All that squishes is not crimped.

So, a while back, there was a bit of a spirited discussion on the merits of some different ways of joining electrical wires.

Toyota, like pretty much all auto/marine/aerospace manufacturers, uses solder within electronic assemblies (think components rigidly mounted on circuit boards, inside rigid boxes), and builds all the wiring harnesses that connect these things together using stranded wire and crimped terminals to hold up in a vibrating environment. Their wire harness repair instructions only advise crimping, and they specify the materials and tools to be used.

The reason they are so picky is that a "crimp connection" is not just a matter of pinching some metal down on some wires. It is a process that works "by applying mechanical force to deform metals past their elastic limits. The mechanical force establishes large area metal-to-metal contact and locks the conductors together through the process of cold welding and the action of residual elastic stresses in the conductors. Cleaning contaminants and oxides from conductor surfaces is also accomplished by a combination of high pressure, metal flow, and gross deformation of the conductors." (Cottrell and Kirejczyk, 1978)

That all happens in a true crimp with the proper tool and parts.


Now, then ... here's a thing devout solderers and confirmed crimpers should all be able to agree on:



These 3M Scotchlok connectors are wonderful things. Telecom companies use them by the gazillions. They're quick. They're easy. They're filled with water-resistant goo. Your tool squeezes down the red cap and the wires are spliced, and the goo squishes around them to make a seal. You don't even have to strip the wires first!

And Toyota automotive wiring happens to be skinny enough to fit into them. It was meant to be! What could possibly go wrong?

These were found in the steering wheel of a Prius that had been repaired after an airbag deployment. They wouldn't have been found, if the airbag light hadn't come on about 150 miles after driving off the lot. All things considered, it's good the light came on.

For starters, even though you use these things by squishing them (and 3M's tool for squishing them even says "CRIMPING TOOL" on the side), these do not make a crimp connection. They are insulation-displacement connectors. Everything said earlier about true crimps? Large area metal-to-metal contact? Cold welding? Nope and nope. All of the contact here comes from some skinny little blades that slip down through the wire insulation and grip the wire inside.



(Keep in mind how magnified that picture is.) Those little knife-blade contacts are all you need for currents on the level of telecom signals.

Also, 3M specifies these for 26 gauge to 19 gauge, solid copper wire—the kind used in telecom.

The Toyota wire harness wires happen to fit (mostly thanks to their insulation formula allowing that layer to be crazy thin), but many circuits in a car carry real current, and as mentioned up top, they are stranded wire. The Scotchlok blades, instead of pinching down on that single, solid, telecom wire conductor, are likely to just damage a few of the outer strands of the Toyota wire, and end up lazily draped over the surviving strands in the middle.

Finally, if you look again at the 3M squishing tool, you see that its jaws are designed to squish the cap of a Scotchlok perfectly straight down, parallel to the body. What happens if you're in too much of a hurry to reach for the right tool, and just squish with your handy slip-joint pliers instead?



Every one of those connectors is squished down cocked at an angle, so even the skinny insulation-displacement blades, pinched down onto the wrong type of wire, are not even pinched all the way down. (In fairness, that's just an installer-technique error and nothing against the Scotchloks themselves, but on top of them being mismatched to the application to begin with, it all adds up.)

This is so, so different from an actual crimp connection made with the materials and tooling specified by Toyota.

These connectors would be misapplied anywhere in a car (even though the wires fit!), but especially questionable in an airbag circuit. The trouble is that the airbag ECU is limited to using tiny currents (that won't deploy the airbag!) for detecting whether the circuit is sound, but it needs to send a real current through there in a real collision, and hope the connection will carry it.

The good news is, it seems pretty easy to check a used Prius for such nonsense behind the steering wheel, just by sticking an inspection camera through the opening around the cruise control stalk. The stuff is pretty exposed and easy to see in there.

Moral: please, use Scotchlok connectors all day long if you're working in telecom.

On a car, Toyota has specified the splicing materials and methods for a reason. (There are posts about those, for the most common, copper-to-copper splices, and about the different tool and materials needed for the few cases where Toyota has used aluminum wire.)