Fractured wires and automotive safety

Article By : John Dunn

Fracturing is an insidious phenomenon to be aware of with stranded wires.

My Toyota Camry’s runaway acceleration problem (see “Runaway acceleration“) appears to have been resolved.

Several fractured wires were discovered at the connector that attaches a wire harness to the throttle position sensor. My repairman disconnected all of those wires, re-stripped them, reassembled everything and now everything works. The car hasn’t gone into runaway acceleration since then so I think my broken ground wire hypothesis has likely been confirmed.

That fractured wire issue needs a bit more attention though. It is an insidious failure mechanism that can take years to manifest itself.

Stranded wire tends to be more flexible than solid wire and is therefore the usual choice for wire cabling. However, in a high vibration environment, stranded wire can gradually be damaged without it being noticed. There can be very effective concealment of wire damage there inside the sleeving (Figure 1).

Figure 1 Stranded wire degradation and failure.

Metal fatigue arising from constant flexure arising in turn from long term mechanical vibration can break down the individual strands one by one by one by … until finally the last strand gets broken.

Even then, the open ends of broken strands can sometimes remain touching each other with the strand breaks going unnoticed. Going unnoticed that is until the wire ends separate from each other by just a tiny little bit and continuity is lost with sudden, possibly life threatening consequences such as sending me and my wife into a parkway exit ramp at out-of-control  speed.

This was not the first time I’d had something like this happen in an automotive context.

I once owned a 1979 Oldsmobile Delta 88 which I was driving westbound on the Northern State Parkway. As I was approaching the exit for South Oyster Bay Road, the engine suddenly died. I managed to coast the car off the roadway and onto the grass where the engine cranked, but would not start again.

After getting the car towed to a shop and having it repaired, the problem turned out to be a single fractured stranded wire underneath the distributor cap.

Figure 2 Stranded wire in my Oldsmobile.

After years of use and automotive vibration, this one piece of stranded wire had fractured open and disconnected the ignition points. Thank goodness it didn’t happen at a critical moment and that I was able to coast the car to a safe stop, without power brakes and without power steering, onto the roadway shoulder.

In both the Toyota and the Oldsmobile, no particular anti-vibration precautions seemed to have been taken for protection of the wires. Instead, the wires were simply placed as was simply convenient.

Eventually, the wires failed.

I have issues with that.

This article was originally published on EDN.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).


Leave a comment