A wheel loader's engine controller (ECM) is experiencing intermittent communication issues with various modules on the J1939 CAN bus. A technician measures the terminating resistors at both ends of the backbone. One end measures 60 ohms, and the other measures 120 ohms. What is the MOST probable cause of the communication issue?

Question: A wheel loader's engine controller (ECM) is experiencing intermittent communication issues with various modules on the J1939 CAN bus. A technician measures the terminating resistors at both ends of the backbone. One end measures 60 ohms, and the other measures 120 ohms. What is the MOST probable cause of the communication issue?

Answer options: ✅ A missing or faulty terminating resistor on one end of the bus.

• A short circuit between the CAN-high and CAN-low lines.
• A disconnected ground wire for the ECM power supply.
• Excessive noise interference from the alternator circuit.

Correct answer: A missing or faulty terminating resistor on one end of the bus.

Explanation: A healthy CAN bus typically has two 120-ohm terminating resistors, one at each end, resulting in an overall bus resistance of approximately 60 ohms (120 || 120 = 60). If one end measures 120 ohms and the other still reads 120 ohms (not accounting for parallel) or if the whole system reads 120, it indicates one resistor is missing or open, which would cause communication issues. A single 120 Ohm reading at one end, and an overall 120 Ohm reading (meaning only one resistor is present) strongly suggests an open or missing resistor. If it's 60 at one end, and 120 at the other, it indicates a fault on the 120 ohm end (e.g. one resistor is present, the other is not or is open). The phrasing 'one end measures 60 ohms and the other measures 120 ohms' is slightly ambiguous but generally points to an issue with one of the resistors.