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Last Revised 22.09.2006

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6 Cylinder Engine Info

The NP 231J was introduced at the 1988 model year in both the XJ Cherokees with the Command Trac option and the standard YJ Wranglers. Later to be called the NV (New Venture) 231J, this transfer case has continued in the trend of chain-driven cases. Its low gear is a respectable 2.72-to-1 (the lowest of all the Jeep transfer cases) and is direct-drive in high. It has a driver's side front output.

This transfer case is identified by its red and silver tag on the rear of the case. There are also very similar versions of the 231 for the downsized Chevy Blazers and Dodge Dakota trucks. Heavier-duty HD and DHD units are available in ZJ Jeeps and Dodge trucks, respectively. OEM 231 transfer cases are capable of transferring from 1600 to 1900 ft. lbs of torque.

The NP231 transfer case has turned out to surprise us in its strength, gearing, upgradeability and servicability. Most of our customers choose to retain the 231 with their conversions, though they do often opt to strengthen the case by switching to a 23 spline input (if currently 21 splines - common with the four-cylinder engine/transmission combos) and doing a heavy-duty tail shaft conversion. Wide chains are also available if even more strength is desired. With such upgrades, a transfer case capable of handling 2300+ ft. lbs. of torque is attainable. This is a result of good engineering and refinement. As such, we and the majority of our customers keep their 231's when performing a conversion.

One easily conquered drawback of the 231 is its length. With the introduction of the planetary reduction assembly and long tail sections, the 231 can be a mild hindrance when performing some swaps with long engines and/or transmissions. This problem is exacerbated by suspension lifts and resulting u-joint angularity. It is recommended that a heavy-duty short tail shaft conversion be performed to reduce the length of this unit. Another thing to be aware of is the tendency of the slip-type yoke to pull and separate from the transfer case during extra articulation of the rear axle. Unless you own a TJ model 231, you will lose the ATF lubricant from the case if this occurs.

Input Gears and Compatibility
The important variations to look for in this transfer case when planning a conversion lie in the input gear. It comes in 21 spline and 23 spline versions with long, medium (rare) and short variations.

The long shafts protrude 2.1" from the front mounting face of the transfer case. The short version protrudes 1.2". The rare medium shafts protrude 1.7". You should be careful when swapping any of these gears due to different bearing widths and gear teeth. The following chart shows how to determine which input gear you should have by transmission. It is always good to verify first hand if possible.

Because of differences in the cut of the gear, as a general rule; one should not interchange pre-'95 gears with post-'94 gears. These years are not hard and fast delineations. If one is not absolutely certain, disassembly and inspection is required.

• Built since 1997 (there Was no 1996 wrangler)
• round headlights again
• first open jeep with air bags
• 93.4" wheelbase
• in 2004 the wrangler "unlimited" was introduced (aka TJ-L) with a 10" longer wheelbase and 10" extra length behind the rear wheels. only a 4-speed automatic is offered.
• more detail of late models is available at jeep.com

• ENgines

• amc 151 cu in (2.5L) i-4 (1997-2003)
• amc 4.0L (241 cu in) i-6 (1997-present)
• daimlerchrysler 2.4L DOHC i-4 (2004-present)

• Transmissions

• ax-15 5 speed manual
• chrysler 32RH 3-speed automatic (1997-2003)
• daimlerchrysler 4-speed automatic (2004-present)

• Transfer Cases

• nv-231j 2-speed (aka command-trac)

• rear Axle

• Dana 35
• dana 44 (optional, some models)
• dana m35 (some newer models)

• front axle

• dana 30

• dana m44 (rubicon model)