MJ 2wd to 4x4 conversion

[lcoutback]

I'm a new member and really like this site.  Great job!

 

Thanks to a knuckle head driver, I totalled my '91 XJ last week.  4.0 HO / 5 spd AX-15 and lift components are all fine, thanks to the Smittybilt that was up front. Dana(s) 30 and 35 are solid.

 

I've always preferred the MJ, and locally can find pretty clean 2 wd models in nice condition.  I know this is a broad question, but how big a project would I be taking on to convert a 2wd MJ to 4wd?  Specifically, I've found an '87 with the 2.8 v6 / automatic, and several '86 four cylinders with automatic.  Are this significant wiring / harness modifications? Are the straight axle 2 wd front ends designed to accommodate the same control arms as 4x4?

 

On a scale of 1 - 10 I consider myself about an eight as mechanic and six as a fabricator. I might score a 3 in chasing electronics and wiring, but mostly because I don't enjoy it. LOL  I have the space and tools to do a little more than just back-yard mechanic but don't know how big a job I feel like taking on.

 

Am I taking on too much at once?

 

 

[Green Mesa XJ]

I think 86 was the only MJ year with the 2.8 v6 which most agree is a underwhelming motor.

 

Do you still have the 91 for a parts source?

A lot of people seem interested in converting 2wd to 4wd, it's do able and there are plenty of parts around for it but if your in the market it might be better to hunt around for what you want to start with rather than try to built it up, unless it's the kind of project you are looking for.

 

For a 2.8 v6 or 4cyl 4wd conversion you'll need a ax5 and np231 then a front Axel with a ratio to match the rear plus a ton of little things nobody thinks about until you are almost finished. Easiest to do if you've got the 91 or another 4wd donor.

 

 

 

 

Sent from my iPhone using Tapatalk Pro

[Pete M]

all depends on how much of a project you want.  all of the front suspension components interchange.  rear leafs are different though.  everything pretty much bolts in, including the engine and wiring and dash and whatnot.  please note that the 2.5L/2.8L engines won't just bolt to a 4.0L transmission.  in a perfect world, I would find a solid 2.5L 87+ MJ and gut it for all of the components of the XJ.  

 

 

also, welcome to the addiction  :thumbsup:

[desbennett004]

The 4x4 conversion is straight forward, it's pretty much bolt in. I would be looking for a 4.0l Comanche though, that would save you an engine conversion as well. Even if you wanted to swap your engine in at a later date, it would make all the wiring and harness work heaps easier.

 

Sent from my SM-N900W8 using Tapatalk

[ARareBreed]

I would put ALLLLL the guts and wiring from your totaled 91xj into the Comanche. The Renix style Comanche is unfavorable compared to 91+. To simply answer your question, the 4x4 swap will be a plug and play from the transfer case forward. Although the leafs are different widths apart from XJ to MJ (which is only concerning if you are doing a rear end swap), you'll be fine for what you'll need. No need to put the XJ rear end into the MJ.

[lcoutback]

Thanks for all the great feedback! (this is why I LOVE a good forum like this)

 

I do still have the '91 XJ.  New twist...

 

I just found a very clean, untouched '87 4x4 MJ with 4.0 / 5 sp.  The catch is the guy says it 'has a knock' and he's finding oil in the air filter.  I'm guessing the oil is just blow-by, but the knock might be a bigger issue. (not sure he is defining a 'knock' or a 'tic') 

 

Are there significant differences between an '87 MJ 4.0 and a '91 XJ 4.0?  I'm going to do additional research, but if it's a simple engine swap and a fresh clutch to have a driver, score!

 

- Bob

[ARareBreed]

Thanks for all the great feedback! (this is why I LOVE a good forum like this)

 

I do still have the '91 XJ.  New twist...

 

I just found a very clean, untouched '87 4x4 MJ with 4.0 / 5 sp.  The catch is the guy says it 'has a knock' and he's finding oil in the air filter.  I'm guessing the oil is just blow-by, but the knock might be a bigger issue. (not sure he is defining a 'knock' or a 'tic') 

 

Are there significant differences between an '87 MJ 4.0 and a '91 XJ 4.0?  I'm going to do additional research, but if it's a simple engine swap and a fresh clutch to have a driver, score!

 

- Bob

I know the Renix 4.0 which is 1987-1990 is significantly different from the non-renix 1991-1992 4.0 high output engines. As for mounting, wiring and whatnot I am not too sure (MAYBE SOMEONE CAN CHIME ON HERE). Don't get anything other than the 4.0 (even if it is Renix), the v6 engines and 2.5l i4 are $#!& power for the Comanche.

[Green Mesa XJ]

The 87 I likely to have that crappy Peugeot transmission ax15 didn't come along until 89-90 I think.

 

On the renix just swap the ecu for a 89-90 for improved performance. The separation between a good driving renix and ho 4.0 is very thin. If this MJ 4.0 is on its way out get it cheap and swap all the 91 ho over with wiring. Better transmission for sure.

 

 

 

Sent from my iPhone using Tapatalk Pro

[Airborne Janitor]

Thanks for all the great feedback! (this is why I LOVE a good forum like this)

 

I do still have the '91 XJ.  New twist...

 

I just found a very clean, untouched '87 4x4 MJ with 4.0 / 5 sp.  The catch is the guy says it 'has a knock' and he's finding oil in the air filter.  I'm guessing the oil is just blow-by, but the knock might be a bigger issue. (not sure he is defining a 'knock' or a 'tic') 

 

Are there significant differences between an '87 MJ 4.0 and a '91 XJ 4.0?  I'm going to do additional research, but if it's a simple engine swap and a fresh clutch to have a driver, score!

 

- Bob

 

A lot of 4.0L motors are susceptible to blowby, and there's a lot of info out there on how to combat it. Not a big deal. As you said yourself the "knock" may be a bigger issue. If you can get the 87 MJ running well I would just swap your front suspension over from your 91 XJ.

 

As far as differences, the big one is the motor. Although they're both 4.0L they have different systems. 87-90 is often referred to as Renix 4.0L, and 91+ are High Output. When you pop the hood you'll see many similarities, but also many differences. Renix motors have a different electrical system and there is no PDC, just a starter relay with many systems plugged into it. Manifolds are different and emissions are somewhat different. Renix motors have a lot more vacuum lines to manage throughout the motor. You'll find the aftermarket is greater for HO motors than Renix motors. Some feel the Renix motors have more torque and reach higher ranges at lower RPMs. Computers are different as well, it's completely different system.

 

Front axle will be different, the 87 MJ will have a 2-piece disconnect passenger axle shaft that is vacuum controlled and the HO years do not.

 

I believe HO years use an electronic speedometer and Renix and prior have mechanical speedo cables.

 

I know there's more I'm sure others will chime in.

[ARareBreed]

I believe HO years use an electronic speedometer and Renix and prior have mechanical speedo cables.

 

This is true.

[Warrior Spit]

I'm currently doing a 2wd to 4wd conversion on my MJ. I had planned on replacing the rear axle, but after doing some reading here, it looks like I may not have to swap out my rear axle????? How do I know if I need to or not?

 

 

Sent from my iPad using Tapatalk

[Airborne Janitor]

I'm currently doing a 2wd to 4wd conversion on my MJ. I had planned on replacing the rear axle, but after doing some reading here, it looks like I may not have to swap out my rear axle????? How do I know if I need to or not?

 

 

Sent from my iPad using Tapatalk

 

You can reuse your rear axle but you will need a new rear driveshaft to attach to your swapped in transfercase.

[Pete M]

I'm currently doing a 2wd to 4wd conversion on my MJ. I had planned on replacing the rear axle, but after doing some reading here, it looks like I may not have to swap out my rear axle????? How do I know if I need to or not?

 

 

Sent from my iPad using Tapatalk

 

replace your axle if:

 

-you need a stronger axle

or

-the gear ratio doesn't match the front.

[Warrior Spit]

 

I'm currently doing a 2wd to 4wd conversion on my MJ. I had planned on replacing the rear axle, but after doing some reading here, it looks like I may not have to swap out my rear axle????? How do I know if I need to or not?

 

 

Sent from my iPad using Tapatalk

replace your axle if:

 

-you need a stronger axle

or

-the gear ratio doesn't match the front.

Here we go.......that's exactly what I wanted to hear. I say that because I have heard and read about this gear ratio stuff but I have never had to deal with it. As a precaution, I got both axles out of the same truck. Should I just continue as planned and put the new rear in? Or can I be lazy and keep the old one there?..........what about my gear ratios? How do identify what it is? Sorry if this is a dumb question, this is my first time this deep and I don't want my project to be waberjawed.

 

 

Sent from my iPad using Tapatalk

[Green Mesa XJ]

Should be a metal tag on the rear cover. Normally see the tag bolted on the axel cover. If it's missing you can count the teeth on the gears, I'm not sure about the exact process .

 

I'd just try to match the new front axel to the existing rears ratio.

 

 

Sent from my iPhone using Tapatalk Pro

[Pete M]

you have to determine all the ratios before you can make a good decision. :thumbsup:  the new axle's ratios could be better! :)  (or worse :( )

 

besides, it's a great idea to pop the covers and inspect the insides before using a used axle.  :yes:

[Shoopaman]

I am thingking about doing the same to my MJ. Does anyone know if the driveshaft to the rear axle is a direct swap from a cherokee to the long bed MJ? I think the frame is a bit longer for the longbed mjs than the xjs.

[Pete M]

the wheelbase of a shortbed MJ is a foot longer than an XJ.  longbed is another 6".  

[Shoopaman]

Sooo custom driveshaft or have the original cut down? Professionally ofcourse.

 

[DirtyComanche]

2 hours ago, Shoopaman said:

Sooo custom driveshaft or have the original cut down? Professionally ofcourse.

 

 

Most (all?) 2wd driveshafts have a rubber elastomeric coupler in them to dampen vibration, and they do not lend themselves to modification.

 

Just have shaft built by a reputable shop.  Tom Woods, JE Reel, High Angle Driveline, etc.  And if you want a SYE, now is the time to do it (I no longer will partake in debates on this subject, I have one personally).

[omega_rugal]

you XJ was 4.0+AX15 so you have factory 3.07 gears

 

automatics with either engine are 3.55, 4bangers may even be 4.1 so you need a 4.0+manual MJ to do the swap unless you can swap the rear axle too...

[HOrnbrod]

You can't be sure of the gear ratios by taking as gospel what they should be from the factory. These trucks are all 25+ years old and could have had multiple gear changes or non-original axles. Always pop the cover and look at the R&P gear stampings to be sure.

 

9 hours ago, DirtyComanche said:

Most (all?) 2wd driveshafts have a rubber elastomeric coupler in them to dampen vibration, and they do not lend themselves to modification.

 

Elastomeric couple? I call it a rubber sheath.    In any event Dirty is correct. Don't try to have them shortened. It may or may not work, and most driveshaft shops won't guarantee them as the rubber tends to break down over time and cause vibes. It's best to have a new one made.

 

[Minuit]

Rubber sheath? I call it an annoying pain in the @$$ that turns your truck into a paint shaker after a while.

[carnuck]

Rubber sheath? I call it an annoying pain in the @$$ that turns your truck into a paint shaker after a while.

Driveshaft shop here in Seattle called it a tube inside a tube with a condom between and the petroleum based lube destroys the rubber over time. (However accidentally it gets lubed, time and wear affect it too )


Sent from my iPhone using Tapatalk

[Minuit]

When I had my new driveshaft built, the guy said in 14 years of business he's never seen one of those vulcanized shafts balance properly. Don't really know what they were thinking. Maybe when the shaft was brand new it reduced NVH slightly? Only thing I can think of.