EFI harness options


Sep 28, 2006
Livonia, MI
Im in the process of gathering parts for my EFI swap (still) and had a question about the harness. Ive asked a few friends locally, and rather surprisingly none seem to have or know of any 89-93 Mustang harnesses. Ive looked in my local yards and the pickings are slim. Plenty of Crown vics, but those are SD and have the 289 firing order, so no good. I see Mark 7s from time to time, and they have the H.O., but speed density. I could just convert it, but Im not entirely sure what that entails. Im sure its not that hard, just havent done it. I was looking at the ford fuel injection website, and it shows the 91-93 t-bird, 94-5 truck, and 94-5 mustang all being the same MAF harness. Are these good candidates? I seem to remember reading that the 94-5 mustang computer doesnt work well with mods, and Im sure the harness wont simply plug into an earlier computer (correct me if Im wrong). Id rather not just buy one off ebay because I would never know it was the wrong harness until I went to put it on the car and it didnt work, or worse yet didnt work right and left me chasing my tail. Also I would like to pull one from a vehicle personally so that I know where everything goes and I can spend all day labeling, taking pictures, inspecting, etc. My other option is much more expensive, but has its merits as well. Flipping through a mustangs plus catalog they have an efi harness from Painless for ~$570. This would be nice because then I wouldnt have to worry that a previous owner let the smoke out of the harness (everyone knows its the smoke that makes the wires work), its the right one, no crispy insulation, and every terminal is labeled, and comes with instructions.

Its going into the car in my sig, which I have no plans to ever sell, will soon have a wide ratio AOD, and currently has what amounts to a rebuilt 96 Explorer motor with mildly ported heads and shorty headers. Once everything gets sorted out and I get financially situated itll get a stroker, but not for at least a couple years.
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Having done the swap and having re-wired a 1993 harness for a 1966 chassis, a few things:

1) Really consider why you want EFI. Performance? Driveability? Economy? Starting? Just for the hell of it?

Performance - You'll get as much or more from a carb setup.
Driveability - Forget what anyone tells you about how much a mustang computer can learn. If you build your motor for performance you'll need a re-tune or you'll have bucking, idle surge, and a host of other issues.
Economy - If you are building a hotrod, gas mileage won't be like an accord either way.
Starting - EFI wins hands down, no questions asked
Just for the hell of it - Go for it

2) Rewiring isn't that hard. Find a harness from any ford eec-iv with a MAF and make sure you can get a wiring diagram - from there you can re-wire it to your needs. My re-wire took 5 evenings after work to strip, and re-wire to fit my 1966 chassis. And I am no genius with wiring. It's really not that hard.

I don't want to rain on your EFI parade, but if I had known the truth when I got started, I wouldn't have gone EFI...just my two cents.

Good luck...
Ive had carbs that work great, but Ive had plenty that didnt. When my Comet was carb'd it never ran very well, though it had other issues as well. I later put that same carb on a stock explorer 5.0 in my Falcon, and after a bunch of work to excersize its demons, it ran much better, but still left me feeling like I was leaving power on the table. Ill probably be putting a lot of miles on the car, so some better mileage would be nice, a little wow factor would be nice, but mostly I like the fact that I can pretty much tune it once and be done regardless of weather or altitude.
I went the used (Ebay) harness route when I did my swap. It's inevitable that you're going to get some broken connectors and questionable lines with any sort of used wiring that's close to 20 years old. Sure you can "freshen" the harness replacing broken connectors and wires, but in the end, some of it is still going to be 20 years old...

If you plan on keeping it forever, I'd suggest going with all new wiring from Painless, Fordfuelinjection.com, Ron Morris or any of the other companies (can't recommend one since I don't have a new harness).

Put some thought into where you want to put the computer, relays, etc before you buy the harness as that plays into whether you buy one of the extended length harnesses or not.

Just my 2 cents.
I was in the junkyard looking at a Mark 7 and assessing how much of a PITA it was going to be to pull the harness, when an old acquaintance texted saying he had just pulled his Mustang harness in the process of converting to a 408 turbo with a blowthru carb. The car had a 5 spd in it, but I seem to remember him saying it originally had an automatic. Since Ill be using an AOD, can I use the a9l manual trans computer with the auto? I seem to remember reading that one computer could be used with either trans, but the other computer should only be used with 1 trans. Its been a busy few months since I was doing most of my research and reading, so my memory of the intricacies of EFI is a little foggy.

I don't know how much this helps you, but I pulled a 88' 5.0 H.O and a 92' A9L ECU and harness, and a F1ZF MAF from a 95' Taurus for $200. The 5.0 was a T5 manual, but we are running a C4, so no electronics. We haven't had any issues, the only thing I needed to add to the harness was the neutral safety switch, so it doesn't start in gear. Other than that, no issues. Hopefully that helps.
you may want to double check to be sure, but as i recall you dont need to rewire the harness, just swap computers and the injector firing order in set in the computer itself. this means you can grab the harness from a crown vic, and use the mustang computer if you like. at least that is what i got from reading charles o probst books on ford fuel injection.
Wouldnt the computer need to know the trans is in gear or not so it can adjust idle speed? Other than that, the trans operates entirely on its own like any other non-electronic trans.

yes, there's a parameter in calibration that tells computer what tranny is used. but really it doesn't have much effect. the computer sences if the engine is in neutral(or clutch is depresed for manual tranny) or in drive (or clutch is released). that is senced via simple switch that shows 0 volts in neutral and 5 volt in drive. you may want to study ford tech paper on computer logic like this ( Strategy Documentation for 89-93 5.0L Mustangs) http://autos.groups.yahoo.com/group/tweecer/files/Documentation/GUFB.zip