Question about harness compatibility, modernizing my 1.8 swap.
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Question about harness compatibility, modernizing my 1.8 swap.
Hello MT.net, this is my first time posting. You all come highly recommended. I have a habit of bring word-y so I'll get to the point.
Last summer I bought a 93 miata that had a spotty history. All that I was really told is the previous owner "set it up for racing" and swapped a late BP into it (I believe it came from a 96). I started to chase and diagnose some running issues, eventually I had the thought to make sure the ecu was stock, and it wasn't. There was - what I discovered to be - an original MegaSquirtPNP which is a really cool piece of vaporware, but is vaporware nonetheless. I then was ignorant, and excited, and ruined the tune. Then trying to fix the tune, I was using a windows 11 laptop further ruining the tune, my coil pack(s), and probably my rod bearings. So, I want to put a modern standalone in it, and replace the harnesses with something I'm certain works, and be able to actually ask relevant questions when troubleshooting tuning if nothing else.
Getting to my question, to my understanding there is a small wiring loom that stays on the engine if you were to like pull it. Then there is a "front chassis" harness, that sits between that and the ecu that also carries the coil pack, o2 sensor, alternator, coolant temp, MAF, radiator fans, headlights, ect.
Would I be able to replace everything between the ECU and the engine with 96 wiring harnesses, without rewiring the rest of the car? Meaning the dash/cabin and rear harnesses, I understand I'll essentially need to disassemble the entire engine bay.
This is my first real project car, so I'm trying to keep things simple when I can even it if costs a little more. Trying to avoid major soldering and involved multimeter testing, for the time being. Baby steps, yanno? I would however, relish the opportunity to strip and clean the engine bay, change subframe bushings, and throw some entirely unnecessary heat tape on the firewall just because that's racecar asf. That's just to say I know its going to be a lot of work, I'm just asking if it will work.
I am absolutely a rookie, so if there are any words of wisdom or any common sense info about these swaps that anyone is willing to throw my way I would be extremely grateful. Thank you for your time.
Last summer I bought a 93 miata that had a spotty history. All that I was really told is the previous owner "set it up for racing" and swapped a late BP into it (I believe it came from a 96). I started to chase and diagnose some running issues, eventually I had the thought to make sure the ecu was stock, and it wasn't. There was - what I discovered to be - an original MegaSquirtPNP which is a really cool piece of vaporware, but is vaporware nonetheless. I then was ignorant, and excited, and ruined the tune. Then trying to fix the tune, I was using a windows 11 laptop further ruining the tune, my coil pack(s), and probably my rod bearings. So, I want to put a modern standalone in it, and replace the harnesses with something I'm certain works, and be able to actually ask relevant questions when troubleshooting tuning if nothing else.
Getting to my question, to my understanding there is a small wiring loom that stays on the engine if you were to like pull it. Then there is a "front chassis" harness, that sits between that and the ecu that also carries the coil pack, o2 sensor, alternator, coolant temp, MAF, radiator fans, headlights, ect.
Would I be able to replace everything between the ECU and the engine with 96 wiring harnesses, without rewiring the rest of the car? Meaning the dash/cabin and rear harnesses, I understand I'll essentially need to disassemble the entire engine bay.
This is my first real project car, so I'm trying to keep things simple when I can even it if costs a little more. Trying to avoid major soldering and involved multimeter testing, for the time being. Baby steps, yanno? I would however, relish the opportunity to strip and clean the engine bay, change subframe bushings, and throw some entirely unnecessary heat tape on the firewall just because that's racecar asf. That's just to say I know its going to be a lot of work, I'm just asking if it will work.
I am absolutely a rookie, so if there are any words of wisdom or any common sense info about these swaps that anyone is willing to throw my way I would be extremely grateful. Thank you for your time.
Last edited by Stalk_Soupra; Mar 25, 2025 at 05:15 PM. Reason: The end felt impolite, sorry.
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Hello MT.net, this is my first time posting. You all come highly recommended. I have a habit of bring word-y so I'll get to the point.
Last summer I bought a 93 miata that had a spotty history. All that I was really told is the previous owner "set it up for racing" and swapped a late BP into it (I believe it came from a 96). I started to chase and diagnose some running issues, eventually I had the thought to make sure the ecu was stock, and it wasn't. There was - what I discovered to be - an original MegaSquirtPNP which is a really cool piece of vaporware, but is vaporware nonetheless. I then was ignorant, and excited, and ruined the tune. Then trying to fix the tune, I was using a windows 11 laptop further ruining the tune, my coil pack(s), and probably my rod bearings. So, I want to put a modern standalone in it, and replace the harnesses with something I'm certain works, and be able to actually ask relevant questions when troubleshooting tuning if nothing else.
Getting to my question, to my understanding there is a small wiring loom that stays on the engine if you were to like pull it. Then there is a "front chassis" harness, that sits between that and the ecu that also carries the coil pack, o2 sensor, alternator, coolant temp, MAF, radiator fans, headlights, ect.
Would I be able to replace everything between the ECU and the engine with 96 wiring harnesses, without rewiring the rest of the car? Meaning the dash/cabin and rear harnesses, I understand I'll essentially need to disassemble the entire engine bay.
This is my first real project car, so I'm trying to keep things simple when I can even it if costs a little more. Trying to avoid major soldering and involved multimeter testing, for the time being. Baby steps, yanno? I would however, relish the opportunity to strip and clean the engine bay, change subframe bushings, and throw some entirely unnecessary heat tape on the firewall just because that's racecar asf. That's just to say I know its going to be a lot of work, I'm just asking if it will work.
I am absolutely a rookie, so if there are any words of wisdom or any common sense info about these swaps that anyone is willing to throw my way I would be extremely grateful. Thank you for your time.
Last summer I bought a 93 miata that had a spotty history. All that I was really told is the previous owner "set it up for racing" and swapped a late BP into it (I believe it came from a 96). I started to chase and diagnose some running issues, eventually I had the thought to make sure the ecu was stock, and it wasn't. There was - what I discovered to be - an original MegaSquirtPNP which is a really cool piece of vaporware, but is vaporware nonetheless. I then was ignorant, and excited, and ruined the tune. Then trying to fix the tune, I was using a windows 11 laptop further ruining the tune, my coil pack(s), and probably my rod bearings. So, I want to put a modern standalone in it, and replace the harnesses with something I'm certain works, and be able to actually ask relevant questions when troubleshooting tuning if nothing else.
Getting to my question, to my understanding there is a small wiring loom that stays on the engine if you were to like pull it. Then there is a "front chassis" harness, that sits between that and the ecu that also carries the coil pack, o2 sensor, alternator, coolant temp, MAF, radiator fans, headlights, ect.
Would I be able to replace everything between the ECU and the engine with 96 wiring harnesses, without rewiring the rest of the car? Meaning the dash/cabin and rear harnesses, I understand I'll essentially need to disassemble the entire engine bay.
This is my first real project car, so I'm trying to keep things simple when I can even it if costs a little more. Trying to avoid major soldering and involved multimeter testing, for the time being. Baby steps, yanno? I would however, relish the opportunity to strip and clean the engine bay, change subframe bushings, and throw some entirely unnecessary heat tape on the firewall just because that's racecar asf. That's just to say I know its going to be a lot of work, I'm just asking if it will work.
I am absolutely a rookie, so if there are any words of wisdom or any common sense info about these swaps that anyone is willing to throw my way I would be extremely grateful. Thank you for your time.
I don't believe there's a sub harness for the engine. IIRC there's a body harness and an engine harness, but it's not common practice to swap/replace them. There are write ups on here about swapping from a 1.6 (90-93 cars) to a 1.8 (94+ cars), so reading those might give you some ideas what plugs/sensors would have been changed in the 1.8 swap. Swapping the harness could be done but doesn't seem like a baby step or simple. If the car was running when you got it, then presumably the wiring has been modified appropriately for the engine swap.
Personally I feel like there are 2 ways you could go about this.
1. Make the MS PNP work
2. Get a new PNP ECU for a 93, figure out what was changed with the swap, and account for that in the new tune.
I'm not sure why you think your rod bearings are toast, but if that's the case then I'd try to get the car at least running on the MS1. No sense spending a bunch of money on a new ECU if you're not even sure the engine's going to run. If you're not familiar with tuning, then starting on a cobbled together car with an outdated ECU is a big hill to climb.
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Ok, thank you I'll take a look at those.
I understand its not common, that why I'm asking the question.
I tried to acknowledge this in the initial post, I understand this isn't exactly a simple task, but going about it this way is a whole lot simpler than diving into the fucked up harness that's in the car. I barely passed my multimeter testing class in school, I know I'm bad at that, that's why I'm trying to avoid that.
Yes the car was running, on 2-3 cylinders because of the coil packs that keep getting bricked because of the shitty harness.
I have already decided on ditching the MS1 in favor of an MS3, but I feel like what's the point of putting a new computer if I know the old harness is a mess.
So do you know if there's anywhere in the front harness where if connects to any other major harness or is it pretty much a direct line between the ecu and the components its connected to? And if not, what is going to stop me from putting a front harness that matches the engine and getting that to run? I'd like to get a new harness either way.
I understand its not common, that why I'm asking the question.
I tried to acknowledge this in the initial post, I understand this isn't exactly a simple task, but going about it this way is a whole lot simpler than diving into the fucked up harness that's in the car. I barely passed my multimeter testing class in school, I know I'm bad at that, that's why I'm trying to avoid that.
Yes the car was running, on 2-3 cylinders because of the coil packs that keep getting bricked because of the shitty harness.
I have already decided on ditching the MS1 in favor of an MS3, but I feel like what's the point of putting a new computer if I know the old harness is a mess.
So do you know if there's anywhere in the front harness where if connects to any other major harness or is it pretty much a direct line between the ecu and the components its connected to? And if not, what is going to stop me from putting a front harness that matches the engine and getting that to run? I'd like to get a new harness either way.
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Well yeah, if your harness is causing issues then replace it. It sounded like the ECU was the issue in the original post.
There are people on here who have dealt with engine swaps and harness replacements, but I'm not one of them. Either hope that somebody with experience chimes in or start doing some searching/reading. Realistically the best path forward would be for both of those things to happen.
My guess is that you're going to have to pull the whole car apart. I would expect that the whole interior (including the dash) would need to come out to swap the harness.
There are people on here who have dealt with engine swaps and harness replacements, but I'm not one of them. Either hope that somebody with experience chimes in or start doing some searching/reading. Realistically the best path forward would be for both of those things to happen.
My guess is that you're going to have to pull the whole car apart. I would expect that the whole interior (including the dash) would need to come out to swap the harness.
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The Miata harness is a single harness from tail lights to head lights, and everything in between. There’s a few exceptions, like there’s a sub harness for the dash, and as you mentioned the injectors (and water temp sensors), but the rest is all 1 piece. The NB technically has an engine harness, but it requires a dash pull to remove it, and obviously the rest of the harness needs to be NB as well.
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Ok, heard that. Guess I'm trying to repair my harness then.
I'll take a look at BP swap writeups to get a better understanding at what I'm looking at. I got a Haynes manual a little while ago so I'll try to track down troubleshooting for specific things (coil pack, dead tach, ect.) in there.
As far as repairing connections go, is it just a matter of cutting the right wire a few inches out from the pigtail, reestablishing a connection, and taping the new wire to the old loom? I worked at a marina for a summer and used to install Garmin systems into personal skiffs, so I understand how to make a connection. It's understanding troubleshooting information that I fall short, hence wanting to avoid this.
It looks like you've built pretty much exactly what I want this to turn into (minus VVT), would you be willing to give me any tips or goals for getting this chassis ready for a new computer?
I'll take a look at BP swap writeups to get a better understanding at what I'm looking at. I got a Haynes manual a little while ago so I'll try to track down troubleshooting for specific things (coil pack, dead tach, ect.) in there.
As far as repairing connections go, is it just a matter of cutting the right wire a few inches out from the pigtail, reestablishing a connection, and taping the new wire to the old loom? I worked at a marina for a summer and used to install Garmin systems into personal skiffs, so I understand how to make a connection. It's understanding troubleshooting information that I fall short, hence wanting to avoid this.
It looks like you've built pretty much exactly what I want this to turn into (minus VVT), would you be willing to give me any tips or goals for getting this chassis ready for a new computer?
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