Down to the Wire

Medium Duty: Deciphering multiplexed wiring and the diagnostic tree.

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.”

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