Question about splicing into engine harness
 

Hey guy's,

I'm about to megasquirt my miata and I'd like to keep the factory engine harness (I'm not looking for a clean engine bay look). I don't want to go PNP because I want to learn the hard way :rofl:. I eventually want to be able to megasquirt anything I want to. I want to be able to go back to stock if I have to. If I make splices like these, I can essentially go back to stock ecu anytime I want. I took a picture of a few splices I made for this post. Are any of these acceptable? Assuming the black wire is original engine harness wiring and the green wire is the megasquirt wire.

The main reason I ask is (I'm kind of a newbie) and wasn't sure if making splices like these would interfere with the electrical resistance, ohms, or other crucial electrical profile.

https://cimg5.ibsrv.net/gimg/www.mia...ffb4a553fd.jpg


Thank you

I went from a stock ecu to a PNP unit on the dyno in roughly 5 minutes, no wiring. What you're planning to do is one of the most risky ways to install a megasquirt. Every single splice is a potential failure point, and you're planning on splicing into every single ECU wire.

That being said, I'll still answer your original question. I don't like any of those. Closest one for me would be #1, however you still have wire exposed. Use heat shrink butt splice connectors, they typically have adhesive in them, so when they're shrunk, it glues the wires in, sealing them against moisture. Here's a kit on amazon:
Another option: get all the OE connectors and pins, and connect your MS harness directly to all of the sensors and outputs, leave your OE harness in the engine bay ziptied out of the way.

yes I didn't put heatshrink tubing or electrical tape in picture, to show the actual splices... but would in real application. I understand it's "risky" but honestly making splices into wiring is pretty easy (just time consuming). The reason I want to stay away from pnp is because I want to be able to megasquirt another car I have that doesn't offer a straight pnp version so this way I can really learn. Thank you!

Why not make your own PNP version. The concept is the exact same without the additional risk. Buy a break-out board, don't use anybody else's PNP diagram (Such as TruboKitty) and pin it out yourself.

​​​​​​​Hmmm, I'll have to look into that idea. I never thought of it. I 'guess' it would be like making my own connector that would fit with the oem ecu connectors? As long as I know the pins of the oem connectors and megasquirt DB37 I should be able to line them up and hook it up that way?

Is that essentially what you're saying? Rather than running into my engine bay and splicing into the wires?

You would buy something like this.
https://www.diyautotune.com/product/...ndenso-76-pin/

The factory ECU harness will plug into the breakout adapter but on the circuit board, you will solder your Megasquirt wiring into it. By not using TruboKitty as a reference, you can get the practice of reading the factory wiring diagram.

In reality, I did the above but I used this website as an "easy mode". It tells me what each pin does and then I matched it to the output/input on my aftermarket ecu.
https://rusefi.com/wiki/index.php?ti...zda_Miata_2003

Ok, so I would essentially buy a MS3x (assembled version non pnp, yes I don't want to assemble/solder the boards myself) - and run the DB37 connector wires directly to this breakout adapter? And then from the breakout adapter, it should plug into the oem stock connector(s)? Essentially making it pnp from that point - which will save me about $200 (not a lot, I know) as opposed to buying a pnppro.

Yes sir, that would be how it works. You didn't list the miata year that you had so I just picked a random breakout adapter from diyautotune btw.

I'm sorry, it's a 99' miata. Cool, now I just need to find the wiring diagram for a 99' miata showing me which pins connect to which oem factory engine wiring harness wires. Thanks again, I might just do this instead of splice into each individual wire inside the engine bay harness..

I picked up this 99 ecu harness used, worked great.
https://www.ebay.com/itm/331950274982

https://trubokitty.com/#/ms3xassembly scroll down to patch harness assembly, the wiring diagrams are below that