Wiring Part 1, Modifying RX-8 and Honda K Harnesses

     Hey guys, I finally got some time to start working on the wiring of this car. Me, having never done a motor swap before, had little knowledge of how you could wire in a motor from a completely different car. But now, after countless hours of research, I have the game plan figured out.

    As I stated previously, I will be using a Hondata K-Pro alongside the stock Mazda ECU. This is the budget alternative to using the Adaptronics ECU, made specifically to interact with the RX-8s canbus system that drives the goodies, like the electronic steering, speedometer, ABS, just to name a few. If you're using a Hondata K-Pro, then this is what you'll want the stock ECU for. 

Now, I also want to retain the factory RX-8 throttle body. This way, I will retain drive by wire, eliminating the need to install a throttle pedal from another car. Also, it's a cheap way to get a 70mm throttle body :) This being said, I need to snag this off of the OEM Renesis engine harness.

When modifying the RX-8 engine harness, there are two things that may be of interest to you. One, the throttle body sensor if you're planning to use the RX-8 throttle body. Two, the neutral safety switch, if you're planning to run it. Both of these are part of the Renesis harness. The reverse switch on the transmission is wired in through the chassis, and the speedometer on manual RX-8s are actually controlled by the speed sensors on the wheels; also part of the chassis.

That being said, here are the wiring diagrams I referenced in order to modify the harness accordingly. I only kept the throttle body wires, so that the RX-8 can actuate the throttle body when I press the pedal. When I finish wiring the car, the throttle position wire will need to be spliced into the Honda harness in order for K-Pro to know what the throttle position is. After removing everything you don't need, you can de-pin the connector to the ECU. To do this, the ECU connectors have a locking tab that you pull up. Once you do that, you're free to pull out the wires you don't need. Here's mine when I was finished. Once you're done, you just push the locking tab back down to secure the remaining wires.

Connector with the locking tab up

All the wires I removed from the RX-8 engine harness ECU connectors

Finished product :)

    After doing this, it was time to work on the charge harness. I needed to figure out how to hook up the alternator and starter, and use it to charge the RX-8's battery. First, I needed to understand the purpose of the current charge harness on the K24. Some of the connectors on it are actually part of the engine, like the knock sensor, alternator plug, and starter solenoid. This post I found labels each connector clearly on the harness. 

    The RX-8's charge harness is already built into the chassis. That being said, we can eliminate almost this entire harness. Following the directions of the KMiata Installation Guide pg.39, I removed the two thicker power wires (white and black) that originally bolted to the alternator and starter. 

    Now, from my understanding, how the starter works is that the starter solenoid wire goes from this charge harness to the engine harness, to the ECU. Then, you have an output wire from the ECU that you can attach to your chassis starter wire from the ignition switch. I'm guessing this is to prevent the car from being started in gear. I wired the starter solenoid directly to the old starter wire in the RX-8 chassis harness, bypassing the ECU altogether. That being said, I removed that wire from the K harness as well. This is it removed:

    Finally, here is my final "charge harness" for the K24:

    All that's left is the alternator connector, knock sensor, and the connector to the engine harness. At this point, that about wraps up this post on modifying the RX-8 and Honda harnesses. The engine harness itself I don't plan to modify much/at all until I get the engine running. Thanks for reading, I hope this helps someone :)


  1. Brilliant post keep them coming. I'm definitely going to do the same converstion

  2. nice article. I want to read more article about automotive electrical wire because I need for my research

  3. Great work!
    I’m thinking about doing this same project and these post are really helpful to get an idea of all that involves!!

  4. Would it be good to just transfer the donor car under hood fuse box to the RX8 instead of doing all of this?

  5. Any updates on this. Going through the same swap and the wiring is the part that stresses me out the most

  6. Great stuff!!! Appreciate the info!

  7. What was the exact k-pro you used?


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