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NP231 transfer case (Command-Trac)

The simple, bulletproof part-time 4x4

The most common transfer case on the Cherokee was the NP231, marketed as Command-Trac: a chain-driven part-time 4x4, simple and rugged, perfect for dirt, mud and snow.

Part-time 4x4

'Part-time' means four-wheel drive engages only when you need it and on loose surfaces: on dry pavement you must stay in 2H, because it has no center differential. Positions are 2H, 4H, N and 4L (low range ≈ 2.72:1), that last one for crawling slowly with lots of grunt.

It's a unit known for taking anything with minimal maintenance. Its weak point isn't strength but the driveshaft angle once you lift the truck.

Lift, SYE and vibration

The NP231 uses a slip-yoke on the rear output. When you lift the XJ more than ~3 inches, the driveshaft angle steepens and vibration shows up. The classic fix is a SYE (Slip-Yoke Eliminator) plus a double-cardan (CV) driveshaft: all but mandatory on any serious lift.

It's one of the items you must budget for when you go from a small boost to a real lift — see the lift kit guide.

What it swaps with (and the splines)

The NP231 was fitted across half the universe: Jeep (XJ, MJ, YJ, TJ, ZJ), Dodge and even GM/Chevy (the «231C»). Most parts interchange within the family, with two important warnings. First, the GM versions have a 5-bolt front face, incompatible with the Jeep/Dodge 6-bolt pattern: if you're hunting a donor, make it Jeep or Dodge. Second, the input shaft comes in 21 splines (pre-1991 XJ, behind the AX-4/AX-5/Peugeot) or 23 splines (from 1991, behind the AX-15 and the High Output AW-4) —it has to match your gearbox or you'll need to change the input gear.

That 23-spline output is the same one it shares with the NP242 (Selec-Trac), which is why the swap between the two is so direct when the donor is from an XJ or ZJ. Both also share the 2.72:1 low range and the same mainshaft weak point that justifies the SYE (above). In plain terms: to go from part-time to full-time you don't change mechanical worlds, just the case.

Specifications