The next step is to tediously un-tape the harness and separate the doomed wires from the keepers. I typically unwrap every inch of tape, rather than relying on shortcuts like slitting the tape with a razor blade. The risk of cutting a wire and causing a diagnostic nightmare somewhere down the road is just too high when the knives are flying.
Generally, I try to cut the wires only when I have to, like when I reached the firewall grommet above, or when the wire was spliced to another wire in the harness.
Once a wire is cut, its much harder to figure out what it was for, so it’s essential to do a thorough enough job identifying connectors that you’re fully confident any cut wire really needs to be removed. Good rule of thunb: i’f you’re not absolutely sure what it does, leave it in….
Following all the wires from their doomed connectors to the other end of their doomed circuit will take you deep into the bowels of the car. Be prepared to take the car very far apart. Fear not, the service manual has very handy information about what order to remove things in so the dash comes out easily. It’s actually quite simple. And if you’ve followed my advice and kept a good inventory of quality fasteners, you won’t have to freak out over the possibility of misplacing a bolt here or there.
Some parts of the harness are tucked into really offensively tight corners. There’s a big bundle, for example, above the clutch pedal. Generally, if you’re struggling to reach something, it’s worth stepping back and thinking about how the car was probably built at the factory. Clearly the wiring harness was one of the first parts installed after painting the bodyshell, so getting to its nether-regions will require getting this corner of the car almost back to that naked state.
It turns out the clutch pedal comes out of the car as an assembly with only three fasteners. One bolt here, under the dash, and the two nuts in the engine bay that also hold the master cylinder on. Removing those and dropping the pedal out of the way is much easier than trying to contort yourself into that little hole.
Third: Make the Hayabusa harness fit:
The Hayabusa’s harness will function just fine in a Miata, given 12 volts and the proper set of signals from the tipover switch, kickstand switch, and silly things like that. Problem is, the harness is designed to fit in a bike, which is an entirely different shape from a Miata.
Look closely in this early picture and you can see the original Hayabusa harness plugged into the coils, then draped over the front of the engine and tangled up under the plenum. Wire routing designed for a Hayabusa just doesn’t lay cleanly in a Miata.
To figure out cleaner routing, I made a harness out of rope, taping together any branches the harness needed.
Next, I laid the rope harness out on a sheet of pegboard and zip tied it down. In retrospect, it might make more sense to make the harness from something less stretchy, like vacuum hose, or *gasp!* wire… Lesson learned.
Firmly zipping the biggest connectors to their spot on the rope harness, I then unwrapped the harness and started re-routing things.
Here’s a handy tip, by the way. If the harness needs to go around a corner to fit on your pegboard, use a long bolt to hold the apex. Both the rope and the wire can be pulled around this corner now without accidentally cutting the corner and ending up too short.
Now, the challenge with re-routing a harness like this, is that some of the wires will need to be longer than they were before. This means you’ll have to splice in extra sections of wire. Sure, you can go buy spools of wire for this, but if you’re anal enough about keeping the scraps you’ve cut out of the Miata harness, you should have plenty of good, automotive-grade wire in a suitably huge array of colors. I keep this bin full of every wire and every connector I’ve ever removed from a car. These came in VERY handy.
And when it comes to doing those splices, take a look a this simple advice for making clean, reliable splices.