NP231 Transfer Case Master Overhaul Kit for Jeep

Command-Trac case leaking at the rear output, whining at highway speed, or sweating at the seam between the case halves? On a Jeep Wrangler YJ or TJ, Cherokee XJ or Comanche MJ, that is almost always the New Process NP231 telling you its bearings and seals are done. This Crown Automotive 231MASKIT master overhaul kit puts the wear parts for a full reseal and rebearing in one box, so the case only has to come out and split once.

What the master kit does in an NP231

The NP231 is a part-time, chain-drive case with a planetary low range of about 2.72:1. Power comes in through the input gear, passes the planetary, and goes straight out the rear output in 2H. Shift to 4H and a mode sleeve locks the drive sprocket, so the chain carries power across to the front output. The input, front output and rear output all ride on bearings that are sealed by lip seals, and the two aluminum case halves are sealed with RTV rather than a gasket. Lubrication comes from a small mainshaft pump, so a case that leaks down starves itself.

The retailer parts list for 231MASKIT includes bearings, seals, shift fork pads, bushings and RTV sealant. The fork pads matter more than most people think: worn pads let the range or mode fork drag and can make the case pop out of gear. The bushings cover the rear output, where a worn bushing lets the slip yoke wobble and chew through the seal.

Symptoms of a worn NP231

• Fluid dripping off the rear slip yoke onto the driveshaft, often coming back weeks after a seal-only fix
• Whine or growl from behind the transmission that follows road speed, not engine speed
• Seep at the case-half seam or at the front output
• Case jumping out of 4L under load, or a lever that is hard to move into 4H
• Clunk on 4WD engagement from a stretched chain, often found once the case is open
• Fine metal on the drain plug magnet and a low fluid level

How to confirm before you order

Check the fluid level at the fill plug and look at what comes off the drain plug. A rear output leak with a wobbly yoke points at the bushing as well as the seal; our NP231 rear output bushing leak guide walks through it. Noise that stays the same in 2H and 4H and rises with speed points at output bearings. If the case is coming out anyway, the NP231 rebuild kit buyer's guide explains why a master kit beats a seal kit on a high-mileage case.

Fitment

Crown 231MASKIT is listed for the Jeep NP231 in the 1987-1995 Wrangler YJ, 1997-2006 Wrangler TJ, 1988-2001 Cherokee XJ and 1988-1992 Comanche MJ. Grand Cherokee ZJ models with Command-Trac also used the NP231, but they are not on the application list, so confirm with the tag before ordering. The tag on the rear case half shows the model and assembly number. GM and Dodge versions of the 231 use different input and output details and are not covered. If the tag is missing, send us the year and VIN and we will check it.

Install notes

• Drain, remove both driveshafts and the shift linkage, support the case and unbolt it from the transmission adapter.
• Split the halves at the seam, lift out the mainshaft, chain and sprockets together, and inspect the chain, planetary and pump while they are out.
• Press the old bearings out and new ones in, drive the new seals square, and fit the new fork pads and rear bushing.
• Clean both sealing faces to bare metal before laying the RTV bead, then torque the case bolts in a cross pattern.
• Refill with ATF; Jeep fluid tables list ATF+3 or ATF+4 for the NV231 at roughly 1.1 to 1.6 quarts depending on year and transmission. Fill to the bottom of the fill hole.
• Replace a stretched chain now with the Alloy USA NP231 chain, and check the NP231 oil pump for scoring. Lifted Jeep with vibration? A slip yoke eliminator kit removes the rear bushing problem for good. TJ owners can add the shift cable conversion kit while the case is out.

Which Jeeps does the 231MASKIT fit?

The NP231 in the 1987-1995 Wrangler YJ, 1997-2006 Wrangler TJ, 1988-2001 Cherokee XJ and 1988-1992 Comanche MJ. Check the tag on the case before ordering for any other vehicle.

Does the kit include the drive chain?

No. It covers bearings, seals, shift fork pads, bushings and RTV. Order the chain, planetary or oil pump separately if teardown shows wear.

What fluid goes in an NP231?

Jeep fluid tables list ATF+3 or ATF+4 for the NV231. Capacity is about 1.1 to 1.6 quarts depending on year and transmission; fill until it reaches the bottom of the fill hole.

Why does my rear output keep leaking after a new seal?

A worn rear output bushing lets the slip yoke move, and it tears up a new seal quickly. This kit includes bushings so both get replaced together.

How long does an NP231 rebuild take?

Plan on a weekend for a first rebuild. The retailer rates it as an advanced job at about 3 to 6 hours of bench time, not counting removal and install.