Allison J1939 Troubleshooting - Fleets

J1939 Troubleshooting

SAE J1939 is a standard used for vehicle communication and diagnostics.
  • Also called a "datalink", "databus", "backbone" or a Controller Area Network (CAN), J1939 is faster and more reliable than previous J1708 and J1587 specifications.

J1939 reduces vehicle wiring and communicates a broad range of information, including:
  • Vehicle operation controls
  • Powertrain interactions
  • Vehicle management information

A). J1939 Construction
SAE J1939 includes two variations – J1939-11 and J1939-15.
  • SAE J1939-15 is often referenced as J1939 "Light" because it uses unshielded cable, this is not Allison-recommended due to increased susceptibility to electromagnetic interference.
  • J1939-11 cable is shielded and includes a 120 ohms impedance twisted pair and a drain wire.
  • Impedance affects the volume and rate of information flow and must be correct for proper databus operation.
  • Mismatched cables, tight bends, long breaks in shielding and improper cable design can affect impedance and performance.
  • The proper wires, terminals, connectors and components must be used whenever building or repairing J1939 cables.

J1939 networks consist of a central "backbone" with "stubs" branching to individual controllers or "nodes".
  • A 120 ohms termination resistor is required at each end of the backbone.
  • Termination resistors can be built into some electronic components, including current 5th Generation Controls TCMs and J1939-based shift selectors.
  • Backbones may be no longer than 40 meters.
  • Stubs may be no longer than 1 meter and spaced at least 10 centimeters apart.

B). J1939 Troubleshooting
Troubleshooting termination resistors:
  • With all controllers connected but powered off and both termination resistors in place, approximately 60 ohms resistance should exist between terminals A and B of a stub connector (measured between terminals C and D on 9-pin in-cab diagnostic connectors).
  • A reading of 120 ohms indicates one of the termination resistors is not installed, or this can also indicate an open somewhere in the backbone.
  • A reading of 0 ohms indicates a short between CAN high and low wires existing in either the backbone or one of the stubs.

Troubleshooting opens and shorts:
  • Opens can affect one or more controllers but will have the most impact on the controller attached to the affected stub.
  • Shorts typically generate multiple controller errors.
  • Opens and shorts can be located by performing continuity checks.


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