I have a 1987 with a 1993 4.0 and I did keep my original dash...
As you've just discovered bolting the engine in and hooking up the big stuff was the easy part
This is mostly to do with the very simple fact that the 4.0
was designed to go where a 2.9 had been.
However the wiring is completely different. particularly across the line
from a Gen1 to a Gen2
First You will need a relatively complete DONOR harness from a 1990-92 Ranger with a 4.0 engine
Start by removing the computer from the passenger side kick panel and shiving the connector out through the hole in the firewall (unseating the giant oval grommet can be "fun".)
Now install the 1991 harness on that hole and working with a razor blade or stanley knife start splitting the two harnesses apart up by where the wire "breaks out" for the passenger side marker light
You will need to do this to BOTH harnesses
work your way around and across the engine bay
until you get to the round firewall gromet where your 1986 harness goes inside the cab on the drivers side.
STOP THERE, you don't actually need to disturb
any of those smaller connectors, cut them off on the engine bay side of the connectors and splice them to wires that you will cut out of the round connector from the 4.0 harness.
there is one wire you will need to ADD and that is the one for the check engine light (when you get to that point I'll discuss it seperately)
There is no way to run a 4.0 off of an '86-88 harness
as it does not include the EDIS module for the ignition system (you'll need to mount this module to the radiator support near the battery)
The advice you have to get BOTH an '86 and a 1991 EVTM (or EVTM to match whatever 4.0 harness you end up with) but my strong advice is to stick to a 1990-92 harness and preferably one from a ranger.
IF you do not have a harness from the engine donor you are basically screwed at this point.... until you get a 4.0 harness.
To be honest I'm a trained electronics technician, used to building
one-off prototypes. and I was trained in the production side of
the aircraft and medical equipment industries.
Yet, with that training it still took WEEKS to get my truck wired
to the point where I was willing to start installing fuses and
powering the various systems one-by-one
What my training and experience allowed me to do was to get it all working essentially on the first try, I had ONE ground wire that was loose and not makign contact and that resulted in the fuel pump relay not turning on.
I still have one "glitch" my A/C systems WOT cutout relay doesn't operate correctly, oh it'll turn the A/C off, the problem is that it never turns it back on again, there is a power feedback somewhere in the circuit that creates
an output feedback to the relay coil that I've simply been too lazy to track down and eliminate. I havea jumper where the relay belongs and so if I want my A/C off when accelerating I must manually turn it off. BFD...
But even WITH another harness there is GOING to be splicing involved, quite a bit of splicing.
I need to phrase it strongly:
Without another harness from a 4.0 vehicle you might as
well quit now. (Call a wrecker and scrap it)
I'm not saying you can't make it work without a proper harness
I am saying there are connectors and components you simply don't have and they are things you cannot do without...
I'm saying that I wouldn't even TRY without another harness.
On my own project I had two pieces of stockade fence, and I used them to "hang" two harnesses that I was not using, one "un-cut" harness from another '87-88 Ranger and an uncut harness from another '93 Explorer.
So if I had a question related to missing to note something or couldn't identify something I had a "reference" harness for each.
The WIRING job took me 3 weeks.
I'm not talking part time, I'm talking 20-odd days where from sunrise to sunset in August I spend every day with the manuals my notes, testlamp, DMM, cutters, strippers, crimpers and a heat gun (I made all most of my splices with the shrinkable butt splices) and soldering iron....
And I'm not counting the days it rained (before I was ready to do wiring inside the cab) or days that I spent dong something else
(even if only throwing rocks at hornets nests to keep from going INSANE!)
But please note I was using a 1993 EXPLORER harness and
I was integrating that power distribution, a new audio system, power feed to communications radios, retrofitting cruise control AND adding dual fuel tanks to the truck... AND converting from 2wd to 4x4....
IT was not just another f'ing engine swap.
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