J1939 CAN Network Operation and Testing
The Society of Automotive Engineers (SAE) developed the J1939 standard to be the preferred CAN (Controlled Area Network) for equipment used in industries ranging from agriculture, construction, and fire/rescue to forestry, materials handling as well as on and off-highway vehicles and transit buses. It is a high-level protocol that defines how communication between nodes (modules) occurs on the bus. The J1939 network is a specific communication system, supporting specific sets of applications and a specific industry, rather than being generalized.
Messages are transmitted between nodes (Modules /ECU/ ECM/TCM/PCM) at 250,000 bps. Any electronic control unit (ECU) using J1939 is permitted to transmit a message on the network when the bus is idle. Every message includes a 29-bit identifier, which defines the message priority, what data is contained within the 8-byte data array that follows the identifier, and which ECU sent the message.
The J1939 layout on a vehicle consists of Backbone that extends the length of the vehicle. The Backbone consists of three wires.
⇒ CAN High + : Yellow wire transmits data
⇒ CAN Low – : Green wire transmits data
⇒ Shield – Connected to ground close to the vehicle center. It does not transmit data, but protects the CAN High and the CAN Low from RF (radio frequency) and electromagnetic interference. The wire is bare and will have aluminum foil around it. It will take any unwanted frequencies and direct them to vehicle ground.
Note: The wires are twisted to cancel out frequencies.
End of Line Resisters (EOL):
The J1939 datalink consists of twisted yellow and green wires.
⇒The yellow wire is J1939 +
⇒The green wire is J1939 –
The J1939 datalink has two terminating resistors, one at each end of the backbone.
The purpose of the terminating resistors is to minimize the reflections of data on the datalink. Collision of reflected data can cause J1939 messages to become partially or completely lost. Data collision can also cause the data to be erratic. Terminating resistors prevent this from occurring. Although the J1939 datalink may function with a missing or failed terminating resistor, data collision can occur and cause problems.
⇒ Each terminating resistor is 120 Ω, but the equivalent of two 120 Ω resistors in parallel is 60 Ω. With both resistors installed in the circuit there should be 60 Ω measured at any two points between J1939+ and J1939– in the circuit, such as between pins C and D of the diagnostic connector.
⇒ But if a terminating resistor is removed, the circuit resistance will be 120 Ω measured at any two points between J1939+ and J1939– in the circuit, such as between pins C and D of the diagnostic connector.
IMPORTANT: It is essential that two terminating resistors are installed in the J1939 datalink. Numerous J1939 problems have been attributed to missing terminating resistors.
Diagnostic 9-pin Deutsch Connector:
Heavy duty J1939 applications use a 9-pin Deutsch connector to interface with test equipment and software to J1939. Communicating to nodes and testing the J1939 can be done through the Deutsch connector.
Operation and Testing
J1939 Resistance Check Operation:
J1939 Resistance Check Open or Missing Resister:
J1939 Resistance Check CAN High Shorted to Ground:
J1939 Resistance Check CAN High and CAN Low Shorted:
J1939 Voltage Check CAN High:
J1939 Voltage Check CAN Low:
J1939 Voltage Check Missing or Open Resister CAN High:
J1939 Voltage Check Missing or Open Resister CAN Low:
J1939 Voltage Check CAN High, CAN High and CAN Low Shorted:
J1939 Voltage Check CAN Low, CAN High and CAN Low Shorted:
Using a Labscope to check the J1939 network:
The network can also be checked with a labscope. The waveform pattern displayed on the scope will be a digital waveform of both the CAN Hi and the CAN Lo data signals. The CAN lo signal will be an inverted image of the CAN Hi signal. To properly check a 2 or 4 channel scope will be needed. For the pictures shown a PICO scope and a Freightliner chassis with an ISB Cummins engine were used.
The picture above shows the typical waveform displayed when checking the network, the two waveforms should be mirror images of each other.
The voltage on the CAN Hi data line should be 2.5 to 3.5 volts, the CAN Lo data line should be about 2.5 to 1.5 volts. The voltage difference is about 2 Volts peak to peak with about 1 Volt between the ON/OFF transition of each data line.
The transitions between ON/OFF of each data line should be crisp with no oscillation or hashing, as shown above. Causes of fluctuations, oscillation, or hashing can be: missing EOL, bad insulation/shielding, network not properly grounded.
The picture above was taken with an EOL resistor missing, as you can see the signal has become somewhat erratic, voltage fluctuations and spikes are noticeable during the data lines ON/OFF transitions.