Rewiring the 1.6l NA Miata … Part 1

The now ST6 Miata has had some serious electrical gremlins from day one. Intermittent starting issues, random fuel cuts, and erratic idling to name a few. I knew at some point I would rewire the car completely. What I didn’t anticipate however, is that I would give the rewire a first pass just days before the regional championship at Nashville Superspeedway.

What was I Thinking?

I have no one to blame for this pickle but myself. I’m the one that decided to marry my love for weight reduction (Converting the Budget Spec Miata to ST6 ) with my disdain for the wiring harness and its associated issues…before the most important race of the season.

In my defense, I honestly thought I had 2 weeks before the championship. Armed with that improper timeline, I dug in and got too aggressive with the wire snips. After all, over the last year, I had been staring at over 20 connectors on this wiring harness that connect to absolutely nothing. Always seeking efficiency in my builds, I got to cutting.

And that is why this is part 1 of a series on rewiring the 1.6l NA Miata MX-5. As you can tell from the pic below, I exhibited a distinct lack of logic to get to this point just days before the event. In this part I will discuss my initial weight reduction goals and how I solved this mess one day before the championship. Part 2 will be a proper guide for anyone looking to simplify the wiring of their NA Miata to the bare minimum and do it the right way.

NA 1.6l Miata MX-5 with the wiring harness and ECU laying across the engine bay for troubleshooting.
I am told this pic is stressful. And although it was at the time, I came out the other side with a wealth of knowledge and a more complete understanding of automotive wiring.

Flawed Process

I began this project by removing/unwinding as much of the harness as I could. I started with the air bag wiring, headlight/popup wiring, and the dozen other open connectors throughout the car. I would trace a sector, such as the headlights and trace them to an origin point. Then I would cut the connector out if it didn’t branch to other circuits. After cutting I would verify the car started.

This worked great until it didn’t. There is a lot of wiring that branches to multiple points in this harness, and very early I caused something to be unhappy. I didn’t want to turn back at this point knowing the ultimate goal was weight reduction. And with the car no longer able to start, I no longer had to be careful. I switched to the always fun, ‘do I need/want this mode’.

How Much Weight Did I Remove from the Miata Wiring Harness?

The Miata has a main relay, fuel relay, and a few sensors that communicate to the ECU to run. Almost everything else is unnecessary in a race car. Things like the diagnostic port, fan relay, gauge cluster aren’t needed/wanted in my case. I cut each associated connector out and the grand total with wiring was 13lbs. I count that a huge victory as I was looking for a 15lbs reduction.

All of the cut wires from the NA Miata MX-5 in a bundle.
This is what 13lbs of wires, fuses, relays, connectors, and electrical tape from the NA Miata looks like.

Getting the Car Running Again

With days before the race, the two previous pics should impress how much I was in crisis mode at this point. In hindsight the solution sounds easy. I poured over the wiring diagram for the car until it started to make sense.

Screenshot of the wiring diagram for a 1991 NA Miata.
Here’s an example of the wiring harness diagram of the 1991 NA Miata MX-5. There are 6 pages of this spaghetti and after a while I saw ‘the Matrix’ and got to repairing the car.

I slowly realized that there was absolutely no way that I was going to trace and verify what was left of the hacked-up harness, so I started verifying every connection that connects the ECU to the engine or power. Then I verified the two remaining relays. Found a few things that weren’t seeing each other. Cleaned up some grounds, replaced a few fuses, and rewired the ignition.

Although it sounds easy, this process took 2 days. Afterwards, the Miata fired right up and idled better than ever.

Conclusion and Part 2

I am the first to admit that wiring the car was outside my skillset. I love learning but appreciate a more relaxed approach than this issue presented. And this issue was a mountain. However, I can confidently say that I climbed this one and now have a very good understanding of the Miata wiring.

During this process, I used a couple of critical resources but found lots of gaps. As a result, in part 2, I will walk through a step-by-step guide to simplify the wiring on the NA Miata to the bare minimum.

By Chris Simmons


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