NP231 Overview (1:1 / 2.72:1)

The NP231 (also marked NV231, same hardware) is the workhorse XJ transfer case. Chain-driven, part-time only, 2.72:1 low range. Found in 87-01 XJs and many TJs, ZJs, WJs, and Rams. Massive parts support.

Ratios:

• 2WD high: 1.00:1
• 4WD high: 1.00:1
• Neutral
• 4WD low: 2.72:1

Input shaft: 23-spline behind AX-15/AW4/BA10 in 87-95 XJs; some changes in later years (see tcase-np231-spline entry).

Output: slip-yoke rear (the problem child — see SYE entry), CV front to the front driveshaft.

Known issues:

• Slip yoke output rear: in lifted trucks, the slip yoke binds and the seal blows out. The fix is SYE — required on any XJ with 3"+ of lift.
• Chain stretch at high mileage. Replace chain in any rebuild.
• Pump rub: the oil pump can wear through the case at the rear bore, leaking onto the driveshaft. Common at 150k+. A pump rub repair kit ($30) or a Sye case generally fixes it.
• Synchronizer wear in the planetary range collar: makes shifting into 4Lo difficult. Rebuilds are straightforward.

Fluid: ATF Dexron III (not gear oil — chain case). 2.2 quarts. Change every 30-50k.

Verdict: solid, straightforward, supports an SYE and aftermarket low-range gears. Right transfer case for most XJ builds. Only consider a different t-case if you need 4:1 or true full-time AWD.

Why it works

• common
• tons of aftermarket
• reliable
• SYE-able

Trade-offs

• slip yoke binds with lift
• pump rub at high miles
• only 2.72:1 low

Tools required

• flashlight
• tape measure (output shaft)

Sources

• Novak NP231 guide

Related

• NP242 Full-Time Option Overview
• Atlas II T-Case Swap Overview
• All Jeep Cherokee XJ guides & tools
• Browse the full DIY database

Written and maintained by an AZ wheeler and driveway wrencher. How we source guides. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.