NP231J advice needed

Gojira94

Active Member
Joined
Apr 29, 2022
Location
Clayton, NC
First, bear with me as my MJ project is my first 4WD anything. Plenty of GM platform 2WD builds, swaps, mods over the last 25 years, mainly 4th gen F-bodies and crazy OBS SBC builds/ LS swaps. First time dealing with transfer cases. Novak Conversions gave me a ton of background info on most aspects of what's what.

Here's what I'm doing: take a basket case 87 MJ 2WD/ 4cyl and convert to 4.0 HO/ 4WD, run it off a Renix 429 ECM. TPS adapter accounted for, none of the engine swap is an issue whatsoever. I don't plan on a lift now, but maybe at some point a 2" lift might be something I'd do to accommodate a larger tire than the present 235/75 R15. I have a re-clock jig on the way for the rear half of the trans case to fit everything into an MJ/XJ tunnel.

Donor vehicle for the engine, AX-15, NP231 is a 93 YJ. I also have a 231J from a 98 XJ I bought from a member here a few months ago. The output shafts are different/ rear housings different length. The thought is the shorter the housing/ shaft, the better for driveline angle at the rear pinion. No matter what, I'll have to shorten the driveshaft I have. The further apart the case output and rear pinion yoke are, the better, right?

The XJ case I got off the forum is locked up tight, no idea how bad it is inside. Was supposed to be a 'working pull from a lightly driven 98 XJ.' Paid $125 for it and put it away since it was for much later in the project. more or less complete driveline from the 93 YJ came in July. I was going to use all of it but then I noticed the output end differences and thought it might make more sense to use the shorter output case I bought before. That's when I noticed the one from the XJ is locked up. So... Reading about going to a SYE, "moving the slip yoke to the driveshaft" I don't know what that means or looks like at the driveshaft. Do I even need a SYE? I'm clear on how to install one, just no sure what needs to be done to the driveshaft to accommodate a SYE. The rear needs to move up/ down (Y axis) but without some sort of slip yoke somewhere it would be bound to a fixed arc of travel, which is a no-go, of course.

My gut says either use the YJ case as-is, since it's definitely shorter overall or put a SYE in it and find out what needs to be done to the driveshaft to run it. And use the 98 XJ case for a rebuild/ spare.

Pics:

93 YJ case:

IMG_5752.jpg


98 XJ case:

IMG_5751.jpg
 
If you go sye you'll need a complete new driveshaft, that is 2 pieces splined together to allow it to compress and stretch. A mild lift MJ should not NEED an sye because it's a fairly long wheelbase. Much more critical in YJ/TJ length.
 
LBarr2002 said:
If you go sye you'll need a complete new driveshaft, that is 2 pieces splined together to allow it to compress and stretch. A mild lift MJ should not NEED an sye because it's a fairly long wheelbase. Much more critical in YJ/TJ length.
Click to expand...
Awesome, thanks for the info. I was looking at a double cardan shaft should it be needed. Sounds like I’ll have plenty length available in the stock driveshaft with the YJ case. And I can have fun rebuilding the other one if it’s not beyond repair. Could be as simple as a seized front output shaft bearing.
 
When my son and I built and converted the mj we used the hesco TPS adapter. The 231 is a very simple case and not much to lock them up. Does the front output turn but not the input? Long drive shafts like to get caught on rocks. There are some changed in the 231 too. Long input short input and of course 21 and 23 spline. If i were me I'd run the 93 case with a SYE.
 
ghost said:
When my son and I built and converted the mj we used the hesco TPS adapter. The 231 is a very simple case and not much to lock them up. Does the front output turn but not the input? Long drive shafts like to get caught on rocks. There are some changed in the 231 too. Long input short input and of course 21 and 23 spline. If i were me I'd run the 93 case with a SYE.
Click to expand...
I'm thinking this TPS adapter: StackPath

Nothing turns on the 98 XJ case. Only thing I can really grab is the front output yoke. Tighter than Dick's hatband...
 
Gojira94 said:
I'm thinking this TPS adapter: StackPath

Nothing turns on the 98 XJ case. Only thing I can really grab is the front output yoke. Tighter than Dick's hatband...
Click to expand...
That is better in price than the one we used. TPS Adapter #640177 | Hesco

Will the shift forks move at all? If they don't move then something is major wrong inside I think. In 2wd ther front output should move freely.
 
ghost said:
That is better in price than the one we used. TPS Adapter #640177 | Hesco

Will the shift forks move at all? If they don't move then something is major wrong inside I think. In 2wd ther front output should move freely.
Click to expand...
Won't shift, though the shift arm will move against the detent spring just a hair. That's it. I suppose I don't have much alternative but to crack it open and see what's what. I'll post up carnage pics, lol.
 
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