J1587 Port – Serial Communications on Heavy Vehicles

This Freightliner chassis has a J1587 Comm port over the radiator at the back of the engine. In general, this would be called an OBD (On Board Diagnostics) system. My chassis doesn’t use the typical OBD-II connector though. Instead it’s a plastic connector like shown below. On my Journey, one of the two installed J1587 connectors is centered on the air dam and has a twist on cap with tether. Here’s a link to a white paper that gives specs: Intro to J1587 & J1708 protocols

The weather cap helps keep the contacts free of corrosion. The connector mounted on the air dam, shown above, is the 6-pin Datalink. There is also a 9-pin used on newer vehicles. In this ’02 Journey, the 2nd data connector is under the dash, just to the left of the brake pedal. It can be accessed by removing the molded plastic cover that backs the dash. It does not have a cap. Note that large vehicle chassis manufacturers were not required to have any system like this until 2005 so your RV may not have anything like it until that year and after. This would have been up to the house manufacturer before that as to whether they ordered the chassis with OBD or not.

——–Note to readers—-I am learning this protocol and I am not an expert—–

In my RV, there are pins in 4 of the connectors holes, and 2 are empty. The empties are for OEM use and since the manufacturer rarely installs them unless they’re having a problem with the chassis the consumer rarely sees them.

A – J1708/J1587 +
B – J1708/J1587 –
C – Battery
D – OEM use
E – Gnd
F – OEM use

(This same connector is used when the J1708 Port setup is used by the OEM).

Protocol SAE J1587 supports up to 6 MID (Message ID) for diesel engines:
128 = Engine
175 = Engine Oil Temp 1
183 = Engine Fuel Rate
184 = Engine Instantaneous Fuel Economy
185 = Engine Average Fuel Economy
186 = Power Takeoff Speed

Most Diesel ECM’s will also respond to requests by the following MIDs, the response takes the form of a PID (Parameter ID) followed by data and control bits:

Typical SAE J1708/J1587 MIDs:
Transmission: 130
Brakes: 136,137,138,139,246,247
Instrument Panel: 140,234
Vehicle Management System: 142
Fuel System: 143
Cab Climate Control: 146, 200
Suspension: 150, 151, 152, 153
Vehicle Navigation: 162, 191
Vehicle Security: 163
Tires related: 166, 167, 168, 169
Particulate Trap System: 177
Refrigerant Management systems: 190
Tractor/Trailer Bridge: 217, 218
Collision Avoidance Radar: 219
Driveline Retarder: 222
Vehicle Sensors to data converter: 178
Safety Restraint System: 232
Forward Road Image Processor: 248

The above MIDs were industry assigned to use the J1587/J1708 protocol but the OEM decides which ones they want to install. For instance, it’s not likely a RV would need #217, 218.

Here’s an example of a message:

MID	PID	Data	PID	Data1	Data2	Checksum
128	21	50	12	05	48	248

Further info available here: Intro to SAE J1587

And here: Truck SAE codes

Here’s a link to a DIY J1587 reader system: Data scanner J1587

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Here’s an overall diagram showing major components, usually in an older RV from about 1996 on, there’s one connector in the back accessed from behind the engine, and another connector under the dash cowling, which has to be removed to access. Many newer RVs have a OBD-II connector next to the drivers left leg, up under the dash.

 

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Just to have it available here for readers with newer RVs, here’s how the J1939 setup looks:

[Edit: May 2019]

I ran into an available J1587 to OBD II cable adapter just recently and immediately purchased it. From Amazon. I hadn’t purchased one up until now because when I searched occasionally over the last three years, I just couldn’t find one at a decent price, and where the seller was located in the US. Those things came together recently and I bought it: Adapter

I am kind of excited it use it. I already have the OBD II reader that uses Bluetooth, and have the Torque app installed in both of my Android phones so I just need to take a trip to test it out which I expect to do this next Wednesday. Torque Pro is a feature rich app and I have high hopes that it will read and display the meager number of MIDs that the engine ECM will output. Torque Pro is only $10 but it’s worth it to have an ad free app. And it works great. I’ve used it for a couple years when I plug the OBD II reader into my Saturn. The OBD II reader’s ad copy indicates that it should be able to read the J1587 protocol, or any protocol from any US vehicle built since 1996…or so it says.

Here’s a couple shots of the adapter and the OBD II reader connected to the J1587 under the dash. It’s a wiring mess under there so this was the best I could do as far as a picture of the installation. The J1587 connector is attached to something under the dash directly in front of the drivers left foot. Had to push the big bundles of wires out of the way to find it. I’ve installed the app on a 7″ Android I got free when I sat for a RV park’s membership pitch. They wanted $9,000 to buy in. Nope nope nope. Now have to find some way to attach the Android to the dash so I can see it while driving.

[Edit April ’19: The Torque Pro does not read the codes output by my rigs computers. When I contacted them, they told me as much and said they had no plans to add that functionality either. ]

Edit: 2021

I did a lot of research after April ’19 but came to the conclusion that no one was going to be interested in going to the engineering trouble of reading these older machines when the market was so small and the coding from the ECMs would be proprietary. So I dropped the idea. I did find a device that was supposed to read truck J1939 codes and output them in the OBD II format for reading on common devices but the stuff I was reading back in ’20 wasn’t helpful so I dropped the idea. I may come back to it here soon. I’m not sure.