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g/auto • Posted by u/scan_tool_sam •

CAN bus fault that only appears above 45 mph — chafed harness behind the exhaust Diagnostics

Posting this because it took me longer than it should have and the technique is worth sharing.

2017 Transit, intermittent loss of communication with the ABS and instrument cluster, U0121 and U0155 stored. Only occurs above about 45 mph and only when warm.

Bus resistance measured 60.2 ohms at rest, which is textbook. No faults with the vehicle stationary at any RPM.

What finally found it: a scope on CAN high and CAN low with the vehicle on a lift, in gear, at speed, and a helper flexing the harness sections one at a time with a plastic stick. When we got to the section behind the driver side heat shield the waveform collapsed.

Insulation was worn through to bare copper against a bracket. At rest the wires sat apart. At speed the whole assembly moved about a quarter inch and made contact.

The reason resistance measured fine: it was an intermittent short between the twisted pair, not an open. 60.2 ohms is what you measure when the fault is not currently occurring, and my test was structurally incapable of finding it.

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u/wrench_monkey •

Wiggle test with a scope is the right answer and the part people skip is doing it under the actual condition. Doing it in the parking lot with the engine off finds nothing on a fault that needs vibration and heat.

Good writeup, especially the last paragraph. Knowing why your measurement could not have found it is the actual skill.

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u/diesel_dan •

The 60 ohm reading is such a trap. It confirms both terminators are present and tells you nothing about the wire between them. I have watched guys measure 60 ohms and declare the bus good three times on the same vehicle.

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u/scan_tool_sam OP •

I was one of those guys for about two hours on this truck. The number is so satisfying that it stops you looking further.

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u/road_warrior_rex •

What scope are you using? I have been saving for one and the range in price is enormous.

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u/scan_tool_sam OP •

Two channel Picoscope with the automotive kit. It is not the cheapest option and I have never once regretted it. For CAN work two channels is the minimum — you need high and low simultaneously or you cannot see differential faults at all.

If money is tight, a used two channel is a better buy than a new four channel from an unknown brand. The probes and the software matter more than the channel count.

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u/millwright_mo •

Industrial side has the same failure mode on every conveyor and every robot cell — a cable that flexes a million times finds the one bracket it can reach. We have gone to routing everything in track with a service loop and a documented flex radius, and chafe faults basically stopped.

Vehicles do not have that luxury, but the diagnostic is identical: find the section that moves.

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