J1939 Communication Link on ALLISON Transmission

SAE J1939 Communication Link

Figure 1.1 5th Gen TCM and Diagnostic Port J1939

The Controller Area Network (CAN) defined by SAE J1939 enables the integration of various vehicle components into an overall vehicle system by providing a standard way of exchanging information between these modules in the vehicle. Allison uses the J1939 communication link for vehicle operation controls, powertrain interaction, and conveying vehicle management information.
Figure 1.2 - J1939 Interface Wiring (TCM Pin-Out)

J1939 networks are laid out on linear fashion, consisting of a central "backbone" with "stubs" branching off to individual controllers or "nodes". Regardless of the cable used, two 120 ohms termination resistors are required, one at each end of the backbone cable. These resistors may built into a receptacle connector or plug connector that contains a blue wedge lock.
Figure 1.3 - J1939 Network Configuration

Typically, all connectors on the backbone and stubs are of the "plug" type. However, "receptacle" connectors may be used in some installations. Stubs and nodes use orange or green wedge locks.

The backbone may be no longer than 40 meters in length. A stub includes the length of wiring on the node, and the length from the backbone to the node must be one meter or less.
Figure 1.4 - Termination Resistors Requirement on J1939 Backbone

The connector for the Allison controller is a 3-way connector configured as follows:
  • Terminal A = CAN High
  • Terminal B = CAN Low
  • Terminal C = CAN Shield
Typically, CAN High is a yellow wire and CAN Low is a green wire.
Figure 1.5 - 3-Way Allison Controller Connector

A-9 way (in-cab) diagnostic bulkhead housing (as Figure 1.1 above), will be configured as follows:
  • A = Ground
  • B = +12V (unswitched)
  • C = High (yellow)
  • D = Low (green)
  • E = Shield
  • F = J1587 + (typically blue)
  • G = J1587 - (typically white)
  • H and J = For OEM use
Figure 1.6 - 9-way (in-cab) diagnostic connector

Wiring and Connector Failures
Wiring and connector are the number one cause of problems in the field. Opens, Shorts, and CAN high being connected to CAN low are among the most frequently encounter issues.

Termination Resistors
A J1939 network requires a 120 ohms termination resistor at each end of the backbone (refer to Figure 1.3) . With all controllers powered off and both termination resistor in place, an ohmmeter should read 60 ohms across terminals A and B of the 3-way connector (refer to Figure 1.5), or terminals C and D of the 9-way connector (refer to Figure 1.6). The test can be performed with controllers connected to the backbone because the impedance at the controllers is much higher than 60 ohms and therefore does not affect the reading.

A measurement of 120 ohms typically indicates that either one of the two termination resistor is not in place, or there is an open somewhere in the backbone of the network.

A measurement of 0 (zero) ohm indicates that there is a short between the CAN high and CAN low wires of the network. The short maybe on the backbone itself, or in one of the stubs connecting it to a controller.

Open Circuits
Open circuits in the CAN high (A) or CAN low (B), sides of the backbone or in any of the stubs can affect one or more controllers on the network. While an open circuit in a stub will have the most impact on the controller attached to the stub, other devices on the network who normally receive information or expect a response from that controller will be impacted as well.

When there are multiple nodes attached to the network, and their connectors are accessible, an open circuit can be tracked by moving down the backbone from stub to stub looking at the datalink information present at each connector. When there is a difference in the amount of datalink traffic between two connection points, there is likely an open circuit somewhere on the stubs or backbone between the two connection points. A DVOM (Digital Volt/Ohm Meter) may be used to detect activity.

Short Circuits 
A short circuit can occur in the J1939 backbone or stubs between:
  • CAN high and CAN low
  • CAN high or CAN low and battery voltage
  • CAN high or CAN low and ground
When a short circuit is present, typically multiple controllers on the network indicate an error on some sort, due to the loss of all communication between any of the nodes. For example, datalink-based instrument clusters will not function properly. Short circuits typically fall into one of the following categories:
  • Mechanical failure - Insulation cut or scraped through, wires pinched, etc
  • Incorrectly wired pins on one or more of the controllers
  • Missing connector seal (s), allowing water intrusion

Inducted Noise
Inducted noise tends to be a much greater issue when unshielded J1939 cable is used. While the following routing tips are a good idea for shielded networks, they are critical when unshielded cable is used. J1939 cable routing must avoid the following by a minimum of 3 to 4 inches of physical separation:
  • Solenoids
  • Alternator
  • Flasher Modules
  • High Output CB Radio
  • Starter Motor
  • Relays
  • Any High-Current Switching Devices
Inducted noise is typically "event driven", or associated with an activity that involves operation of a high-current load near the network wiring.  To find noise sources, monitor datalink traffic under the following conditions:
  • With the keyswitch ON: operate every input the driver has access to, such as the CB radio, blowers motors, fans, A/C, flashers, turn signals, lights, horn, brakes, etc
  • With the engine running: Exercise every function on the vehicle as is possible, such as engaging the engine fan, turning on the A/C compressor, operating the dump bed, etc
If errors or pauses in datalink traffic are noted during any specific activity, investigate the network wiring near the associated component (s).

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