Ive got an 88 sport with a motor swap. 305 chevy small block and ax15 trans. I blew up the transfer case and the number plate disappeared. What I believe i need is an np231j with limited slip yoke installed. Looking through parts online I’ve found there is a short and long input shaft. How do i tell with one i need?
NP231J Transfer Case
- Thread starter DBobCrane
- Start date
You need an NP231J transfer case with a slip yoke eliminator. Or you can buy a transfer case without one, and then buy the SYE kit and add it yourself. It’s not hard to install.
When people say “input shaft,” they are incorrect. They mean the input gear, and those have different stickout lengths as they go into the seal on the back of the transmission. So stick out length is based on transmission. An automatic places the seal further forward, so you need the long input gear to reach to the seal. A manual almost always needs a short input gear. And by “need”, I mean that is all that is required. A long input gear can be installed onto the AX15 transmission, it’s just longer than necessary and the seal will still ride close to the transfer case as if the gear were a short one.
for some unknown reason, Jeep shipped 98-99 TJs with AX15s using long input gears. Not because they needed to, but they just did. No clue why. YJs and 97 TJs all shipped with short input gears on AX15s.
Next up is input gear spline count. AX15 needs a 23 spline input gear. So in your case you need a 23 spline short gear, but you could do a 23 spline long if you needed to.
Worst case, take the gear out of your current transfer case and install it into the new one if you can’t find a case being sold with the proper gear. Only downside with that is its more work to disassemble the transfer case that far, and you’re basically doing a whole rebuild at that point. No big deal though.
also keep in mind, an old style input gear (1994 and prior) will not work in a 95+ transfer case, and vice versa. So try and find a guaranteed YJ case if you can. Otherwise if you get a TJ case, that’s fine, but if the input gear were wrong, you’d need to buy a 95+ input gear (short, 23 spline, long in a pinch) that would work in the 95+ YJ or TJ transfer cass. It has to do with gear cut, the input gear’s teeth were cut differently in 1995 and the transfer case and planetary assembly teeth were modified to match that.
The year designations of 1994 vs 1995 are not dead set either. Some 94s got the newer style. My 94 was built in oct 1993 and it had the old style. Some ways you can tell are caged needle bearings on the output shaft, and also you can tell by the bearing style on the input gear. A narrow bearing is later, wide bearing is older style. You can figure out what bearing you have by taking off the 4-bolt front cover so you can see the input gear bearing.
Good luck, post up if you have questions. If you need more detailed help you can pm me and I’ll try to walk you through it to get the details sorted. I recently learned all the ins and outs because I had to figure out if my 94 was new or old style before I bought parts for it.
When people say “input shaft,” they are incorrect. They mean the input gear, and those have different stickout lengths as they go into the seal on the back of the transmission. So stick out length is based on transmission. An automatic places the seal further forward, so you need the long input gear to reach to the seal. A manual almost always needs a short input gear. And by “need”, I mean that is all that is required. A long input gear can be installed onto the AX15 transmission, it’s just longer than necessary and the seal will still ride close to the transfer case as if the gear were a short one.
for some unknown reason, Jeep shipped 98-99 TJs with AX15s using long input gears. Not because they needed to, but they just did. No clue why. YJs and 97 TJs all shipped with short input gears on AX15s.
Next up is input gear spline count. AX15 needs a 23 spline input gear. So in your case you need a 23 spline short gear, but you could do a 23 spline long if you needed to.
Worst case, take the gear out of your current transfer case and install it into the new one if you can’t find a case being sold with the proper gear. Only downside with that is its more work to disassemble the transfer case that far, and you’re basically doing a whole rebuild at that point. No big deal though.
also keep in mind, an old style input gear (1994 and prior) will not work in a 95+ transfer case, and vice versa. So try and find a guaranteed YJ case if you can. Otherwise if you get a TJ case, that’s fine, but if the input gear were wrong, you’d need to buy a 95+ input gear (short, 23 spline, long in a pinch) that would work in the 95+ YJ or TJ transfer cass. It has to do with gear cut, the input gear’s teeth were cut differently in 1995 and the transfer case and planetary assembly teeth were modified to match that.
The year designations of 1994 vs 1995 are not dead set either. Some 94s got the newer style. My 94 was built in oct 1993 and it had the old style. Some ways you can tell are caged needle bearings on the output shaft, and also you can tell by the bearing style on the input gear. A narrow bearing is later, wide bearing is older style. You can figure out what bearing you have by taking off the 4-bolt front cover so you can see the input gear bearing.
Good luck, post up if you have questions. If you need more detailed help you can pm me and I’ll try to walk you through it to get the details sorted. I recently learned all the ins and outs because I had to figure out if my 94 was new or old style before I bought parts for it.
Wow, just wow...hows that for details. 
Some of you guys are incredible.
Some of you guys are incredible.Much appreciated. So essentially, if I find an np231j that is 23 spline short it will work in my yj no matter what year, and if it doesn’t have the slip yoke i can just buy the kit? Would the one on my current case(destroyed but not the slip yoke) work with any of the years, or would this part be year specific as well?
And again thank you for getting me this far.
Pretty much yeah. SYE kits are not year specific, they fit everything. The install is a bit different depending on new vs old but it’s just a matter of hammering out some quick caged needle bearings out of the old sprockets before you move them to the new SYE shaft. Easy with a screwdriver and hammer and about 3 minutes of time.
also I forgot to mention, TJs use a double cardan front driveshaft, similar to what you have in the rear. So you would need to remove that and install your yoke or a new one before you put that transfer case in your YJ, assuming you got one from a TJ.
and if you get one from a TJ, you also need to unscrew the vacuum switch out of your YJ case and move it to the TJ case. Unless you no longer have the vacuum operated CAD.
If Macho does nothing else, he does do his homework!Wow, just wow...hows that for details.
Gotsta give him credit for that.
True that.Both of you and a few others know this stuff inside and out through trial and error. That's what it really takes Id say and several of you have been there done that
You and I are bit older and we've both got the miles to prove it!
When I was younger, Friday mornings I could almost always be found grazing at my local Junk-Yard.
Looking for the best parts from whatever to adapt to the Jeep, and God usually let me trip over them!
Man, I miss that...These days it's just not the same.
Ha...always wanted my own junk yard. Figured I could create some pretty interesting things. Like you I used to wander around looking and picking thru stuff. I just wished some of those vehicles could talk and tell me their story. Man, would that be something.
Not to hijack Gilaguys thread, but I have a question. In his pics of the fragged t-case you can see his speedo cable go into the t-case with a big 1” nut holding it to the t-case. Mines the same on my NP231J t-case. But lots of pics I’ve seen show an electrical connection/plug in that same location with two bolts holding the plug to the case. Why the difference? I’ve not seen many t-cases with the 1“ nut going into it like mine and his.
1987-1990 were all cable only. 1991 was a hybrid cable setup with a two wire plug to send speed feedback back to the PCM so it knows how fast you’re going. 1992-1995 is a three wire all-electric sensor with an all electric Speedo gauge.
They aren’t all interchangeable but not because of that. You can remove all that stuff and drop in the full sensor from a later year and vice versa. What isn’t interchangeable are things like the spline counts in the input. But all of that can be changed by opening up the case and swapping parts.
So I finally got a new t case made installed it yesterday. After putting everything back together, neither shaft will engage the axles. Rear wont work in 2 and 4 wheel and no 4 wheel at all. When rolling the jeep forward or backwards the shafts will spin. Any input would be greatly appreciated.