A triple-container handler was suffering from "Ghost" diagnostics. The dash would flicker, the traction would cut out momentarily, and the fault codes would be generic "Communication Errors" (J1939 CAN Bus errors). The shop replaced the heavy-duty cab display and the main controller, but the glitches continued.
The CAN Bus is the nervous system of the truck. To ensure data signals don't bounce back and forth (reflection), the network requires two 120-ohm "termination resistors"-one at the beginning of the line (usually at the display) and one at the end (usually at the rear axle or controller).
We disconnected the battery and measured the resistance across the CAN High and CAN Low pins on the diagnostic port. A healthy system reads 60 ohms (120 ohms in parallel with 120 ohms). This truck read 120 ohms. This meant one of the terminators was missing or open-circuit. We traced the harness to the rear light bar controller. One of the terminal resistors inside the controller was unplugged due to a loose connector. Without the termination, the signal reflection was corrupting the data packets, causing the computer to "hallucinate." Reseating the connector and restoring the 60-ohm resistance stopped the ghost faults.
