Jeep NP231 Rebuild
I finally decided to do my Jeep NP231 rebuild. One of the first steps is to locate the badge on the back of the NP231 transfer case. This is the information you will need to know when ordering the replacement parts for your NP231 transfer case. I recommend changing all the bearings even if they look good. Replace all the seals. Inspect the other components for wear. In my transfer case the shifter fork was worn and needed to be replaced.
Ensure that your work area is clean and free of any debris. Small particles can ruin new bearings and surfaces. Scrape and pressure wash the transfer case. Some detergents or solvents may be used. The tools you will need will include a pair of snap ring pliers, a bearing puller, a hydraulic shop press, a soft mallet, a pry tool or three jaw slide hammer. You will also need some Permatex, and grease as well as transmission fluid (2 quarts) to refill the NP231 transfer case when you are done and for some lubrication during assembly.
I am going to assume that you already have the NP231 transfer case on your workbench and that you have already removed the drive shafts, vacuum lines, linkages, wires and cradle needed to remove the transfer case from your transmission.
Remove the extension housing. Remove the 3 – 15mm bolts using a 15mm socket. Use a rubber mallet to release the RTV seal. Now remove the output snap ring with your snap ring pliers. Next remove the main output housing bolts. There are 5 – 10mm bolts remove them using a 10 mm socket. Now pry the housing off the case. The rear output bearing will come off with the housing.
Now it’s time to split the NP231 case. Remove the eight – 15mm bolts using a 15mm socket. Gently pry the case apart. Pull the rear case off of the output shaft. The pump and pick up tube will come off with it. Note the pickup tubes filter you will be cleaning this later. You may need to replace it if it is torn or broken. Remove the shifter fork spring and range fork and collar.
Next remove the front output yoke using an impact gun and a 28mm socket.
Now grab the front and rear output shafts to pull them out this should bring out the chain fork and collar. Flip the case upside down and remove the four input retaining plate bolts with a 10mm socket.
Remove the front output seal using a screwdriver and small hammer or mallet. Now remove the snap ring that holds the bearing in place. You can now drive the bearing outward using a press or a hammer and drift. To remove the bearing on the rear case half use a screwdriver to break the cage of the bearing along with the needles. Using the slide hammer with the three jaw head fasten the jaws on the inside upper lip of the bearing. You may have to use a torch to heat up the aluminum to successfully remove this bearing.
Now we are going to disassemble the main shaft of the NP231. I used a vice and put the shaft in the vice with the input side up. Then remove the snap ring located on the front end of the shaft. The sprocket gear and should now slide off the shaft. Remove the large snap ring holding the planetary assembly together. There are three thrust washers one steel and two plastic. Gently remove the pilot bearing located in the input gear out with your press, drift or large screwdriver.
I was not writing this down for my NP231 dis-assembly therefore remove any bearings or seals that I have neglected to mention. You should always replace all the bearings and seals as well is the chain. Many people only replace the chain and windup having to disassemble and rebuild the transfer case all over. You can obtain all the necessary parts including the chain for about $200. I also needed to replace my mode fork which cost me around $40.
Jeep NP231 Rebuild
Reassembly – Reference Diagram
Jeep NP231 Rebuild – Reassembly
Make sure all your parts are clean. Do not begin reassembly unless your parts and work area are clean. First install the new front output shaft bearing onto the front case. You can use a bearing installer or a brass drift and hammer but I carefully used my press. Reinstall the snap ring. Install the new front output shaft seal.
Install the new input gear bearing. Be sure to install the snap ring onto the bearing. I used my shop press. Press the bearing in carefully until the snap ring rests against the case.
Install the main shaft pilot bearing into the input gear. I used my shop press. You can use a sturdy socket to tap this bearing in place if you do not have a shop press. Make sure the socket is the same diameter as the bearing.
Install the first thrust washer into the gear assembly. Install the input gear into the planetary gears. Now install the second thrust washer. Replace the snap ring that holds the assembly together. Lubricate the input gear with some automatic transmission fluid. Place the input gear and low range gear assembly into the front case. Install the snap ring that holds the assembly in the case. If the gear assembly was properly seated the snap ring groove will be fully exposed. Install the oil seal in the front bearing retainer plate. Apply RTV silicone to the retainer plate mating surface. Lubricate the seal with ATF and install the plate. Torque the bolts to 16 foot pounds.
Put the range fork onto the shift hub. Be sure to replace the fork pads. Put the range fork shift hub assembly into the case with the fork and pin in the shifter slot. Install the drive sprocket airings. First install the front bearing flush with the edge. Install the rear bearing leaving 4.6 mm below the edge. Lubricate the sprocket bearings with automatic transmission fluid.
Install the drive sprocket bearings stop ring and the synchro hub. Install the synchro snap ring. Install the synchro sleeve onto the synchro. The beveled edge faces stop ring. Install new pads onto the mode fork. Install the mode fork assembly onto the synchro sleeve. Now place the main shaft assembly into the front of the case. Be sure that the range fork pin does not come out of the slot.
Install the NP231 drive chain. Install the mode spring. Install a new seal on the oil pump. Be sure to replace the o-ring in the pickup tube. Be sure that the NP231 pickup tube and filter screen are clean. Reinstall the pickup tube and filter. Install the magnet making sure that it is clean. RTV the front case mating surface and install the rear case onto the front case. Be sure that the oil pump is properly on the main shaft and that the tube is inserted into the pump. Torque the case bolts to 30 foot-pounds and be sure that the case bolts at the locating dowels have washers.
Install the new bearing on the rear retainer. RTV the mating surface and install over the main shaft. Torque these bolts to 18 foot-pounds. Pull up on the main shaft and install the rear bearing snap ring. Install the extension housing oil seal. RTV the mating surface of the extension housing and torque the bolts to 30 foot-pounds. Install the front yoke and torque to 110 foot-pounds. Install your transfer case back in your Jeep. Fill your NP231 transfer case with Quarts of automatic transmission fluid.(Dexron II ATF)
You are done with your Jeep NP231 Rebuild !!