Automotive

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End of the Line

Why Do Automotive Connectors Fail?

One simple failure resulted in a lengthy tow and a lot of frustration.

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Episode Summary:

Wiring harnesses are the central nervous systems of modern vehicles. They’ve evolved far beyond simple bundles of wires and encroach on every system and every panel in both body and chassis. Conductors now carry data as well as power, but the common element is the need to terminate those conductors in some kind of electrical connector. They are normally trouble-free, but when they fail, the result is at best inconvenient and at worst, crippling to the motor vehicle. Why do they fail? Jim Anderton shows us one connector that simply didn’t measure up. 

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Transcript of this week’s show:

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Take a look at this connector. To those of you in the automotive, electrical or electronics industry, you know what this is. It’s a connector but to me it’s not just any connector. This particular connector represents a 75-mile auto club tow and a lost weekend. Why? Because this charred, melted chunk of resin sat midway in the power feed circuit to the in-tank electric fuel pump on my vehicle. What brand you may be asking? I’m actually not going to say, because my point is not about any one OEM, although this one was from a large Michigan-based automaker, but instead is about how things like this little connector are engineered. 

This is part of what the industry calls a wiring harness, one of several in modern vehicles that form the central nervous system of cars and light trucks. Cars have always had them, for over a century in fact, and for much of that time they were little more than taped bundles of wires with crimped ring terminals at the ends. The development of modern injection molding and low-cost thermoplastic resins around World War II however let engineers design connectors that pushed into place and locked, dramatically reducing the installation time on the assembly line. 

And these things brought other benefits as well. Connections were positive and repeatable, wouldn’t vibrate loose and weren’t subject to the skill of the individual installer. In theory, they’re just as durable, but like so many things in design engineering, the move to this technology from spade lugs and ring terminals moved responsibility for quality and durability out of the hands of the line worker and onto the desk of the engineer. What does it take to design one of these? It all starts with questions:

  • How much current will the circuit carry? 
  • What are the physical constraints on the metal contacts inside the injection molding? 
  • How big, or how small must the connector be? 
  • How much insertion force is required to seat and latch the connector? 
  • How much vibration must the completed connection withstand? 
  • How much heat?
  • Is exposed to corrosive liquids or vapours? 
  • Does the connector have to be keyed or indexed to prevent incorrect assembly? 
  • Should it be color-coded?  
And these are just the initial functional considerations. In choosing the plastic resin, depending on the application it may be as simple as a straight commodity grade polyethylene, a commodity resin with additives for things like flame retardancy and colour, or it may be a filled resin or even an engineering grade. The other end of this harness has a connector that spends its life immersed in gasoline, so you can bet that it’s designed and tested differently from this under dash component. And of course, if possible it has to be designed at the lowest possible cost, and with the greatest possible commonality with existing parts. It’s a tough order. 

So why did this fail? It was installed in an inertia switch, a safety device designed to shut off power to the fuel pump in case of a collision. That switch is installed on the passenger side toe board in a location where it’s exposed to dust, dirt and moisture. It doesn’t take much to corrode the contacts, and with corrosion comes resistance and with resistance comes heat. Can a major OEM simulate a kid with muddy boots kicking his feet up under the dash? Probably not, but today, this kind of thing had better be an exceedingly rare event. Seeing this is a highly competitive industry, and I’ve never seen this happen on my Honda. 

Written by

James Anderton

Jim Anderton is the Director of Content for ENGINEERING.com. Mr. Anderton was formerly editor of Canadian Metalworking Magazine and has contributed to a wide range of print and on-line publications, including Design Engineering, Canadian Plastics, Service Station and Garage Management, Autovision, and the National Post. He also brings prior industry experience in quality and part design for a Tier One automotive supplier.