Down to the Wire

Medium Duty: Deciphering multiplexed wiring and the diagnostic tree.

In practice, the capabilities of multiplexed wiring are limited to the imagination of the maintenance manager who spec’s the truck. He could set the truck’s radio to chime audibles when there’s something wrong, instead of putting separate speakers in. He could spec’ two 20-amp circuit breakers to protect the truck’s tail lamps, and then spec’ running lamps that have lower protection. Additional modules can provide customized control and support for auxiliary add-ons, like a PTO circuit, body interior wiring, or control for a salt spreader.

“The key is going to be understanding what sort of circuits you need to protect,” Matsil says, “and understanding how much a module might cost in order to obtain the objective.”


As Matsil says, this complexity can simplify the technician’s job, as long as that technician understands the technology.

“There’s a methodology, when you look at the decision tree, for how you’re able to determine what the problem is,” he says. “There’s a series of steps that you can learn that I don’t think are that tough for a qualified mechanic. Perhaps it’s going to take some training, some people more than others, but I think it’s something that can be taught.

“Of course, some people are more adept at, and more comfortable with, the electronic tools than others, but if they’re willing to learn and use what they’ve built up over the years in terms of intuition and experience, they can be much more productive now in terms of how much time it takes,” he goes on.

“For example,” he says, “a typical decision tree may be: a vehicle’s running rough, or it’s in limp-home mode; you’ve got a check engine light and/or reduced power light, or both. You can start thinking about what parts you can change—fuel pump, trying to understand whether you have low compression, if the engine’s running rough, if you’ve got wires that are grounded, or their insulation’s work down, or any of the peripheral sensors, whether they temperature or O2 sensors, are defective.

“Now, you can change a lot of parts before you come to the right answer; but here you plug into this ALDL connector, you boot up a computer, and it walks you through this decision tree with a printout of the codes that are affected.”


“Fleet customers are generally satisfied with multiplexing,” says Mike Eaves, product manager, medium duty trucks, for General Motors. “What multiplexing does is it really makes the whole system more reliable. Because the engines are now so sophisticated, technicians can diagnose things that you couldn’t diagnose before. So, it’s working out well, because you pick up on faults that probably would have gone unobserved before until there was a major problem. Now you get a fault code, you know something’s wrong, and you can react more quickly than you could in the past. So rather than have a catastrophic failure, or the truck stops working for some reason, maybe you’re avoiding that, and getting more uptime with the vehicle.

“Here’s a good example of what we can do,” Eaves says. “We can set the exhaust brake up now in a way we couldn’t before. Before it was just ‘on’ or ‘off.’ Now, we can set it up to operate within certain specific parameters, so when you’re almost at a stop we back the exhaust brake off. So it sets up a speed signal to turn itself off. We also integrate it into the automatic transmission, so that when you select the exhaust brake and you take your foot off the throttle, the vehicle will sense that and it will immediately take a downshift whenever it can. I mean, if you’re going too fast you can’t downshift, but the exhaust brake will come on and as soon as you can take a downshift, it will downshift. In a lot of driving, particularly out west, you can go up and down grades without touching the service brakes. So, that is a nice feature that gives you more control over the vehicle, and it can do that because everything’s talking to one another.”

Despite these obvious benefits, when GM designed the current Topkick and Kodiak in the early 2000s, they limited the multiplexed wiring system to the truck’s onboard systems.

“We integrated the electronic engines with the Allison transmissions, that were electronically-controlled, along with the instrumentation and the brake system, so they can all talk to each other. So the vehicle is talking to itself,” says Eaves.

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