NP231 Rebuild Kit
Buyer's Guide

The NP231 (sold later as the NV231) is the part-time, chain-drive Command-Trac case behind most Jeeps built from the late 1980s through the mid 2000s: Wrangler YJ and TJ, Cherokee XJ, Comanche MJ, and the Command-Trac versions of the Grand Cherokee ZJ. GM and Dodge ran their own versions in S-series trucks and the Dakota. It has a 2.72:1 low range, a simple planetary, one drive chain, and a mainshaft oil pump. It is one of the easiest cases to rebuild at home, which is exactly why buying the wrong kit is such a common, avoidable mistake.

The case usually comes apart for one of four reasons: a rear output leak that will not stay fixed, a bearing whine that tracks road speed, a stretched chain that clunks on engagement, or a case that jumps out of 4LO. The kit you need depends on which of those you have, so diagnose first and order second.

The Three Kit Levels

If the case has more than 100k miles and you are splitting it for any reason, the master overhaul kit is the right call. The labor to pull, split and reseal the case is the same whether you replace one seal or all of them, and the bearings are cheap compared to doing the job twice. Our NP231 master overhaul kit (Crown 231MASKIT) covers the Jeep Command-Trac case with bearings, seals, gaskets and RTV.

Match the Kit to Your Case Before You Order

Read the tag

The aluminum tag on the rear case half carries the model and assembly number. NP231, NP231J, NV231 and NV231J are the same family, but GM (231C) and Dodge (231D) versions use different input and output details. If the tag is gone, see how to identify your transfer case.

Input spline and input bearing width

Jeep NP231s came with 21-spline and 23-spline input gears depending on the transmission in front of them, and the input bearing comes in narrow and wide versions. Bearing kits are sold to match. Count the input splines and measure the old bearing before you order a bearing kit, not after the case is in pieces on the bench.

Slip yoke or SYE

A stock case has a long rear output housing and a slip yoke. If the case has been converted to a slip yoke eliminator, the rear seal and output housing are different from stock and a stock kit's rear seal will not fit. Check our NP231 slip yoke eliminator guide if you are thinking about adding one while it is apart.

What No Rebuild Kit Includes

This is where most home rebuilds go wrong. The kit makes the case seal and spin quietly. It does not fix worn hard parts. Inspect each of these during teardown:

• Drive chain. Lay it on its side and push the ends together. If it bows noticeably or the case clunked going into 4WD, replace it. See the NP231/NP233 drive chain and our NP231 chain replacement guide.
• Oil pump. The gerotor pump on the back of the mainshaft is the only thing feeding the chain and bushings. A worn pump, or one that has been rubbing a groove into the rear case half, is the root cause of a lot of dead chains. The ZTNP16209 oil pump is cheap insurance while you are in there.
• Shift fork pads. Worn range fork pads are the usual reason a 231 pops out of 4LO. Pads are inexpensive; a fork with a worn slot needs replacing.
• Rear output bushing and the yoke itself. On slip-yoke cases a grooved yoke chews up new seals in weeks. Run a fingernail across the seal surface; if it catches, the yoke goes too.
• Planetary and annulus. Look for chipped teeth and excessive end play. These rarely fail on stock trucks but do on lifted, big-tire rigs.

Bench Notes

• Drain the case before you pull it. Metal fuzz on the drain plug magnet tells you a lot about the chain and pump.
• Remove the case-half bolts and pry only at the designated pry points. Prying at the seam gouges the sealing surface and you will chase a leak forever.
• The mainshaft, front output and chain come out as an assembly. Do not try to pull the chain off in the case.
• Clean both halves down to bare aluminum, use a thin continuous bead of RTV, and let it set up before you fill it.
• Refill with the fluid on your case tag or in your owner's manual. Most Jeep NP231s call for ATF+4 (earlier trucks were spec'd with Dexron-type ATF). Fill until it just starts to run out of the fill plug hole. See our transfer case fluid guide.
• Cycle the lever through 2H, 4H, N and 4L with the truck stopped before the first drive.

FAQ

Is a master overhaul kit worth it over a seal kit for an NP231?

If the case is coming off the truck and getting split, yes. The labor is the same either way, and a master overhaul kit replaces the bearings that cause whine along with every seal and gasket. A seal kit only makes sense for a quiet, low-mileage case that is just leaking.

Does an NP231 rebuild kit include the chain?

No. Bearing, seal and master overhaul kits do not include the drive chain, oil pump, shift fork or planetary. Inspect those during teardown and order them separately if they are worn.

What fluid does an NP231 take after a rebuild?

Use what your owner's manual or case tag specifies. Most Jeep NP231 and NV231 cases call for ATF+4; earlier trucks were spec'd with Dexron-type ATF. Fill until fluid just starts to run out of the fill plug hole.

How do I know if I need a narrow or wide input bearing kit?

Count the input gear splines and measure the old input bearing when it comes out, or match the kit to the tag numbers on your case. Jeep NP231s used both 21-spline and 23-spline inputs depending on the transmission, and bearing kits are sold to match.

Why does my NP231 pop out of 4LO?

The usual cause is worn range shift fork pads, followed by a worn fork slot or weak detent. A rebuild kit alone will not fix it; replace the fork pads, or the fork, while the case is apart.