How-to, Part 1: Disassembly & Inspection of NP/NV
231 Transfer Case
1: The New Process/New Venture type 231 transfer case is
identified by its distinct I.D. plate. The 231 is popular on the Jeep YJ Wrangler, Jeep TJ Wrangler, the Jeep
XJ Cherokee and some Jeep Grand Cherokee models. Read the plate carefully for details on type and
This is a two-part
'how-to'. See the steps below then click here for Part 2 on the following
2: This '99 Jeep XJ Cherokee features the NV231 transfer case
coupled to an AW-4 automatic transmission. The Aisin-Warner automatic transmission has a proven track record
for durability plus a versatile 4th gear overdrive. The AW-4 is only used in the XJ Cherokee and MJ Comanche
models. NP231 or NV 231 usage includes YJ and TJ Jeep Wranglers and some Jeep Grand
Note: Follow the steps described in a factory-level workshop manual
to remove your Jeep Wrangler, Jeep Grand Cherokee or Jeep XJ Cherokee's NP/NV231 transfer case. After more
than four decades of professional Jeep 4WD work, I still refer to the factory workshop manuals when wrenching
on my Jeep 4WD projects!
3: Drain fluid from the transfer case before removal. When
removing the transfer case, cover the transmission output shaft and seal area to prevent contamination. Avoid
binding as you slide the case rearward.
4: I use a transmission jack that provides
careful tilting and protects the transmission and transfer case during removal and installation. Note the long,
splined output shaft. This is the shaft that accommodates a slip coupler. The long shaft and slip coupler extend
the overall length of the powertrain.
5: This is the only way to prevent damage to the aluminum
cases and flanges: Use a transmission
6: Prevent contaminants from entering the rear of the
transmission. Place a lint-free rag or plastic cover over the output of the transmission while servicing the
transfer case on the bench. An automatic transmission is high sensitive to contaminant
7: These are
the Advance Adapters’ Fixed-Yoke Kit’s components. They readily match OEM quality castings, shafts and
machining. The new output shaft is massive, providing 50% more
stamina to better handle oversized tires, high torque loads and a chassis lift with steeper driveline
8: Mark the speedometer housing and clamp position before
removing these parts. This electronic speed sensor unit still uses a conventional drive/driven gear mechanism
on a '99 Jeep XJ Cherokee. 'SYE' kit comes with a new output shaft drive gear that works with this unit. I
mark the location of the clamp and housing for reassembly later.
Note: Since the slinger (rusted piece at output shaft) on the
original output shaft will not be reused, it can be removed with a long, two-jaw puller or similar means. I
use the Jeep factory tool #MD998056-A for this chore.
9: The stock front yoke nut comes off readily with an impact
gun and 1-1/8” deep impact socket. You can hold the yoke with a large pipe wrench or special holding tool.
Use of air impact force requires nothing more than light holding pressure to prevent the yoke from rotating
as you remove the nut. Use impact sockets for this kind of work.
10: When installing an SYE kit, the slinger stop spacer will
not be reused. If rebuilding the transfer case "stock," protect the slinger and use the recommended
11: This is the first snap ring to remove. I use quality snap
ring pliers to prevent distortion of rings during removal and installation. The Advance Adapters kit includes
new snap rings to replace originals.
12: Remove the tailhousing/retainer seal with a suitable
puller or by tapping at the seal’s outermost edge while staying clear of the aluminum retainer material. The
rear retainer tail-housing is aluminum and will not tolerate prying with screwdrivers or awls. Always protect
the aluminum castings.
13: Use an eye-type snap ring tool for this snap ring. This
internal snap ring requires a heavy-duty tool designed for internal snap rings. Use the correct tool to
prevent parts damage or physical injury.
14: Removing this snap ring at the output bearing enables removal of the
tailhousing/retainer. Slide the snap ring free of the output shaft splines.
15: Remove the five
bolts holding the housing in place. When separating castings, pry only at the factory prying points or the pry
slots provided. If all hardware is loose, there should be no resistance beyond the
Warning: Do not try to wedge these pieces
apart with a screwdriver or other pry tools. Never gouge or pry against the mating surfaces of aluminum
16: With the tailhousing removed, the oil pump is accessible.
I slide the pump rearward, carefully, to disengage the oil pump tube from the pump unit. Tilting the pump
will detach it from the tube inside the rear case housing.
Caution: Do not allow the pump to become contaminated during this
work. I wrap the pump assembly in clean, lint-free shop towels and set it aside until
17: Remove the case bolts and note their positions. The bolts
with washers go to the dowel locations. Keep track of hardware. When prying the case apart, look for
screwdriver slots at opposite ends of the case. These serve as pry points. Use only these points and pry
evenly, end-to-end. You are separating the sticking adhesive sealant.
18: Once you separate the case halves, you can see the oil
pump feed tube and oil filter assembly. Use of an oil pump and filter is a higher-tech design. Clean the
filter carefully during the rebuild process.
19: The front drive sprocket should slide out of its support
bearing with light pressure. I use a plastic hammer, if necessary, to lightly tap on the threaded end of the
shaft. There should be little resistance.
20: Once the front drive shaft is free of its bearing, the
slack in the chain will permit removal of the chain from the output shaft sprocket. Do not force any of these
parts. The NP/NV 231 is a precise, close-tolerance assembly.
21: You now have access to the main drive assembly. The shaft
and drive sprocket will separate from the case as an assembly. Include the shift fork and shaft.
22: Output shaft and shift fork with rail are now removed
from the planetary gearset. Note the relationship and fit of the outer shift sleeve and
hub. Unless you have a damaged planetary, this
is the extent of disassembly required to install the Advance Adapters 'SYE' kit. Advance Adapters provides
thorough details and instructions.
Note: If you intend to completely tear down the transfer case, use a
factory-level workshop manual. Here, I have removed the output assembly and shift fork from the
removal from the output shaft begins with snap ring removal. Invest in quality snap ring pliers if you plan to do
this kind of work regularly. Simply removing the snap ring will free
the mode hub and sprocket from the original output shaft. You will not reuse this shaft, and a new snap ring is
furnished with the Advance Adapters kit.
Note: On earlier 231 transfer cases, there is a caged bearing inside
the drive sprocket bore. You must press this out so that the original sprocket will fit on the new shaft. TJ
and later design XJ sprockets (shown) do not use the caged bearing. The drive sprocket fits directly on the
24: Inspect the drive sprocket, mode hub, shift fork and shifter rail
for damage or excessive wear. Forced shifts and abuse of the transfer case are the most common causes of
damage. These parts are rugged. The chain is a more common wear item, and even the NP/NV231 chain lasts a
very long time.
parts except the electronic speedometer drive (speed sensor) assembly and electrical switches have now gone through
the parts washing cabinet. Inspect parts carefully for damage. This is the time to repair or restore pieces,
while the unit is apart on the bench.
Note: There are other SYE approaches, including hacking off a section
of the original output shaft, then drilling and tapping the end of the shaft to accept a stock spline count
yoke. I like the Advance Adapters approach far better—at a 32-spline count, the Advance Adapters' SYE kit is
good for 50% more strength than stock...See Part 2 for assembly details, including the Advance Adapters
*This is a two-part
here for Part 2 on the following page!