JCB U0029-88 – Engine ECU, Aftertreatment ECU, Dosing Control Unit (DCU): CAN Bus Communication Failure at 250kbps

The system has detected a CAN bus communication failure at a speed of 250kbps, indicating a loss of data exchange between essential electronic control modules. This 'CAN bus off' error prevents proper communication within the affected circuit, which can lead to significant operational issues and often causes the machine to enter a Reduced Power Mode or derate to prevent potential damage.

• Backhoe Loaders
• JS115
• JS130
• JS145
• JS160
• JS180
• JS190
• JS200
• JS210
• JS220
• JS235
• Wheeled Loaders
• Telescopic Handlers

1. 1

   Record and clear codes using a diagnostic tool, then operate the machine to see if the fault returns immediately (rules out temporary electrical surges or 'ghost' codes)

2. 2

   Check battery voltage to ensure it is stable and above 12.5V for a 12V system

3. 3

   Inspect all main battery and chassis ground connections for cleanliness and tightness

4. 4

   Visually inspect wiring harnesses associated with the engine, aftertreatment system, and DCU for rubbing, chafing, melting, or vibration damage

5. 5

   Unplug connectors for relevant ECUs and sensors to inspect for corrosion, bent or pushed-out pins, and moisture ingress

1. 1

   Check CAN bus resistance

   With the ignition and all systems powered OFF, measure the resistance between the CAN High (CAN_H) and CAN Low (CAN_L) pins. A healthy bus reads ~60 Ohms. ~120 Ohms indicates a missing terminating resistor, ~40 Ohms indicates an extra resistor, significantly less than 40 Ohms suggests a short circuit or faulty ECU, and high or infinite resistance (OL) indicates an open circuit in the CAN wiring.

2. 2

   Isolate the fault

   If the resistance is 120 Ohms, a terminating resistor is likely missing or faulty (one may be in a harness, another in an ECU). If resistance is otherwise incorrect, disconnect modules one by one from the affected CAN bus (e.g., Engine ECU, DCU, instrument cluster) and re-measure resistance each time. The last module disconnected before resistance returns to normal is likely the source of the fault.

3. 3

   Check for shorts to ground

   With power off, measure resistance from the CAN_H pin to chassis ground, and then CAN_L to chassis ground. Readings should be very high (mega-ohms or open). A low reading indicates a short to ground in the harness.

4. 4

   Check CAN bus voltage

   If resistance tests are inconclusive, power on the system and measure voltage between CAN_H and ground (expected ~2.6V), and CAN_L and ground (expected ~2.4V). Incorrect voltages point to a faulty ECU transceiver or wiring issue.

5. 5

   Repair or replace affected components

   Repair any chafed or broken wires. Replace missing or faulty terminating resistors. If a specific ECU (like the DCU or Engine ECU) is proven defective during isolation testing, replace it and ensure the new part is programmed for the machine.