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Building A Better Transfer Case: NP231J - 4-Wheel & Off-Road Magazine

Kevin McNulty Former Editor, Mud Life

The NP231 transfer case was introduced in the mid '80s on Jeeps and has been the staple in a number of four-wheel-drive vehicles ever since. The NP231 has 2H, 4H, Neutral, and 4L. When Chrysler took over Jeep this transfer case was coined the NVG 231J, or simply the 231J. The transfer case is a light-duty chain-driven unit that is very popular amongst four-wheel enthusiasts because of its rugged dependability.

131 1010 Building A Better Transfer Case Np231j transfer Case Kit Assembled Photo 34301847

The fulltime NP249 transfer case in our '97 Grand Cherokee works well in stock form, but needed to be swapped out because it's not as good for medium-to-hardcore wheeling. The NP249 can be found behind '93-'98 Grand Cherokees and has 4H, Neutral, and 4L. Some of the earlier units are known to have issues with viscous coupling, which acts as a limited slip inside the case. We personally don't care to drive around in four-wheel drive in normal driving conditions either, as it's tougher on the tires and drivetrain and can effect fuel economy.

We contacted JB Conversions regarding the swap for our Grand Cherokee. The company recommended the complete HD 231 transfer case, including a slip yolk eliminator tailshaft. Since we love to put things together ourselves, JB Conversions sent us everything needed to assemble the 231J, including new case halves. The result of this build is a high-quality transfer case that's perfect for a high-horsepower engine and tough four-wheel conditions.

PhotosView Slideshow JB conversion sells completely assembled transfer cases or master rebuild kits with all new parts for the 231J/D, 233, 241C/D, NVG241OR, Dana 300, and NP205. We were impressed with the 231 transfer case kit due to the high-quality parts and ease of assembly.

JB conversion sells completely assembled transfer cases or master rebuild kits with all new parts for the 231J/D, 233, 241C/D, NVG241OR, Dana 300, and NP205. We were impressed with the 231 transfer case kit due to the high-quality parts and ease of assembly.

It's necessary to know the input gear's spline count before ordering a transfer case or rebuild kit. The Grand Cherokee fitted with a 5.2L engine and automatic transmission usually came with a 23-spline input gear. It's a good idea to remove the transfer case from any vehicle to verify the spline count before ordering any parts. Available input gear spline counts are 21, 23, 27, 29, and 32. Also, inputs gear depths change throughout the years.

The input shaft is installed in the planetary gear with a large snap ring. This is an all-new, high-quality planetary gear designed with exacting tolerances for long life, and it will keep the HD transfer case quiet compared to other transfer cases.

The chain sprocket assembly should be liberally coated with transmission fluid before being installed on the main shaft. The new main shaft is heat-treated and machined to exacting tolerances.

We have seen a number of transfer cases explode on the trail under extreme use. The usual culprit that causes this disaster is almost always due to chain failure. The HD 231 kit includes an HD wider chain that is superior in strength compared to a factory chain.

Once the new HD chain is installed on the main shaft assembly and the front output shaft and sprocket, they can both be lowered into the transfer case. Both shafts should slide easily into place if both are lowered in evenly.

A good-quality silicone sealant like Great Stuff should be used to seal the case halves. There's nothing more irritating than a leaking transfer case and an oily driveway!

With the case assembled and all fasteners tightened to the proper specifications, we bolted the HD transfer case into the Grand Cherokee. The 231HD from JB Conversions is a direct, bolt-in replacement for any Dodge or Jeep NP231 transfer case.

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Building A Better Transfer Case: NP231J - 4-Wheel & Off-Road Magazine

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