What would be indicated when an ohmmeter connected across the J1939 backbone reads higher than normal resistance?

When an ohmmeter connected across the J1939 backbone reads higher than normal resistance, it indicates that one terminating resistor is open. The J1939 network operates with a pair of terminating resistors at each end of the backbone to ensure proper signal integrity and reduce reflections. If one of these resistors is open, the overall resistance across the backbone will be higher than expected, reflecting the absence of that resistor. This results in improper communication on the CAN network.

In a properly functioning J1939 network, you should observe a consistent resistance that is indicative of both terminating resistors being intact. An open resistor means that the circuit is incomplete, leading to signal issues that can affect data transmission between electronic control units (ECUs). Thus, identifying an open terminating resistor through a high resistance reading is critical for diagnosing and resolving communication problems on the network.