NP231 Rear Leak
Bushing and Seal Fix

The NP231 is one of the best transfer cases Chrysler ever put in a Jeep. Wrangler YJ and TJ, Cherokee XJ, Dakota, and Durango all ran it, and most of them still do. Its weak spot is the rear output: a slip-yoke design where the driveshaft yoke slides directly into the tailhousing and rides on a bronze bushing. The seal only seals. The bushing carries the load.

When that bushing wears, the yoke wobbles, the seal lip cannot follow it, and fluid walks out the back. The drip lands on the exhaust or the crossmember and owners chase it for months thinking it is the transmission.

Why the Bushing Wears

• Lift kits. Lift a Jeep and the rear driveshaft angle steepens. The slip yoke pulls partway out of the housing and levers against the bushing. This is the number one cause.
• Driveshaft imbalance. A bent tube or worn U-joint shakes the yoke at highway speed.
• Low fluid. The bushing gets less oil on its surface and wears faster. A slow leak becomes a fast one.
• Mileage. At 150,000 miles even a stock-height XJ is usually due.

How to Check It

With the vehicle safely supported and the driveshaft still installed, grab the rear driveshaft right at the yoke and push it up and down and side to side. A good bushing allows barely detectable movement. If you can see the yoke move in the housing, or hear a clunk, the bushing is worn. Then look at the seal area: a wet tail with a ring of grime, fluid on the driveshaft tube, and drips forward of the rear axle are the signature.

Pull the driveshaft and look at the yoke's sealing surface too. A groove worn by the seal lip will leak no matter how good the new seal is. Light grooves can be covered with a thin repair sleeve; deep wear means a new yoke.

Repair Options

Bushing and Seal Replacement, Step by Step

• Mark the rear driveshaft to the axle pinion yoke so it goes back in phase. Unbolt it at the axle and slide it out of the transfer case. Have a drain pan ready.
• Drain the transfer case through the bottom plug. Always crack the fill plug first so you know you can refill it.
• Pry out the old seal with a seal puller. Do not gouge the bore.
• Remove the bushing. A blind bushing puller is the clean way. Many techs carefully collapse it with a small chisel, taking care not to cut into the aluminum housing.
• Clean the bore, then drive the new bushing in with a correctly sized driver until it sits at the original depth. Line up any oil hole with the passage in the housing if your bushing has one.
• Lightly oil the new seal lip and drive the seal in square.
• Inspect the yoke, then reinstall the driveshaft in its marks.
• Refill through the fill hole until fluid just starts to come out. NP231s take roughly 1.2 quarts (about 2.5 pints) of ATF+4 or the fluid listed for your year.

A proper driver set makes the job far less frustrating. Our 52-piece bushing and seal driver kit covers the NP231 tail and most transmission bushings you will meet.

When to Skip Straight to an SYE

If the Jeep is lifted more than about two inches and you have already done a bushing once, stop throwing bushings at it. A slip yoke eliminator replaces the long tailhousing and slip yoke with a short housing, a longer fixed output shaft, and a fixed yoke. The slip moves into the driveshaft, the bushing problem goes away, and you usually gain driveshaft length that cures the vibration too.

We carry the NP231 slip yoke eliminator kit (32-spline) and the Rough Country NP231 SYE kit, plus a TJ rear driveshaft for SYE and lifts. Older NP231s with a 21-spline output need the matching kit, so count splines or check the tag first.

If the case also has chain slap or bearing noise, do it all at once with the NP231 master overhaul kit and a new drive chain. You will already be halfway in.

Mistakes We See on This Job

• Seal only, no bushing. The most common one. The new seal is fine for a few weeks, then the worn bushing lets the yoke walk the lip off its surface and the leak returns.
• Seal driven in crooked. A cocked seal leaks immediately. Drive it with a flat driver the full diameter of the seal, not with a socket on one edge.
• Bushing driven too deep. Some bushings have an oil feed. Drive it too far and you block the passage, starving the new bushing.
• Driveshaft reinstalled out of phase. If you did not mark it, you can create a vibration that chews up the new bushing.
• Overfilling. NP231s vent through the top. Overfill and it pushes fluid out the vent and makes the case look like it is still leaking.
• Ignoring the yoke. A grooved or rusty yoke surface leaks no matter what you do to the housing.

Also check the front output seal and the rear case-half sealant line while it is apart. Fluid from either of those runs back and drips at the tail, so the bushing gets blamed for a leak it did not cause. Clean the whole case with brake cleaner and recheck after a drive before you order parts.

Is the Case Worth Saving?

Usually yes. Unless the case has been run low on fluid long enough to wear the pump or chew the housing, a bushing, seal, and possibly a chain will give an NP231 another long service life. They are simple cases that respond well to fresh parts.

FAQ

Is it the transmission or the transfer case leaking?

Clean everything and drive a few miles. A tail leak shows up at the rear of the case and on the driveshaft. A transmission leak at the adapter shows up at the front of the transfer case.

Can I drive with a small rear leak?

For a short time, if you keep it full. Run it low and the pump starves, the chain stretches, and the bushing gets worse.

Does an SYE need a new driveshaft?

Yes. It needs a double-cardan (CV) rear driveshaft and the pinion angle set to match.