Hello all!

I am purchasing a drop-in '03 VVT engine for my '90 Miata that comes with an 03 ECU, harness and basically everything else needed to run the engine on day 1. In theory, can I completely replace my current harness with this new one and purchase an MS3 pro for an '01-'05 chassis instead? I want to make this swap as simple as possible and if I have access to this complete harness I figured I would ask.

FYI, this will be a boosted engine if that changes anything.

Thanks a ton!

 

Theoretically you can use the VVT harness, but practically you're usually better off using your original harness and running an extra wire or two to operate the VVT. Read FM's 1.8 swap guide in full for a decent starting point.

 

MT.net Definitive VVT into NA thread

 

It can be done, however it's not ideal. You have 3 options.

1. Splice each connector individually. Cut your coil harness off, splice in the VVT coils, etc. Repeat for cam sensor, crank sensor, TPS, IAC, etc. Only one you'll really be adding is VVT, running this to your ECU.

2. Do the same as #1, but further down the harness. The entire VVT harness eventually joins into one somewhere behind the intake manifold. You can cut it here, strip the insulation back, and make all of your connections here. You have to reroute parts of NA harness slightly, as it's split in about 3 sections. You'll be dealing with only wire colors, so you better have a really good understanding of wiring diagrams from both cars. You can also open up the harness and remove unneeded connectors, such as EGR, VCTS, and o2 (you'll probably be using an aftermarket wideband sensor). I've done this method a half dozen times or so, it can come out really clean, but takes a lot of work.

3. Make the complete VVT harness work. This is probably the hardest, because the VVT engine harness integrates with the chassis harness and battery harness. I've done this once on a bare shell when I already had an engine harness, and it was easier to buy the used VVT harnesses off ebay than buy an NA one and figure out how to make the two work.

 

+1 for megathread. its like magic.
unless you have an ulterior motive for replacing the harness, in which case I don't see why it wouldnt work.
The immobilizer/key dont matter once you go MS, right?

 

Cool, thank you everyone for the help!! I have seen the megathread and the section on wiring but I might have missed anything related to a later harness. One more question then. Is there a demand for VVT stock ECUs and Harnesses? If not I will hack up the harness for wiring purposes but I don't want to ruin a perfectly good and needed item in the community.

Again, thanks for the help! You'll hear from me more when my engine gets to its new home!

 

The stock ecu is near useless without the matching key and immobilizer. Harness is worth $100 or less, usually used for what you need it for, so keep it unless you want to source all new pigtails.

 

Sounds good, thanks for the good info!

 

Like others have said, yes its possible but not worth it. I was in a similar position as you when i swapped 2 years ago.

My +1 here is to keep the old harness. I just so happen to throw mine in a corner and forget about it, but man has it saved me. Come in useful many many times for connectors and matching oem wire color/gauge when adding things!