Medium Duty: Body, Say ‘Hi’ to Chassis

TMC RP 1507—The scoop on the new J1939 Network Interface Strategy.

How many times have you seen a science-fiction movie where the hero can miraculously communicate with an alien life form because he’s brought along his trusty “universal translator?” How cool would it be to have that in real life?

Okay, maybe that’s far-fetched, but the vocational truck industry may soon have something like that, in the form of the Technology & Maintenance Council (TMC) Recommended Practice (RP) 1507: Recommendations for a J1939 Network Interface Strategy. According to TMC, the purpose of RP 1507 is “…to recommend key elements of a J1939 network interface strategy for use by vocational/specialty vehicle OEMs.”

“The current PR is a first step,” says Cindy Florek, who chaired the RP 1507 Task Force for TMC. “Not all the issues of J1939 implementation are addressed in this RP.”

A senior project engineer with Allison Transmission, Florek is well-versed in J1939 communications, and the need to get body builders talking to chassis OEMs. And, after serving as Task Force Chair, she is equally well-versed in the challenges of getting both chassis OEMs and body builders to the table. “We attempted to address a common OEM and installed body builder connector and connector location recommendation and could not get consensus,” she says. “We also attempted to engage body builders in the development of this RP. We wanted to understand what J1939 messages they are currently using and what messages they would need to have identified by the SAE J1939 committee.”

Fortunately, several fleets were more than happy to participate in the Task Force, and let the manufacturers know what they wanted and needed in a communications standard. “Fleets were primarily interested in making sure they get a reliable network system that can be tailored to their needs,” Florek says. “The network troubleshooting and repairing should happen without having engineers from OEMs, suppliers, and body builders in the service bays.”


The J1939 standard came about because chassis OEMs and body builders were trying to address cost and reliability, according Bob McClure, co-founder of telematics company, and a RP 1507 Task Force member. “One of the advantages of J1939 is that you can use it to do multiplexing,” he explains. “And as such, that allows you to cut the cost of copper—you get rid of a lot of wires, and copper is extremely expensive. For example, I can buy a computer chip cheaper than I can buy a connector. A good connector will run you $3 to $50; a computer chip will run you $3 to $20.”

According to McClure, wiring is and always has been a truck-building nightmare. “You’ve got all those wires and the cost of a wiring harness is unbelievably expensive,” he says. “And from a maintenance standpoint, when one of those wires or one of those bundles breaks, or has a bad connection, it can literally take you days if not weeks to find an intermittent connection.”

Multiplexing replaces all the wires in a traditional harness with only two wires running the entire length of the truck. Costs are reduced, maintenance is simplified and diagnostic capabilities are increased. “From a build point of view, the cost drops dramatically, because you get rid of all those wires and connectors,” McClure says. “And from a maintenance point of view, you have the advantage of having distributed processing, so I can have the processor run diagnostics on itself and power up and tell whether it’s got an intermittent connection, and if so, where. I can have the computer tell me which one of those wires is getting flaky on me.”


McClure credits engine, transmission, and brake manufacturers for pushing J1939 as a way to get a communications technology in place where they could fully integrate a vehicle’s driveline.

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