So you’re sick of that POS 249 and want a real t-case? Here’s how to do it. First, I’ll run through some of the reasons why people swap in a 231.


The 93-95 249’s still rely on the viscous coupler to transfer power while in 4Lo. This presents a major problem off road as the front and rear driveshafts are NOT locked together. This issue is most apparent when attempting to climb a steep obstacle. With the weight off of the front tires, a majority of the power is sent to the front because they are easier to turn, but with no weight on the tires, you get no traction up front!

Viscous Coupler Failures

The Viscous Coupler in all 249’s is prone to failing around the 75,000 mile mark. An easy test for the VC is to do several figure 8 maneuvers in a parking lot after driving the Jeep for 30 minutes or so at highway speeds. If the vehicle can idle through the maneuver, chances are the VC is okay. If it binds or needs additional power, your VC is likely on its way out. A VC from the stealer-ship will run you about $800, so it is usually MUCH cheaper to swap in a 231 or 242.


At extreme lift heights, the rear driveshaft can cause vibrations. Currently there is no slip-yoke eliminator for the 249, and probably will never be one. An SYE creates a fixed rear output shaft on the t-case, much like the front output. This strengthens the rear output, as well as lengthens the rear driveshaft, lessening the angles on the joints, which can usually eliminate the vibrations.

Now, onto the swap itself:

Where to get your 231:

Most 231’s will work, but may not necessarily bolt in. Ideally you will find one with a 23 spline input shaft that matches the length of your current input shaft. There’s no for sure way that I know of to determine the length of your input shaft without pulling the t-case and measuring it. You can find 231’s in YJ’s, XJs, and TJs, but they are pretty rare to come stock in ZJs. On the YJ’s, they use a different front output yoke that will not work with your driveshaft. You can find the proper output yoke at most tranny shops.

Try to get a t-case out of a vehicle the same year as yours, or at least close to it.

So now I have my 231, what do I do with it?

Start by taking out your old 249. I find it easiest to drive the ZJ up onto 4 ramps when doing t-case work, or putting it on a lift. Be sure you are in 4WD if you pull up onto the plastic ramps though. It’s very hard to push the front tires up onto the ramps and you don’t want to shoot the ramps out from under the rear tires. Don’t ask how I know this!

It helps to have some people helping, as the t-case is pretty heavy and not fun to maneuver by yourself. So raise the vehicle and secure it in place with the e-brake (putting it in park will obviously not work as you are removing the drivetrain!) and other methods if you so desire.

  • remove front and rear driveshafts (fluid will probably leak out the rear output when your pull the rear shaft, so you may want a drip pan)
  • place a jack under the tranny pan using a block of wood or some other method to keep from damaging the pan.
  • Disconnect all hoses, linkage, and other fittings on the t-case
  • Drain the fluid of the 249 (30mm socket or wrench to open drain plug)
  • Remove the 4 bolts holding the crossmember to the frame rails (be sure your jack is holding up the tranny and t-case or things will get very bad VERY quickly)
  • SLOWLY lower the jack under the tranny pan until you can access all 6 nuts holding the t-case to the tranny. These should be 9/16” or their metric equivalent. They are a MAJOR PITA to get to, so be patient. Leave a couple bolts on the mounting studs to hold the t-case on until you’re ready to yank it. Raise the jack back up once you have the upper bolts removed
  • Using a tranny jack or pure muscle, now remove the t-case from the tranny. You will need to slide the t-case back, and then it will come down. Tranny fluid will probably leak out a little bit, so again a drip pan is a good idea.

NOTE – on some models the tranny mount may be in the way of some of the nuts holding the t-case on. In this instance, remove the crossmember from the tranny and set it aside.

So now you have the 249 out of the vehicle, how do I get all this stuff back together??

Now that the case is out of the vehicle, you can examine the input gears to make sure they are the same length. If they’re different, don’t fret too much. This is where a Factory Service Manual is worth its weight in gold. You’ll need to tear down both cases and take the input gear from your 249 and put it in the 231. It will work, assuming that the gear cut is the same. There was a change in gear cuts around 94-95, and the gear cut MUST match what the case originally had in it, or again, very bad things will happen very quickly. The difference in gear cuts is easily identifiable simply by looking at the two input gears. If the cut of the gears looks different at all, time to look for a new input gear. I’d avoid the dealer, as you’ll pay more for the input gear than you probably paid for the whole t-case. I found one at the same tranny shop that had the correct output yoke. It ran about $150, which still isn’t cheap, but it’s half of what the dealer wanted.

Personally, I would rip down the 231 regardless if I was swapping input gears. It’s a good idea to do a full rebuild on it, or at least clean it out and get an idea as to what goes on inside of the t-case. Once again, the FSM is a great tool to have for this.

One other step that I would take before putting everything back in is getting the proper shift linkage for the 231. The tab that bolts to the outside of the case is probably going to need to be changed. It’s a cheap part, dealer only. Order the part for a 93 ZJ with a 231, you’ll need a case ID# too. The one I used that I found on here is 5209 8319. Simply unbolt the old tab, and put the new one on. Your factory linkage may need to be lengthened or shortened, but with this tab the linkage will work without any major modifications.

So now you’re 231 is ready to be installed!

Simply reverse the steps you took to remove the 249 and you’re good to go! Don’t forget to put new fluid in your 231 if you haven’t done so already.

Double and triple check everything to make sure you’ve got it all tightened up and ready to go before driving the vehicle. Don’t go right out onto the highway, tool around on side streets for a bit to make sure everything is working properly.

Additionally, here is a brief summary of issues that I ran into with the swap, and how to solve some of them:

Input Gear
Needs to be a 23-spline, but the length is not known until the case is actually removed from the Jeep. You can make an educated guess as to what it will be, but there is no for sure way of knowing what it is. Usually you can swap the input shaft from the 249 into a 231, but there was a change in the gear cut right around 94 or 95. I got my case from a 90 YJ and needed to swap input shafts. The one from my 249 (1995) was the wrong gear pitch, which would’ve seriously screwed up the t-case. I had to purchase a new input gear to match the t-case and tranny. They are NOT cheap!! I paid more for that gear than I did for the whole case. The original 231 I put in has since been retired due to the front driveshaft coming apart and knocking a hole in the case, and the 231 I replaced it with already had the correct length shaft on it. So . . . I have two spare input gears for jeep t-cases. Both are the longest there is, one old gear cut, one new. If you do the swap and run into the same problem I did, and need a long input gear, let me know!!

Shift Linkage
More than likely will not work correctly. I can’t remember the correct name for this part, but there is a tab on the outside of the case that the linkage hooks into. All you need to do to correct the linkage is get a new tab. It’s hard to describe in words, but I’ll see if I can find a pic of it for you.

Vehicle Information Center
I personally did not solve this “problem” as I know when I’m in what mode on the case. Apparently you can change the wiring up a bit and this will work, but I haven’t messed with it at all.

Shifter Bezel
The “diagram” next to the t-case shifter will obviously still be for the 249. If you want to replace this a FEW early ZJs with the 4.0 came with a 231 and you may still be able to order this part, or the bezel from XJs might fit there too. Again, an “issue” I didn’t deal with.

Front Output Yoke
Depending on what vehicle you source your 231 from, you may run into this problem. I got mine from a YJ and YJ’s use a single u-joint at the t-case end of the front driveshaft, whereas ZJs, XJs, and TJs all use a double Cardin joint.

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