Touched a little bit on this in my thread in the Single Turbo section, but wanted to make a post here for some more visibility and hopefully some more opinions.

Bought my car as an empty bay roller, finally got an engine last year, and bought a single turbo kit from Turbosource this year. Since I don't have a harness and plan on running a Haltech, I was thinking about picking up their flying lead harness when I order the ECU. However I've found a good condition stock harness for sale for much cheaper. I'm not a total novice when it comes to wiring, but doing all the work necessary to properly setup a flying lead harness seems more daunting the longer I think about it. But it might just be a cause of "afraid of what I dont understand", so before I commit to either option I'd like to know:

What modifications would I need to make to the stock harness?
What would the roadblocks and limitations be?
For anyone who's used the Haltech harness, what was your experience like?
How difficult really is it to properly wire a flying lead harness?
What resources do you recommend for learning how to do this right?

I dont want saving a couple hundred buck now to be the cause of tons of issues down the line. I've dealt with trying to diagnose wiring gremlins in very simple systems and the last thing I want to do is deal with them in a system with actual complexity.
Appreciate any advice as always.

 

The Haltech ECU shouldn't care if there are unused twin-turbo solenoid connectors in the stock harness, it just needs the ECU pins connected to the correct sensors, injectors, etc. If you use a plug-and-play adapter harness that should be minimal wiring work needed. The main disadvantage is the adapter harness takes up space, so it will be hard to fit everything where the stock ECU went.


If you're planning to cut the stock ECU connectors off of a factory harness and crimp new pins to go to a Haltech ECU, it might be tricky to get the lengths correct enough to plug in nicely since the Haltech ECU connector won't end up in the exact place as the factory ECU connectors did. Be aware that there are four plugs on the stock ECU and only two of those plugs come from the engine harness. The other two stock ECU plugs come from the dash and the body harness. I would not modify the stock dash or body harnesses for an aftermarket ECU, I would try to find the mating connector and build a short adapter harness so you don't need to cut any of the factory wires.

Building a flying lead harness will take a lot more time, it's always surprising how long it takes me to do wiring work. I think I spent 30-40 hours helping a friend wire up a Miata engine swap using a flying lead harness (not Haltech). Just one improper crimp can make the engine run badly so you need the right tools and some knowledge, plus plenty of patience and planning. The advantage to the flying lead harness is the wire will be newer and probably better quality than the 30-year-old stock harness, and you can route things where it makes sense for your car. The factory harness is sealed by wrapping it in a couple layers of electrical tape. On a budget, you can duplicate that using good quality electrical tape (3M/Scotch Super 33 is rated to 105 degC) without the extra time and cost of doing a high-effort harness with nice Raychem heat shrink and booted/epoxied connectors. If you do your own, be careful to avoid exhaust heat and abrasion and pinch points. Adding 6-10 inches of wire length to keep the harness a safer distance from the hot turbo and exhaust parts is a good tradeoff. If the Haltech doesn't supply twisted/shielded wire for the crank and cam sensors, I would buy something like this, one for the 2-wire 'crank' sensor and another for the 2-wire 'cam' sensor. https://www.prowireusa.com/TSB-2C


If you want information overload about motorsports wiring techniques, this webpage is a great resource and they have been updating it for years and years now. Each time I revisit it, there's something new to learn.
https://www.rbracing-rsr.com/wiring_ecu.html


Rob Dahm has made a couple good videos about wiring harnesses, but IMHO some of his materials are overkill for a DIY'er with a budget, and he sometimes misses simple concepts like 'make the harness longer so you can keep it 10 inches further from the exhaust and avoid it getting melted', or 'add a P-clamp for mounting to avoid abrasion'.

 

Yeah if I go stock harness, I would be running the Haltech PNP sub harness so I wouldn't be chopping off connectors. Specifically I'd say my two paths are either:

1500&PNP Harness + stock harness

1500 + Flying lead harness

This is what concerns me the most. The last thing I want is to dedicate a lot of my (very limited) time to learning and working on this then find out 2 years down the line that I fucked something up when I finally go for the first start.

 

That Haltech terminated flying lead harness looks like a good option. It looks like all the ECU pins are populated, and also the engine bay sensors, injectors, IAC, etc. When I hear 'flying lead harness,' that usually means only the ECU part is terminated and you need to find lengths and crimp pins on all the engine bay sensors and injectors and outputs. You will still need to find the two plugs that connect to the factory dash harness and chassis harness (those connectors send 12V power and the ignition switch signal and brake switch / clutch switch signal to the ECU, and receive the tachometer signal from the ECU to make the tacho work), but it looks like most of the work usually involved in a flying lead harness is already done for you.

 

In this era, we don't really see the point in wiring in ECUs anymore. The vast majority of use cases are covered by a off the shelf harness or a plug and play ecu or a jumper harness of some kind. It's a tremendous effort to build a harness correctly and hope you did it all right in both actual construction and wire location.

Unless you absolutely have to, do not build a harness. Purchase one, use an adapter or buy a plug and play ecu. Other than the self gratification of doing it, there's just no reason to build a harness yourself when making a street car with standard equipment.

 

+1 building a harness takes a LOT of time.

 

Yeah it seems like stock harness is my answer. At least now I know what I need to start researching.

 

Have you thought about an aftermarket harness? For example, Rywire makes a few options for FDs with a Haltech:

https://www.rywire.com/rx7-harnesses-s/1832.htm

There may be other aftermarket options, as well. I am by no means an expert and I don’t have direct experience with the Rywire harness - just wanted to mention it as I think there are options besides a stock harness or flying lead.

I liked the aftermarket option because it updates the fragile old stock harness without having to build / terminate your own harness. Obviously you pay for that convenience, but it may be worth it if you have limited time, experience, etc. with wiring.

 

Flying lead != terminated. People seem to be interchanging these.