Jeep Cherokee NP231 Transfercase Partial Overhaul
	Category:	Drivetrain
	Author:	MachineMan
	Date:	May 9th, 2005

This write up explains how to replace the front case oil seals and bearings, shift fork pads, replace the chain and general assembly. The write up is based off a 1990 Cherokee 4x4 with AW4 auto transmission; other years may vary slightly including GM and Chrysler models. Note that this procedure starts from an already removed transfer case.

On my rig I was installing a newer (1995) transmission which required me to change the TC (transfer case) input gear from a 21 spline to a 23 spline. Since I already had the case open I decided to replace the shift fork pads, shaft oil seals, and bearings. The back end of my TC had recently been upgraded with a SYE (slip yoke eliminator) including new seals and bearings so I left it alone and is not covered in this write up.

Parts
Pricing out all the parts I needed exceeded the cost of buying a master overhaul kit. I bought a complete overhaul kit from rubicon4x4.com (P/N 231-MASKIT) for $89. The kit included more parts than I needed consisting of several oil seals, bearings, and the shift fork pads. It does not come with a new chain, speedo assembly seals or shift rod bushings. Here’s what the kit comes with including all the extra parts I did not use.

Tools
Other then a set of standard tools, here are a few you will need if you don’t already have them:
30mm socket (drain and fill plugs)
1 1/8 inch socket (yoke nut)
Lock Ring pliers
Rubber mallet (for pounding seals and bearings)


Case Disassembly

1. Start draining all the fluid out by removing the 30mm drain plug.
   
2. Remove the front shaft yoke with a 1 1/8 inch socket. An impact wrench makes this easy but if you don’t have one then you will need to get some leverage using the u-joint bolt holes. Put 2 sacrificial bolts in holes on the same side of the yoke and put a long rod in between them and lay it up against the TC to prevent the shaft from spinning. Then loosen the yoke nut with a breaker bar. You will have to do the same thing when putting it back together.
   
3. Remove the all the 15mm bolts holding the case together. One of the bolts on mine was a 12pt 10mm socket. Note for reassembly that the 2 bolts that have washers on them go into the holes that have the guide pins.
   
4. Split the case slowly. Once open slide out the shift fork assembly noting the position of the mode spring, synchronizer sleeve and range fork hub. From here seals and bearing can be replaced in any order.
   

Shift fork removal

NP231 Exploded Diagram

Input Gear / Shaft Seal and Bearing Replacement

1. Remove the input shaft retainer plate using a 10mm socket or wrench. Knock out the oil seal in the plate and replace it. Using a rubber mallet pound it in evenly flush to the retainer plate outside surface. Clean up any residual silicon sealant.
   
   
   Front Seal Retainer Plate Removed
   
   
2. Using lock ring pliers remove the input shaft lock ring and drop out the input gear and planetary gear assembly.
   
   
   Input Shaft Lock Ring Removal
   
   
   Input Shaft and Planatary Assembly
   
   
3. Using a flat head screw driver remove the snap ring holding the input gear in the planetary gear assembly. Note the position of the thrust washers for reassembly.
   
   
   Input Gear Removed from Planatary Gear
   
   
4. Choose the biggest deep socket you have that fits inside the spline shaft and pound out the input gear plug and pilot needle bearing. First note the position of the bearing so you can install the new one to the same depth.
   
   
   Input Gear Plug and Pilot Bearing
   
   
5. From the shift hub side put in the plug followed by the new pilot bearing and using a large deep socket to pound them in straight. The pilot bearing should sit just above the rim inside the input gear. Use the bearing to finish pounding in the plug so it goes in straight and no deeper than the back of the bearing.
   
   
   Input Gear Plug and Pilot Bearing Replacement
   
   
6. Now remove the main input shaft ball bearing. Start by prying out the bearing snap ring and pounding the bearing out into the case.
   
   
   Input Bearing Removal
   
   
7. Insert the new bearing with the snap ring already attached to it from the outside in. Mine was tight so I put the bearing in the freezer for a while and used a torch to heat up the opening to aid in putting it in. I still had to pound it in and it took a couple times before I got it in straight. Don’t hold the torch in one place for long or you could damage the aluminum case.
   
   
   Applying some heat to expand the input bearing opening
   
   
8. After the bearing is in place assemble the input gear back into the planetary gear with thrust washers and snap ring. Put the assembly back through the input shaft bearing and put the lock ring back on. Don’t forget to put some ATF on the thrust washers and gears before assembling.
   
9. Put a very thin bead on Silicon sealant all around the front retainer plate and bolt back on with the four 10mm bolts. Be sure to align the oil hole on the case with the notch on the retainer plate and put silicon on the outside edge of the notch.
   

Front Seal Retainer Complete

Front Output Shaft Seal and Bearing Replacement

1. Pry out the oil seal and remove the snap ring above the ball bearing.
   
2. From the inside pound out the ball bearing.
   
   
   Front Output Shaft Output Housing
   
   
3. Put in the new ball bearing and snap ring. I found that using a section of 2inch ABS pipe was a perfect fit to evenly pound the bearing in.
   
   
   Front Output Shaft Bearing Installation
   
   
4. Now evenly tap in the shaft seal. For extra leak prevention apply a super thin layer of Silicon around the seal casing.
   

Front Output Shaft Bearing and Seal Complete

Chain Replacement

1. With the case already split, this is the easiest part. Remove the lock ring around the main shaft.
   
   
   Main Shaft lock ring
   
   
2. Slip the drive sprocket from the main shaft with chain at the same time pulling the front output shaft out of the rear case bearing. Note the orientation of the sprockets and synchronizer stop ring for reassembly. Inspect rear needle bearing and replace if needed. (The bearing is included in the master kit)
   
   
   Chain Removed
   
   
3. Put on the new chain and reassemble sprockets and snap ring. Oil up the parts prior.
   

Shift Fork Pad Replacement

1. With the mode and range shift forks removed from the case, replace all 6 shift pads. The outers on both forks are the same but the inners are different. Apply some grease to hold them.
   
   
   Shift Fork Pad Replacement
   
   
2. Put the Synchronizer sleeve and range fork shift hub back into the fork and they should look like this. Don’t forget the mode spring.

Assembled Shift Forks

Case Reassembly

1. Double check all snap rings are in place and all gears are oiled up.
   
2. Wipe out the case for the last time removing any debris.
   
3. Before reassembling remove the oil pump screen and filter to clean. Wipe out the rest of the case and interior parts.
   
   
   Oil Pump Screen
   
   
4. Oil up and install the shift fork assembly into the front of the case. Make sure the shift pads stay in place. The mode fork pin goes into the shift sector and the synchronizer sleeve fork ride on the outside of it.
   
   
   Installed Shift Fork Assembly
   
   
5. Lube up all the gears and shaft seals.
   
6. Apply a bead of silicon sealant around the case mating surface. There’s no gasket. Align shafts, shift rod and mount both halves together. Remember that the bolts with washers go in the 2 holes with guide pins.
   
7. Tighten up case and install the front shaft yoke.
   
8. Let cure overnight before filling with 1 quart of ATF. Recheck fluid level when mounted in rig.
   
9. Rotate the shafts while shifting the lever and make sure each gear is working properly.
   
10. Install on transmission.

NP231 intsalled on AW4 Transmission

Torque Specs
Front Shaft yoke - 130ft-lbs (175n.m)
Case Bolts - 30ft-lbs (40n.m)
Front bearing retainer plate bolts - 20ft-lbs (27n.m)
Drain and Fill Plugs - 30ft-lbs (40n.m)


Links
www.rubicon4x4.com (where I bought the master kit)
http://www.novak-adapt.com/knowledge/np231.htm (good general info about NP231)

Tom K. May 2005