Understanding data lines Part II

By Larry Turay

Part I of understanding data lines was in the last issue, coverering data line structure and communications. Part II will cover troubleshooting and diagnoses of the J1587/1708 and J1939 data lines.

As with any repair, following a logical set of steps will help to find the problem quickly and avoid missing the cause of the failure. There are seven common steps to follow in this process: 1. Verify the complaint, replicate the condition if possible. 2. Obtain pertinent information: driver input, repair history, check fault codes, clear history, check TSB’s. 3. Determine potential cause: component, wiring, programming/ECU, possible “normal” operation. 4. Narrow the list of potential causes, understand how the system operates, test components, test accessible connections. 5. Test to determine the root cause, make use of available tools: multimeter, computer/software, service information including wiring diagrams. Divide the system and close in on the fault, for example, tractor and trailer have no clearance light, unplug the trailer and the tractor lights are OK. 6. Repair the root cause and any progressive damage it caused: corrosion in connector or wicked into the harness. 7. Verify the complaint has been eliminated, road test, cycle the device or circuit off and on.

            Cues that there may be a data line problem: unit may crank and not start, A/C compressor will come on for 10 seconds when the A/C is first turned on then shuts off and will not turn back on, even though the system has a full refrigerant charge. The compressor control head, ACP switch and evaporator temperature sensor has been replaced! The ABS light is on, the four-way flashers are on and the right headlamp is on, or a message states that the J1939 is missing.

            Databus Diagnostics and Troubleshooting tips for J1587/J1708. 1. If a single ECU is not reporting on the bus, check:  Source voltage supply, wake up (usually ignition supply), ECU grounds, databus wiring from problem ECU to where it connects into the databus, ECU module. 2. If no ECU’s are reporting on the bus then disconnect one ECU at a time until a list of ECU’s start reporting. When that happens, go to the ECU that was disconnected last, and look for a problem in the databus wiring from that ECU to where it connects into the bus. 3. If only a partial list of ECUs’ shows up on the laptop, look for a problem in the databus between the data junction blocks. ATTENTION! Cycle the key off and on before each reading to ensure the data link activity.

            Databus Diagnostics and Troubleshooting Tips for J1939.  If an onboard ECU is reporting a databus-related fault code or the complaint is some electronic component is not responding correctly, apply the following guidelines: 1. Disconnect the batteries and check for 60 Ohms of resistance across pins C and D in the Diagnostic connector. 2. If the Ohmmeter reads 120 Ohms, check for an open in one of the terminating resistors. 3. If the Ohmmeter reads 40 Ohms, there is likely an extra terminating resistor on the backbone. 4. If the Ohmmeter reads open (OL) across pins C and D, check for an open in the branch being tested. 5. If the meter reading is 0 or very close to 0, the J1939 wires are shorted together. 6. If the reading is 60 Ohms, then the J1939 backbone is not at fault. The problem lies between where the bus connects to the backbone and the ECU. Next issue will get into CAN, Sub bus and LIN data lines.

Stay tuned!

This article appeared in the November/December 2020 issue of MACS ACTION magazine. You can view the entire magazine issue here.