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NP242 transfer case (Selec-Trac)

The full-time 4x4 that's happy on pavement too

The NP242 (Selec-Trac) was the Cherokee's upper transfer case: it adds a full-time 4x4 mode with a center differential, safe to run on pavement without hurting anything.

Best of both worlds

Unlike the part-time NP231, the NP242 has a center differential that enables a full-time 4x4 mode: you can leave it in all-wheel drive all the time, even on dry or wet pavement, without binding the drivetrain. And when the terrain gets rough, you also get part-time 4H and 4L (low range) with the diff locked.

In plain terms: for anyone daily-driving the XJ on mixed surfaces —rain, gravel, a bit of mud— the Selec-Trac is a huge convenience. It's a touch heavier and more complex than the 231, but far more versatile.

The 231↔242 swap and donors

The NP242 was used in Jeep (XJ/Wagoneer 1987–2001, MJ, ZJ/WJ, KJ Liberty), in the Dodge Durango and even in the Hummer H1 (an HD version with 32-spline shafts, stronger). The most common swap on the XJ is exactly to replace the part-time NP231 with a 242 from another XJ or a ZJ: the 96+ ZJ's 242 is practically the same case. They share the 2.72:1 low range, so the swap doesn't change the reduction —it just adds the full-time mode.

Two things to get it right: that the input-shaft splines (21 pre-91 / 23 from 91) match your gearbox, and —from 1992 on— that the speed sensor is the three-wire digital VSS rather than the old mechanical cable. What does not work as a donor are the 242s from a Dodge or a WJ (they're different): always look for XJ or ZJ.

Specifications