hakukamana Posted July 21, 2013 Share Posted July 21, 2013 Brand new to the forum, and just picked up a new Jeep MJ project. Only problem is I had to tow it home. 90 MJ, broken NP231, it was removed and the case was disassembled kind of. Driveshafts were on the ground and it had not been started in 5 years. Got it started, I have a NP231 from a 99' TJ wrangler, and I was looking at the old case which I think came out of the MJ but am not sure. It has the short shaft. The NP231 from the TJ is a long shaft, the new case is in cherry condition. I am no stranger to Jeep's, YJ's are in my blood but I just had to have a MJ. I really need to know short shaft NP231 for the AX15 in the MJ or Long shaft? Both are 23 spline. Link to comment Share on other sites More sharing options...
JeepcoMJ Posted July 21, 2013 Share Posted July 21, 2013 IDK how your 90 xj is 23 spline if it's automatic...only if it's a 5 speed with ax15. They will directly swap as far as bolting up if they truly are same spline count. Whether they seal up on the back of the trans depends on whether they are both the same length shaft or not. sometimes that doesn't even matter. you will have to eliminate the vacuum hoses going to the central axle disconnect on the axle, and shim it over, or posi-lock it with a cable. Link to comment Share on other sites More sharing options...
skidoo_j Posted July 21, 2013 Share Posted July 21, 2013 I just installed a '97 np231 and it had a medium input shaft length. The OD's were the same and there was plenty of space in the trans to swallow the extra 1/2". I did replace the seal on the transmission just incase there were slight difference in the running areas of the seals. Link to comment Share on other sites More sharing options...
hakukamana Posted July 21, 2013 Author Share Posted July 21, 2013 The 90' has a AX15 in it. The NP231 shaft is longer than the case I found in the back of the truck. The spline counts are 23, it is more of the issue of input shaft length. The newer 231's have an electric shift switch, that can be changed out to the Command Trac vacumn switch and the existing harness will plug directly into that. The new tranny seal is a good idea just to make sure of the sealing on the shaft. Link to comment Share on other sites More sharing options...
skidoo_j Posted July 25, 2013 Share Posted July 25, 2013 I'd keep the electric switch and get rid of the vacuum disconnect, either convert to a single passenger, lock the collar, or make it a posi-loc with a cable. Wire up the switch to your dash. Link to comment Share on other sites More sharing options...
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