Medium Duty: Body, Say ‘Hi’ to Chassis

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

“If you have a car that’s built in 1995 or later, with anti-lock brakes, and the engine and the brakes and the transmission are all talking together, you’ve got a truly integrated vehicle,” he explains. “You know that the transmission, the brakes and the engine are all working together. If you step on the brakes, the transmission, engine and brakes will all decide how hard to brake you safely. People today just don’t worry about skidding like they used to.”

For a long time, however, truck and bus operators couldn’t take advantage of this technology. Heavy duty engine, transmission and brake manufacturers simply didn’t have common language.

“That’s how J1939 came to be,” says McClure. “This is how (these components will) talk together. The whole goal was to get an integrated truck to meet the safety requirements of the Federal Motor Carriers Safety Administration and the emissions requirements of the EPA.”

Today that goal has been reached, and a heavy vehicle of today using J1939 is very different from a heavy vehicle of ten years ago. A former Allison man, McClure naturally uses an example from his days with the transmission manufacturer: “Their transmission will actually control the torque of the engine during the shift,” he says. “So that on a passenger bus, there used to be a jerk there and if you had a passenger standing up it might knock them down. They will not feel that shift today.”


The problem faced by the RP 1507 Task Force, McClure explains, is that not everyone adheres to J1939. Body builders for vocational trucks don’t understand the protocol, he says, and so they don’t use it, even when told that it can cut costs and increase reliability.

The fleets get it. Why don’t the body builders?

“It (J1939) has cut the fleets’ time for repair immensely, because they get their PC, they roll it out there, and it does a lot of automated diagnostics for them,” McClure says. “If they can start going to the body builders and saying, ‘You see the kind of test integration we have over here for diagnostics and troubleshooting? Why can’t we have that across the entire vehicle? What can you do to give us those services?’ that’s going to lead to maintenance costs going down, and sales going up.


Will RP 1507 lead to better integration between chassis builder and body builder? It must, according to Mike Ehrenhaft, manager of technical maintenance—chassis for Waste Management, Inc, and another RP 1507 Task Force member.

“I’m coming at this specifically from the refuse application, but there have always been communications issues that exist between the first stage manufacturer—the OEM—and the second stage manufacturer—the body company,” he says. “When these two don’t talk, out jumps the devil.”

The problem, as Ehrenhaft sees it, is that the OEMs and body builders are constantly improving their products, to grow with the needs of the customers and remain competitive —a good thing—but, for a variety of reasons, they do their product development in a vacuum—not always a good thing. “So, they might develop an idea, but they keep it internal until it’s time to implement it in production,” he explains.

“If these changes aren’t communicated between the chassis OEM and the body OEM, there are interface problems,” Ehrenhaft says. “And either one of them can be guilty: the chassis manufacturer, the first stage, can evolve their electronics and not let the body companies know, hoping they’ll catch on, and the body company can do the same. The bottom line is, one company’s success can cause a problem for another company to solve.”


But companies can’t be counted on to solve each others’ problems, and that’s where RP 1507 comes in. Two major objectives were for the establishment of a standard connector between chassis and body, and a standard location for that connector on the chassis.

“We wanted to provide a standard location for a plug, or connection, that would connect the chassis to the second stage, or body,” Ehrenhaft says. “We spent several days trying to determine where, on a particular type of chassis—either conventional or cab-over—to put it. Should we be back of the cab? Should we be a firewall location? Should we be left or right? How high? What’s the design of the plug? As inconsequential as those things sound, they are significant when the manufacturer has to source the components and build them into the truck plants.”

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