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99 NP231J XJ to 94 NP231J YJ?


NAXJA Forum User
La Center, WA
I found a transfer case out of a '94 YJ and wanted to put it in my '99 XJ, well, I already put it in... unfortunately, the rear output shaft is a couple inches shorter and so when I went to test it out, it wasn't long before I heard the rear driveshaft fall to the ground. Do I have to take the T-Case back out and swap the output shaft (I really don't want to do that) or is there an extended yoke or something I could use to add about 1-2" of length to the driveshaft?

I also realized that the input shaft is a "short-input" as opposed to the "medium-input" on the AW4 (https://www.novak-adapt.com/knowledge/np-nvg-input-gears/). I am pretty sure the spline count was the same, again, I'm making an assumption because I was too stupid to check before putting it in, but it fit the shaft on the transmission and it worked in 2wd for the short distance I had tested it. Are the spline counts on these the same based on the specs below?

There's also a 4-prong Vaccum switch... Can I just leave this disconnected or do I need a vacuum to engage 4WD? I have no experience with T-cases w/ a vacuum switch.

Are there any other modifications that are necessary to make this swap work?

What I'm driving:
1999 XJ Sport 4.0L w/ AW4 and NP231J T-Case.

Specs for the transfer case I'm swapping out:
Model: NP231J
Assy: 5209-9212
Serial #: 929982C
Ratio: 272
Transmission: AW4

Specs for the transfer case I'm trying to swap in:
Model: NP231J
Assy: 5209-7535
Serial #: 322941
Ratio: 272
Transmission: AX15
The YJ case has the older, "internal" slip yoke (inside an extension on the tail housing), the 99 had the later "external" slip yoke design. The rear driveshaft off an 87-95 XJ with the same rear axle as you already have in the XJ is probably the length you're looking for, unless you want to just bite the bullet and do an SYE while the replacement case is on the bench.

As to the vacuum switch, from what I found with a quick online search, that's just for routing vacuum to the disconnect "motor" on a disconnect-type front axle, something Jeep got rid of in the early 90s. Means that when the case is in Neutral, the outputs aren't tied to the input but are still tied to each other.

You can test this by actuating the shift lever on the case until it's in neutral, then try spinning one output and seeing if the other turns with it.

Really, though, all this means is that if you're going flat-tow the Jeep once the 94 case is in, I'd pull the front shaft and then put the case in neutral.
The other questions, I can't really help with.

Good luck!
X2 on what Rob said about the outputs. Don't worry about the input length. I've had a short input 231 (from an 89 manual) mated up to an 98 AW4 (normally mated to a medium input) with no problem for years. Also, the drive shaft from a manual for the same year should be a bit longer as the AX15 manual is about an inch shorter than the AW4 auto.
Also, I thought the YJ 231 was clocked a bit different, and didn't hang as low on the drivers side? I could be wrong though.
You're correct about the clocking, but the trans made this difference not the tcase.

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