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Discussion Starter #1
I have a 66-200 with an operational but hacked up wiring harness.

Long term, I want to swap the 200 for a 5.0 with EFI and an AODE or 4R70.

I’d like to replace the birds nest of wiring under the dash with an updated harness and fuse panel. I’ve got everything working that wasn’t, but it’s only a matter of time before something else blows or shorts out. My confusion is on what I need so that when I get down the road for the engine swap that my harness is compatible with the EFI and Trannie.

Seems like the EFI harnesses by Painless and Ron Francis are at least partially stand alone. If I install one of the 64-66 American Wire or Painless harnesses should I be good with compatibility?

Please forgive the ignorance and thanks for any advice or help.
 

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Discussion Starter #3
Only thing is long term will likely be really long term. I’m probably on a 5 year plan that could have the engine swap fall in the 4th or 5th year.

If a main harness swap isn’t wise until I’m ready for the engine swap then I guess I’ll need to find some more permanent solutions for the hacks I used to pass inspection.
 

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If you replace your main harness now I believe you would only need to get the conversion EFI-MAF 5.0 harness later and blend it into your system. Certain things would drop off from the old harness and some new EFI harness wiring would need to be grafted in but it is not that bad if you have decent wiring skills. You do wind up with some cut off wiring unless you strip the jackets off everything, pull the old wires completely out and put new tape or wiring loom over the converted wiring system.
 

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Using hacks to pass inspection isn't the most advisable route. Wiring is not something to try and MacGyver in an old car. Especially if you're planning on driving it for a few years before doing an engine swap. I watched my mint condition 1971 Bronco go up in flames on a busy road in Boston due to bad wiring by a PO years ago.

That being said rewiring the entire car is pretty involved. I did mine last year. The easiest way to do it I found was to blow out the interior (seats, steering wheel, column, heater box, etc.). I wired mine with an American Autowire harness and ran the wires close to stock locations with some modifications here and there. I am very pleased with the results. I got rid of the voltage regulator and have a 160 amp one wire alternator which can handle anything I decide to add. I also had an electric fuel pump at the time so I wired in a fuel pump relay for added piece of mind.

This year I added a Fitech fuel injection system which was an easy install since I already had an upgraded harness and the fuel pump relay in place. I think it literally only took me a couple of hours to get the carburetor off and the Fitech system installed, wired and fired. I also have wires ready for adding fog lights, AC, an electric fan and even power windows if I ever want them. You can remove wires from the harness for things you don't want but I left all the unused wiring intact just in case I want to upgrade anything else in the future (like the Vehicle Speed Sensor (VSS) wires). Any wires that I am currently not using (I don't even have a heater box in my car right let alone AC, that's next years project) I have capped off with wire nuts, heat shrink and the fuses pulled if applicable. I would suggest doing the same if you rewire your car before the engine swap with any unused wires.
 

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Discussion Starter #6
Using hacks to pass inspection isn't the most advisable route. Wiring is not something to try and MacGyver in an old car.
That’s why I’m hoping to find a middling solution. I’m confident in the reliability of my hacks. My last hobby was building tube amps. I don’t MacGyver because 400 volts across your heart will stop it, and it hurts like hell if it doesn’t. Ask me how I know. And my hacks aren’t so much hacks as reading the schematic and cutting out what shouldn’t be there and replacing it with what should. My favorite is improper grounding. I’d dress up like MacGyver if I could get the brake lights to stop going out every other time I close the trunk.

Anyway, my concern is the rest of the birds nest. I either need to spend the time to pull everything out and replace everything that’s worn, frayed or spliced together with electrical tape, or do the full modern rewire. I hacked things together for my son to get things legal last summer with the intention of putting in an American wire harness last winter. He lost interest in the car last fall and this summer it became my car.

I’ve come to the conclusion that I’d like the eventual engine trannie combo to be ECU(?) tunable. I don’t really know how the harness for a modern nineties 5.0 and trannie ties into the main harness for one of these older cars. Hence the question. In some ways it’s kind of a “keep your options open” kind of question. I don’t want to put in a harness that definitely won’t be compatible.

Just as a sidebar. I just recently found out this was becoming my car so I haven’t quite got the whole “typing the right thing into the search bar to find out what you actually want to know” figured out. So please forgive me if I ask a few obvious questions as I dig back in to this whole thing.

Thanks
 

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If you get a conversion harness specifically for the mustang which is what you would want, I would expect it comes with the instructions on how to blend it into your system.


I found this website that goes through the wiring and engine upgrade. You have plenty of time to study up on all of the details of the conversion.


Create a Custom EFI Installation


That is just one way to do it and I think he is using an 87 to 93 5.0 harness actually.
 

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The link above is a good primmer to what you'll need but considering electric fans, stereo/amp, headlight relays and trans or ignition what-nots I bet any"kit" you could buy will need to be modified before you are done with your setup anyway.
I would start with a new fuse/relay box under the hood and work out from there. Eventually eliminating the under dash block will keep you from having to contort yourself to check a fuse or add anything else down the line. 6 relays and 12 circuits should about cover it. New box under the hood on the PS inner fender and a new penetration and grommet under the heater will keep the new separate from the old till its all done.

Wiring is too complex to list here but it isn't hard once you get the hang of it. Big hot from the battery goes to buss bar in the new box then through a fuse/breaker in the box then out in a circuit to an on demand object like the lighter or to a switch(like the headlight switch or ignition switch), then out of the switch to the application. Or from the switch via a "light wire" to a relay which sends power out to the object in a "heavy wire."

Drawing a schedule and have a dozen different colored wires would be ideal. Not too expensive considering each color only needs to be about 10' max.
No crimp connectors or twisting and taping. Twist then solder then heat shrink the splice where they are needed. Split loom is nice so you can add to the loom in pieces over time. When all done you can wrap it with nice cloth tape and get pro results for the fraction of the price of a kit that still needs some of the same basic steps done to it.
 
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