Down to the Wire

When is the last time one of your technicians “interrogated” a truck? According to Steve Matsil, General Motors’ chief engineer for medium duty commercial trucks and rear-drive vans, it happens all the time. No, it doesn’t involve a dark room with blinding spotlights, or a “good cop/bad cop” routine. It simply requires a “sophisticated” technician who knows his or her way around a computer, says Matsil.

We’re talking about multiplexed wiring, something Matsil knows quite a bit about. He has developed GM’s LAN (Local Area Network) architecture, the SAE J1939 standard communication protocol by which the major system control modules in GM’s medium duty commercial trucks communicate with each other. When asked how multiplexing has simplified the life of the medium duty technician, Matsil hedges a bit.

“When you say ‘simplify...’” he chuckles, then, apparently unwilling to redefine the word ‘simplify,’ he pauses and tries a different tack. “The complexity of the module now requires a very sophisticated microprocessor, which will allow it to think for itself, instead of just reacting to stimuli. You’re allowing someone to interrogate the system to determine why you have a check engine light on. The technicians now have a lot more diagnostic tools at their fingertips to sort which module it is, what is the code that is causing the error to be communicated, and that code now takes you pretty close to the route of the problem.

“So, It can simplify the technicians’s work,” he says, “but now the technician has to be more than a parts changer; he has to be able to take a tool, like a Tech Two or a diagnostic tool, plug it into the ALDL connector, and interrogate the system, and basically say “I want to know a whole bunch more about why I have this check engine light on, and/or another form of malfunction, like an ABS light, a brake light. It will allow you to do a lot just by sitting at your computer and interrogating, rather than taking things apart, jumping wires, and using the old backyard mechanic approach. So while it may make the job easier, the sophistication of the technician’s training and skills is higher.”


On a Chevrolet Kodiak or GMC Topkick, a network of control modules governs the performance of the engine, the Allison transmission, the crash sensor and diagnotic module (or black box), the radio, the anti-lock brakes (ABS) and the instrument cluster.

“All have unique addresses, and they can receive and transmit messages,” Matsil explains. “So they are all connected by this bus called GM LAN. They’re compatible, they can talk to each other, and when a message is sent out from the engine control module that may tell the instrument cluster to illuminate the check engine light because the engine experienced a misfire or a low oil pressure or high coolant temperature, then the modules are now communicating via this bus or GM LAN architecture.

“If you look at a GM LAN, we would anticipate up front that we would have certain federal requirements relative to monitoring and reporting, and we would have other ones that would help us with diagnostic time and repair labor costs,” he says. “So, we would put them in there relative to breaking them down into individual codes that would define the error.”

The result, Matsil explains, is a fairly complex list of things that the technician could confirm very quickly, via electronic verification, as being operational or non-operational, and needing to be serviced.

“When you turn on the system every module will check in, say, ‘Here’s my address,’ the bus will recognize it, and it will communicate a state-of-health message,” he says. “That state-of-health message is like a handshake; it says, ‘I’m online, and I’m ready.’ You do this in some cases many times a second, and in some cases less frequently.”

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.

“What we did not do is multiplex the wiring for the lighting systems and the upfitter integration. That is still traditional, and for many of our customers that is what they want. Many of our upfitters are not high-volume, not high-tech, and it works right for them.”

In the next redesign cycle for the trucks, Eaves says, General Motors will look at providing multiplexing for upfitters.


For smaller fleet shops, multiplexing can be a mixed blessing. Because much of the diagnostic work enabled by multiplexing involves proprietary software and, thus, requires proprietary scan tools and specialized training, some will have to bring their malfunctioning trucks back to the dealer.

But because multiplexing is becoming so prevalent, even the smallest fleets will have to get with the program sooner or later.

“I think that in the last few years, because of everything that’s changing in the automotive industry, you really have to be on-board,” says Eaves. “You have to understand what’s going on with the electrical systems, and now with all the passenger cars and light-duty trucks going this way, there’s a lot of momentum in the established shops to understand how to do this correctly.”

Those that do get onboard can look forward to another potential benefit to multiplexing: more advanced telematics capabilities.

“You see it on the light-duty vehicles from GM, the interface with OnStar,” eaves explains. “We don’t have that on our medium-duty trucks now, but we can certainly see that as a possibility in the future. You can pick up these signals, you can alert people with a warning light, and with OnStar they can diagnose things over the line with the customer. You can tell the customer, ‘You need to go to a dealer now,” or, ‘It’s okay, complete your journey, but then take it to the dealer at your earliest opportunity.’

Does that mean that in the future fleets will be relegating their decisions to a disembodied OnStar operator? Not necessarily, but it does underscore the benefits of multiplexing: greater diagnostic power, and more options. And that’s not a bad thing.