Rear Run Box Connectors

[ak49er]

2008 Beaver Marquis Caterpillar C-15 ACERT

During my first trip I noticed this connector that is on the Passenger Side of the engine was unplugged, it looks like it terminates into a Diagnostic Plug on the side of the Rear Run Box. Can anyone tell me what it does. I plugged it in and haven't noticed any changes.

[Ivan K]

It's a J1939 termination resistor and should be plugged in

[DavidL]

The reason it "doesn't make any difference" is because it's there to eliminate "reflection" on the CAN bus.  Most of the time not needed.  But once in a while, an unterminated CAN bus will do some funky things and you will see an intermittent issue in a myriad of devices on the CAN bus (engine / trans controllers, shifter, lots of other things).

The Diagnostic connector is a "9Pin Deutsch" connector to connect a diagnostic scan tool for powertrain diagnostics.

One of the very first things a good Technician will do to diagnose a vehicle problem is measure that there is 60 ohms resistance between CAN High and CAN Low wires.  If the termination resistor isn't plugged in, the Tech will see 120 ohms and know either the CAN bus has a break, or the termination resistor is missing on one end or the other.

Edited April 7, 2023 by DavidL

[Scotty Hutto]

To add to Ivan's post, that three connector system is the J1939 data bus.  It connects to the engine ECM, transmission ECM, and Brake ECM, along with Aladdin (or Silverleaf) and a few other things.

For that diagnostic port, it allows someone with the Cummins Insite program (or the Allison or Bendix programs) to connect to those computers.  To leave it unconnected could cause errors on the data bus and erratic behavior of the affected systems.

If I have any of this wrong, @Frank McElroy can correct any errors.  😉 

[Frank McElroy]

19 minutes ago, DavidL said:

The reason it "doesn't make any difference" is because it's there to eliminate "reflection" on the CAN bus.  Most of the time not needed.  But once in a while, an unterminated CAN bus will do some funky things and you will see an intermittent issue in a myriad of devices on the CAN bus (engine / trans controllers, shifter, lots of other things).

The Diagnostic connector is a "9Pin Deutsch" connector to connect a diagnostic scan tool for powertrain diagnostics.

One of the very first things a good Technician will do to diagnose a vehicle problem is measure that there is 120 ohms resistance between CAN High and CAN Low wires.  If the termination resistor isn't plugged in, the Tech will see 60 ohms and know either the CAN bus has a break, or the termination resistor is missing on one end or the other.

Each of the two terminating resistor is 120 ohms.  When both are on the data bus, the tech will measure 60 ohms between data high and data low lines with the power turned off.  If one of the resistors is missing, or a line is broken the tech will measure 120 ohms.

[DavidL]

7 minutes ago, Frank McElroy said:

Each of the two terminating resistor is 120 ohms.  When both are on the data bus, the tech will measure 60 ohms between data high and data low lines with the power turned off.  If one of the resistors is missing, or a line is broken the tech will measure 120 ohms.

Typed too  quick 🙂  corrected...thanx

[Frank McElroy]

1 hour ago, Scotty Hutto said:

To add to Ivan's post, that three connector system is the J1939 data bus.  It connects to the engine ECM, transmission ECM, and Brake ECM, along with Aladdin (or Silverleaf) and a few other things.

For that diagnostic port, it allows someone with the Cummins Insite program (or the Allison or Bendix programs) to connect to those computers.  To leave it unconnected could cause errors on the data bus and erratic behavior of the affected systems.

If I have any of this wrong, @Frank McElroy can correct any errors.  😉 

Scotty - That is correct. 

What can confuse folks is that on older coaches, you might have multiple data busses including slower data busses like J1708 and J1587.  The latter is used for the Aladdin and the old SilverLeaf VMSpc systems.  So when connecting to a databus you need to know the system(s) you coach has.  And it could be multiple systems and dedicated sub systems like the transmission to engine to gear selector.  Connecting into the 6 pin or 9 pin data service port will sort out what system your coach is using based on pin output configuration.

But, don't be surprised to find a 9 pin Deutsch connector on the Rear Run Bay but only a 6 pin connector under the dash.  The 9 pin connector has the high speed J1939 data bus while the 6 pin connector only has the slower J1587/J1708 databus.  That's what Scotty had under his dash.  But he also had a J1939 data bus for the Bendix ABS control module nearby.  So he was able to tap into it from the dash.  That gave him high speed engine, transmission and Bendix ABS diagnostic data and real time data recording while driving if he wanted to simulate performance testing like on a dyno.

[ak49er]

Ok, thank-you, this all makes sense, now that I've thought about it, I do remember the ABS light coming on randomly when I switched the ignition on after plugging this in, at the time I thought it had to do with unplugging the Bluefire Device.