Graham, good to see you posting.  Glad you are still working on your Mustang.  Hope to see my old wheels in a pic someday.

I've never worried about the Ford color coding, I just strip the harness and make sure each pin of the PCM is running to the right pin of the various sensors.  The way I do this is by using the diagrams available in the Charles Probst book.  I recommend the EVERYONE who is messing with these systems get that book...last time I checked it was still available through Amazon.  "Ford Fuel Injection and Electronic Engine Control"  by Charles Probst.  This is the bible on EEC-IV systems from 88 - 93 and has all the wiring diagrams you are likely to need to modify a Ford harness or build a new harness specific to your car.

You say you bought an "early" Fox as a donor.  What year?  if it's pre 88 (IIRC), 49 state it will likely have a speed density system.  California 88 and all 89-93 HO Mustangs were Mass Air which is really the system you want.  A speed density harness can be easily modified to accommodate the Mass Air PCM/system...I've done it.  Actually, using the diagrams in Probst you can make nearly any EEC-IV harness work for an HO 5.0. 
Buy a good digital VOM (Volt-OHM Meter) to ring out the wires in the harness from end to end.  I like to lay the harness out on a sheet of plywood as it will be used in the car.  That way I can shorten or lengthen the various harness legs so the finished product will fit the car as it should.  If you completely strip the old harness of all wrapping or sheathing this is a fairly simple job.  Getting the harness stripped it the hardest part of modifying an old harness. When it's finished you can re-wrap the harness with the loom material or you choice.
Be especially careful with the connectors.  If you remove the terminals and wires from the connectors you need to be very careful.  The little locking tabs in the connectors will be brittle from the years of hot/cold cycle and are easily broken.  If you brake a connector do not try to use it as the terminals will not stay anchored.  Go to Rock Auto or other source and purchase the connector and pigtail you need.  Standard, Echlin, and others make the connectors with about 6" of wire.  These can be spliced on to the old harness wires and insulated with adhesive lined heat shrink tubing.  I prefer to use non-insulated crimp or solder barrel connectors for these splices.

Take your time and you will end up with a set up that will serve you well for many years/Kilometers.  And have fun!

Can't wait to see some pix of your project.

BB1

PS:  Feel free to PM me with questions, sources, etc.

 

Mustangs were carbs until 1985.
86-88 were speed density efi...no MAF.
89-93 were MAF efi.

http://www.veryuseful.com/

I used above website to help me  as well as a couple of others.
This one has information for efi wiring.

Last edited by Nos681 (4/22/2020 4:21 AM)

MAF corresponds to a '91 Mustang, but that doesn't mean the harness or even the MAF came from a Mustang.  Mustangs often have parts that have truck, T-bird, etc. part numbers on them.  The part number just follows the original application. 

Did the MAF and harness come from the same donor vehicle?  Be aware that SD and MAF harnesses are largely the same, and plenty of SD harnesses were converted to MAF.  There are some pinning changes at the ECU plug.  Also, O2 sensor wiring changed over the years.  The plugs are the same, but the pin assignments are different.  Here's a link that covers the differences:

https://www.ronfrancis.com/images/FH026-INST.pdf

Graham;
The 83 mustang had a carburetor so who know's about that harness...probably been hacked on.  I would suggest you use a diagram from a 90 or 91 Mustang 5.0 and just take all the pieces you have and build a harness that fits your car. 
The PCM...should be an A9P or an A9L...will take a signal from the VSS.  You can also drive your speedo from the same signal.
I believe you should absolutely keep the OBD1 port.  That's the only way to see what might be making the system sick.   Gotta have it and the MIL (check eng. light) which, for some reason, many folks eliminate....?
Ignition power is supplied to the coil/TFI/HEGOsand to trigger the EEC relay.  The EEC and Fuel Pump relays should get battery power through separate fused ckts.  All of this is easily seen in the diagrams...or should be.  I haven't looked at the Moss diagrams but they are probably very well done.  I just like Probst since the book is also full of useful info.
I did and you can also use the stock Ign switch.  The only real load you will add to the Ign switch is the HEGOs unless you use a relay to power them.  I've done it both ways, the HEGOs do not pull a lot of current.
While on the subject, I do like to cut the accessory ckt loose from the stock IGN switch and let the ACC lug on the Ign switch simply trigger one or two Accessory relays which take fused power directly from the battery.  I like to remove all the load I can from the old IGN and headlight switches.

Sounds to me like you won't have much trouble with this since you aren't afraid to tackle wiring.  That's usually what panics most folks when it comes to EFI.

BB1
 

Last edited by Bullet Bob (4/22/2020 8:38 AM)

I have 90% of what you need I even have a new in box painless harness for the fuel injection system GT 40 powder coated upper and lower intake for sale Lmk Covid 19 sale LMK

Last edited by Coupedaddy (4/22/2020 2:36 PM)

Details from my PCM:
EEC-IV
GSKT NPF 1F181
*9L2B0323MWXH*

F1MF-12A650-HA
1D1F18
Theres also some pink writing on the back which I think is part No's,
9123 1009 E9AF14A624 AA

gkm24 wrote:

Details from my PCM:
EEC-IV
GSKT NPF 1F181
*9L2B0323MWXH*

F1MF-12A650-HA
1D1F18
Theres also some pink writing on the back which I think is part No's,
9123 1009 E9AF14A624 AA

I went through all the PCM codes...such as A9L, A9P, W4HO, etc...in the data I managed to get from the old RJM website before Ford shut him down and I can't find a reference to anything you have here.  I'm sure he didn't have everything listed but quite a bit of the more common ones.  Is any of this printed in bold in the upper right corner of the label?  That's where the code that we are looking for should be.  The common Mustang 5.0 HO codes are A9L, manual trans;  A9P, AOD; A9M, AOD.  I think the pink printing on the off side of the connector is a Mfg. code.  I have two '93 Explorer boxes and they both have that same code...one says GTE also. 

BB1

I was digging around last night and i found an exact replica of my PCM advertised for sale which is reputed to be out of a 91 T-Bird so that makes a lot of sense.
Thank you very much Bob. I know it worked and I can make it work but for me, I like to know where it came from so that there are none of those awkward questions with vague answers when problems arise.
I have ordered the book that you spoke about. Im sure it will be a very interesting fountain of knowledge.
Regards.

The motor mounts to use are the ones that came on your car.  The 5.0 bolt pattern is same as a 289 or 302 or a 351w.
Use a 1/8” pipe plug to fill the dipstick hole in the block when you change to front sump pan and timing cover.  Parts from your old v8 will bolt right up. But you have to use the 5.0’s harmonic balancer.

Bob,
Do you mind if I ask where you found the Thunderbird wiring diagram and whether you can direct me to it?

No problem if it was a site that I am unable to visit.

If you know that the system you have was running okay, and if you are planning to keep the engine relatively stock, I can't see where you would be any better off going to the Mustang PCM.  Just clean up the harness and make it fit your 66.  If you ever needed to change to an A9L or P the required modifications to the harness would be relatively simple.  And, the A9L/p/m boxes are getting a bit pricey these days.

BB1

Yup...L1C = 91 - 93 T Bird 5.0/Auto.  Just sent you an email with a photo of the diagram.  Should print out on 8.5X11 or "A" or whatever you folks call standard letter size. 

BB1