88 MJ Renix swap to 1991 XJ HO4.0L Qs

I know this topic has prob been beaten to death, but I wanted some honest advice...

Here's my situation.
I want to upgrade my 88 MJs driveline, to a HO4.0L especially because I need a 23 spline output AW-4 In order to upgrade to a Dana 300.

I'd like to basically gut my entire MJ rig & start fresh....

I have a 1991 XJ for parts, with 180,000k on it.

Basically what I'm thinking is rebuilding the 91 HO 4.0L, reusing the 23 spline output 91 tranny, and rebuilding & flipping a Dana 300.
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As far as my mechanical skills, I built a 1 ton MJ buggy from scratch, I can do pretty much anything chassis/suspension/axle-wise with no issues.... I can follow directions and rebuild an engine & transfer case... No worries.

BUT electrical shit scares the hell out of me.
So... Pulling the HO 4.0L from the '91, along with the wiring harness & computer...
THEN installing it into my 88 is a very daunting task to me.

What route should I go here? Is this something that is even doable by a engine-swapping first timer? Is it as complicated as it seems?

I'm willing to read & do as much research as need be, I'm just worried I would commit to this and get everything in and it not run or there are electrical bugs I cannot figure out.

I'm assuming the more time I spend carefully labeling wires when I pull the '91...
the easier it'll make this...

Or when it comes down to it... would it be better to go bigger and upgrade to an EFI V8 and heavier duty tranny since i intend on gutting the whole thing anyway?

i did it on my 90 xj, the donor was a 94. the easiest way to do it is to use everthing from the donor.

Use the complete wiring harness, dash, fuel system, evap and fuel tank if possible. I don't now the differences between mj's and xj's but it should be easy but time consuming.

Interior harnesses are different from XJ to MJ. You either have to replace the rear body connector with the correct MJ HO one or make an adapter for the MJ to XJ wiring.

I'm gutting the entire MJ and using everything from the XJ

I didn't plan on having to do the dash too though

The XJ dash harness will not directly connect to the MJ rear harness. You will need the XJ dash harness to make the gauges work. RENIX an HO gauges use different pot values. Good luck finding a fuel sender for a HO era MJ. It took me almost 6 months to find mine. I have done this swap only from a 86 2.5 to a 93 5.2 V8. Either way it is basically the same swap from a RENIX to a HO.

why not just use the 23spl tranny in your MJ? .is there a reason you could not do that?91 is still cable spedo right? . seems like a lot of work to just get the tranny?

My engine runs hot. I think someone rebuilt it and the timing is somehow advanced.
It's the only explanation.... I've tried everything.

I want a new engine.

I don't care about the OEM guages... I'll put aftermarket ones on.
It's a woods buggy.

Maybe I'll build a V8 and do propane. This sounds expensive....

I put a '93 XJ harness onto a '92 engine and into a '90 MJ.

It really wasn't that bad. I stripped the whole '93 harness down (it had power everything and all the extra shit I didn't want) and got the engine/interior side all done.

I didnt put in a dome light, didn't use the door jamb switches, and didn't spend time mating up the brake light switch. Sounds like you're in the same boat.

The only thing you tap into the rear for at that point is the fuel pump. You can easily add in your own taillights to the brake light switch.

Everything else would just be using the XJ harness/parts (gauge cluster, etc.). It's not that daunting of a task trust me. Do yourself a favor and buy an FSM for the year harness you're getting.

After all that shit I scrapped it and dumped in a big block :laugh:

Just curious, why didn't you just use the 92 harness?

What do you do with all the wires you aren't using? Do you remove them all together?
As far as the new H.O harness, Id like to keep the bare essentials only... To simplify the process as much as possible.

Speedo doesn't matter, I'll prob run aftermarket guages grafted into the dash. Besides the guages, all I need to function is the Headlights, blinkers controls, cigarette lighter/power point to plug in an Audiopipe self enclosed sound system. I don't have any interior lights, door dingers, heat/AC controls, speakers or anything like this. It's stripped bare.

In the rear, all I have is a Jeep YJ fuel pump in my fuel cell & two 3 phase round LED taillights. This should simplify things

I plan on finishing this build this summer, wheel it this year... And next summer. If I can't solve the overheating issues, OR of I blow out the AW-4 or 231... I am going to go through with the HO engine swap along with a 23 spline AW-4 & Dana 300.

I only had the '93 harness available to me.

Just get an FSM and go through it if you have any electrical knowledge. It's real easy to follow/understand.

You basically just pull wires out of splices, pull terminals out of connectors, etc. Very simple, just time consuming and make sure you label stuff so you don't get confused.

Since you're there. Toss a GM LQxx in there..

Im currently doing the same swap with an 87 MJ, i also have a 91 as a donor. i thought it was going to a little easier than it is but after reading a little i think i can manage it. im just not looking forward to stripping both these trucks down.

I'm doing a similar build but with a 95 XJ to 89 XJ Currently the entire airbag system is removed on the 95 as it was a reconstructed title. I also have wiring issues with the 95 but I believe if I pull the entire harness I could possibly find melted wires somewhere. I think I will also have to use a relay as the 89 has power windows and my 95 does not, then tap into the fuse box for an activation feed but really the main feed would be coming off the battery. I'm worried though that my ignition harness will be completely different which will cause complications I'm sure.

Mines a buggy pretty much. All I have in back is the brake lights & fuel pump.
I have no doors, no dome, no radio, no AC/heat, no guages (except aftermarket Temp & Oil Press guages). Mines stripped down...
I want to strip out all Renix & all the wiring out of my 88 MJ
and start fresh.

I basically want to pull the H.O. out of my '91 parts XJ,
Rebuild it completely. Then strip down the 91 harness removing everything I don't need.
Then use everything from the 91, transferring everything into my MJ.

Im a great mechanic, fabricator, suspension guy...
I just have never done anything like this before, and it makes me nervous like I'm getting in over my head.

Honestly it's not hard, it's just time consuming and you need patience.

I'd plan on two full weekends worth. It took me a day to remove the wiring harnesses from both vehicles (donor and MJ), and about two full days to go through everything (both 14 hour days).

You can pretty much junk the rear half of the harness at the black connector by the drivers side kick panel (just save the other side of the connector). Since all you have are brake lights and fuel pump, you can just tap into those pins and run them back.

There's not a whole lot that gets stripped out of the engine side harness. Just things like horn, brake light switch, etc. if you don't want them.

Make sure to ground out the NSS as well while you have the whole harness apart.

The steering column is different also, so swap those out otherwise non off the ignition or switches will mate up to the HO dash harness.

96 system into an 87 body here.

As long as you swap everything in, you shouldnt have any problems.

I did a similar thing to my '89 XJ, picked up a donor '98 and swapped everything! and I mean everything, took a week and a half working everyday to swap everything over, including rain delays working outside. Like prev posts, just take your time. That was only my second engine swap in my life, and first time I swapped harnesses and dashboards before, not as hard as it looks.