Down to the Wire

Medium Duty: Deciphering multiplexed wiring and the diagnostic tree.

“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.

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