ECU wiring issues with mid year '95 motor in '94 body
 

I posted this in the miata.net forums as well, but figured you guys might have more experience with this kind of thing.

So I got a '94 roller, and I had bought a mid-year '95 engine for it that came with the ECU and a chopped up harness. The car still has the old harness. All the connectors were just pulled off the motor and labeled, so I'd like to keep that in the car rather than having to swap out the wires, and extend what was chopped off the new motor. The problem is the ECU from the new motor is the 3 plug version, whereas the harness from the car goes into a 2 plug ECU. Diagrams I've found show that a lot was moved around on these plugs while Mazda prepared to update to OBDII. I don't believe the motor is the OBDII version, but an in-between version from mid-late '95 that only had some changes. I can't remember exactly what gave that away, but I believe it was the coil pack connectors or some missing sensors that would be on a 96 OBDII motor.

Is anyone familiar or have experience with these motors and their wiring? Ideally, I'd like to disconnect or chop off both at the ECU connector, and just reconnect the right wires to the right places on the new connector. But I honestly don't know if that will work or what additional wires I might need to run, if any. Before I go comparing every wire path in the diagrams, I'm hoping someone here might know more about it. I did the DIY megasquirt and DIYBOB on my '92, so I'm comfortable with messing around with the wiring, I just don't want to spend all the time to rewire the plugs if it's going to be a waste of time or potentially fry something that I won't notice until I've redone everything else 5 times. (ask me how I know)

I've never heard of a mid 95 wiring change. What makes you believe there was one? Are you simply looking at the build date on the car it came from? Built in 95 doesn't mean model year 95.

Why aren't you just using the 94 harness and ecu that are in the car to run the 96 engine you have?

I got the motor already pulled so it didn't have all the info from the donor car. It came with the ECU still attached on the wiring harness, looks like whoever pulled it only cut what they needed to and pulled the whole thing with the harness and trans still attached. I looked up the model number from the ECU when I noticed the difference between the harnesses being 2 plug on donor vs 3 plug on motor. It also has 4 available plugs on the ECU itself, but the harness it was pulled with only uses 3. The model number is BPS5 18 881A. I couldn't find anything on that exact model number but seemed to point to '96 as per this post. https://forum.miata.net/vb/showpost....20&postcount=4 Looking into 96's, they should have a sensor and trigger wheel on the crank, but I don't see that on the motor. So after digging some more, I came across this thread ( https://forum.miata.net/vb/showthread.php?t=462753 ) which mentions a split in '95.
Specifically,
which is the same as the motor I have. 3 wire coil but no sensor on the crank.

I'd like to use as much of the 94 harness as possible since it's intact and in the car already, but the roller didn't come with an ECU. That thread also mentions an issue with the 3 wire coils and compatibility with tach and ignition. I'd love to not rewire anything and just slap a '94 ECU in there if that's gonna work, I just personally wouldn't know if it would.

I’m in Australia so perhaps not relevant.

I can 100% confirm that mid 95 was the computer change along with a few small other changes such as door cards with pockets and an interior light on the windscreen surround like NB’s.

I have an 11/95 NA8 and it’s got the different computer ect to my friends 3/95.

When I get my MS I require the 96-97 style.

I believe the ecu went from 8bit to 16bit and the tune was improved, My Miata Yoda who has been an Mx5 driving instructor for 2 decades said you can feel the difference.

Good luck with it all.

I don't know how it works in australia but in the USA a 11/95 build date would be a 96 model year car. It does not go by calendar year.

From what I gather in the thread you linked you can run the 95 engine with a 94 ecu and harness and not touch any wiring. The coils are listed as interchangeable for 94-97 so the tach should be driven by the coils without issue.

Thanks Mate, that’s the confusion. In Aus the plate year is the year model.

Not to go too far off topic but might interest some.

“Down under” you can get great deals early in the year on last years models.

TV is full of “2019 plate clearance on now!” ect For example.
Not uncommon to buy a November plated car in late January early February for $3,000-$4,000 AUD less than the exact same car with the current year stamped on the plate........ they even throw in free floor mats!!! lol.

when swapping engines, no need to swap in a whole ecu/harness

drop in the engine and it will plug into all your existing 94 plugs/senors/ecu

That's what I'd do

As Borka and Sixshooter have said no need to change harness.

If I’m correct your 94 roller has the wiring but no computer but you have the wiring that’s hacked up a bit and the computer for the donor motor, 96.

I think the easiest is get a 94 computer or a Megasquirt for the roller since the wiring is all there.

Plug it in and your golden.
If it’s just to get it running you may be able to pick up an older MS cheap and upgrade later.

Thanks so much for the helps guys. Swapping the ECU to a '94 one sounds like the solution.
So it seems like the motor came out of a '96? model year that was manufactured in late '95, or a foreign late '95 model? Either way the weird "OBD1.5" time period I guess I'd call it.
I found a 94-95 ECU with 2 plug on Ebay for $50 so I'm willing to just buy that and throw it in. Is there any difference in ECU for the manual vs automatic? The listing doesn't say but it's model number BPL9-18-881. I can't find any more info on if there's an ECU difference for transmissions in these years or what the model numbers would be.

From what I can tell from this thread (https://forum.miata.net/vb/showthread.php?t=180191) there is also some confusion around this time about the 3 vs 4 wire coils so I still think I'd need to swap out the coils though, right? The roller didn't come with a motor or ECU, just the harness, so all sensors are from the donor motor (the 95/96 or whatever). It's got the 3 wire coil, but the '94 harness is 4 wire, so the ECU will be looking for the tach signal from there? I'd just have to swap out the coil for the 4 wire version as well then, right?

Yes exactly. No motor or ECU from the 94 car, just harness still installed. New 96 motor came with chopped harness still plugged into the 96 ECU. I'm just trying to get it running for now. It still needs a rad, coolant lines, and a couple things here and there anyway. Just trying to get it running stock so I can get it moving and finish up whatever else it needs.

The auto was electronically controlled I believe so the computer would be different, ask the seller if it came out of a manual which let’s be honest over 90% of miata’s would be.

I can’t help much on the coils but the way I look at it there is a possible 3 options.

Firstly check out the DIY COPs wiring all years on this forum, it’s a sticky somewhere but has the wiring diagram. I did my COP’s from this. COP is coil on plug if you didn’t know.

From there either.
1, see if you can run a jumper to trigger tach, the 3 wire I would imagine is 12V, pulse and trigger. Trigger would go to tach I imagine but I’m talking out my arse here so do some research.

2, Buy the 94 coil pack I’m sure they would be cheap.

3, Do the COPs conversion now, have stronger more reliable spark and never buy leads again, this is what I would do. You guys are lucky on parts prices there, it was cheaper for me to buy used Toyota coils from the U.S and ship here than get second hand in Australia.