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Article 1:  Swapping out your NP-249 for an NP-231

Article 2:  Warn Hub Conversion


Swapping out your NP-249 for an NP-231

The ZJ I have is a 1994 4.0 and uses the NP-249 which is a full-time transfer-case that utilizes a viscous coupling to act as a limited-slip between the front and rear drive-shaft. One of the main drawbacks of the '93-95 ZJ version of this case is the lack of a low-range lock. In other words, if you get into a sticky situation and need both drive-shafts turning to get you through, this transfer-case will only act as a limited-slip and not mechanically lock the two drive-shaft to turn in unison.


Photo by: A Photographers Credit should usually go here in the lat tag for all photographs Photo by: A Photographers Credit should usually go here in the lat tag for all photographs Photo by: A Photographers Credit should usually go here in the lat tag for all photographs
Basic NP-249 transfer case with input on the right and slip-yoke on the left. Measure your input length here. I will be eliminating the slip yoke arrangement with the new NP-231 unit


We pulled the 249 t-case out and by chance it had the same length input shaft as out donor NP-231 from a '03 TJ. These shafts have to be the same length.. The 249 input shaft will fit in the 231 case but you have to be sure of the right gear cut and also the spline count on the input shaft. There seems to be an older 21 count spline and the newer 23. The 249 and 231 I had both had the 23 spline count.

The shift levers for these cases are different so you will have to use the lever that came with the t-case. The short one came with the 231 and the longer one is the 249. You can not use the longer lever from the 249 on the 231� it will fit but will not allow the 231 to shift right.

I do know that they make a couple of different gear cut and they would not fit or work together and they also make 3 different input shaft lengths. I don't know if these are different because the 5.2l and 4.0l are different.


Side view of the new NP-231 transfer case pulled from a '03 TJ Measure your input length here. Another view of the input shaft.


I decided to install a SYE kit to help alleviate any driveline vibes and to eliminate the pesky slip yoke that is always letting dirt in and fluid out. the SYE let me increase the length of my drive-shaft by over 4"..

Pulled the 249 out�installed the SYE kit in the 231�installed the 231 in the ZJ. Measured for the new drive shaft and called it in to the driveline shop. Had it back the same day and installed it. Filled the transfer case with Dexron III, ordered the new shift pattern plate from the dealer and everything worked great.

Set the new pinion angle in the rear and took it for a test drive…no vibes at all.


Warn Hub Conversion

I have installed 5 sets of the hub bearings and have been needing the Hub Conversion for a very long time.  Not only does it solve the hub bearing problem but it allows me to free wheel the entire front end as well.

I am not going to cover all the instruction because Warn sends out a very detailed instruction manual...but I wanted to add a few pictures and a few notes about the install.  I will say you will increase the front track width by 3/4" on each side.  That also helps for side hilling as well.  You MUST also use the correct front rotor.  Some of the stock rotors do not have a thick enough flange so be sure to read about this on the Warn site or just be sure to follow the install instructions.  You MUST also open the hub flange hole but a god machine shop can help with this.

The install is very easy if you just follow the instructions and only took a couple of hours at a slow pace.  I went ahead and upgraded my axles at the same time and will keep the inner axles as spares but I feel the hub itself will be a good fuse instead of an axle.  The hub itself is very easy to trail fix and not that expensive so it will be a great fuse.  

After the install I could really tell out on the road that the front shaft was not spinning like crazy.  It will have a very easy life as far as on road goes.  

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