electrical components,” says IC Corporation’s Keith Kladder. “This includes temperature, vibration, salt spray, etc. In addition, we have real-world experience with over four years of multiplexed buses in the field.”

In the first year or so, service will probably be done under warranty by dealerships, but then what? IC Corp. representatives said multiplexing will probably not result in more frequently replacing expensive components rather than repairing them. That’s because multiplexed systems will help identify the failure, allowing for simplified repairs. Another benefit is that there are no moving parts, such as mechanical relays.

“Customers have all the tools available to them to service multiplexed buses themselves,” Kladder adds. “Generally, if customers take their vehicles to dealerships to service the engine or body, they will likely continue to do so for the electrical system.”

IC Corp. expects no changes to servicing due to the multiplexed electrical system, and all the needed diagnostic software is available to school district mechanics.

Siuru is an automotive freelance journalist based out of Temecula, Calif.