Intermittent Dropouts on the J1939 Backbone
2020 transit bus, J1939 backbone at 250 kbps, terminated at the ECM and at the rear bulkhead. Complaint: intermittent ABS and transmission warning lamps, a dash gauge sweep, and a check-engine lamp; worse over expansion joints and in wet weather. Three visits, no active fault when the technician road-tests. Stored codes differ by module: the ABS ECU logs lost communication with the ECM, the transmission ECU logs lost communication with the ABS ECU, and the body controller logs a lost engine message. Key off, all modules asleep, resistance across CAN-H and CAN-L at the diagnostic connector is 41.2 Ω. Key on, engine off: CAN-H 2.51 V, CAN-L 2.49 V. A telematics gateway was added by the operator six months ago and tees into the backbone at the rear bulkhead.
1. Interpret the 41.2 Ω reading. What does it tell you, and what does it not tell you?
Read the worked answer
Two 120 Ω terminators in parallel give 60 Ω. 41.2 Ω is close to three in parallel (120 ÷ 3 = 40 Ω), so there is almost certainly a third termination on the bus. The gateway added six months ago is the obvious candidate: many of these devices ship with an internal 120 Ω terminator that nobody removes for a mid-bus tap.
What it does not tell you:
- That the gateway is the fault. Three terminators alone usually still communicate; they reduce signal amplitude and noise margin, so marginal wiring elsewhere then pushes the bus over the edge - which fits dropouts that only appear over bumps and in rain.
- Where the third terminator is. Only disconnecting modules one at a time finds it.
- That the harness is sound. Wet-weather sensitivity points at a chafe or water intrusion that must be found separately, and a low-resistance reading says nothing about an intermittent CAN-L short to ground.
2. Give the isolation procedure for the extra terminator and the rule you must follow while doing it.
Read the worked answer
With the key off and the bus fully asleep - confirm modules have gone quiet, because an awake module biases the bus and the reading lies:
- Record 41.2 Ω at the diagnostic connector as baseline.
- Unplug the telematics gateway and re-measure. About 60 Ω confirms the gateway's terminator was the third one; leave it disconnected or remove/disable its termination per the OEM procedure, then re-measure.
- If still near 40 Ω, disconnect backbone modules one at a time starting at the far end, re-measuring after each. A jump toward 120 Ω means you removed a legitimate terminator; a jump toward 60 Ω means you removed a module that was internally terminated when it should not have been.
- Rule: never road-test with a terminator removed to "see if it helps." A single-terminator bus reflects and produces exactly the intermittent faults you are chasing, and you will create a new complaint while closing this one.
Document the final value: 60 Ω ± a few ohms with two terminators installed.
3. Resistance is now 59.8 Ω, but the dropouts still happen over bumps in rain. What is your next test plan?
Read the worked answer
Move to a live test, because the fault is physical and intermittent.
- Break out the bus at the diagnostic connector with a breakout box or CAN analyzer, drive the same rough section, and log error frames, live CAN-H and CAN-L voltage, and which node drops first. The first node to drop and the segment it sits on are the lead.
- Wiggle test with a scope connected: flex the harness at the rear bulkhead, at the re-clipped sections, and at the gateway tee. Keep the J1939 drop cable short (on the order of 1 m or less); a long stub causes reflections.
- Look for water intrusion: green pins, swollen seals, a connector that fills at the bulkhead. Check CAN-L for an intermittent short to ground (bus stuck dominant) and CAN-H for a short to power.
- Verify the shield drain is grounded at one point only; multiple grounds create a loop that reads as noise when the chassis flexes.
- After repair, road-test the same section with the analyzer running and confirm zero error frames before releasing the bus.