Transfer Case Teardown
The New Process (or New Venture Gear-same guys) 231 transfer case has to be one of the most popular factory-equipped T-cases ever used in a 4x4. It's found under a multitude of Jeeps (Wranglers and Cherokees), Dodge, and Chevy 4x4s. It's a chain-driven transfer case with an aluminum housing and a 2.72:1 gear ratio (though these three traits are common with many transfer cases built today). If you find an NP231 under your vehicle, it will likely have a "J" (Jeep), "C" (Chevy), or "D" (Dodge) after the numbers. All of the versions of the 231 T-case can be improved, but some are stronger than others, and some have better qualities than others. Dodge, for example, used an NP231HD with a bigger chain, and sometimes six planetary gears instead of the normal three found in most 231 T-cases.
We have an NP231J case installed under one of our XJ Cherokees that was working just fine, but we nonetheless felt the need to beef it up before we caused any serious issues with it. By building the transfer case up now instead of waiting until something breaks, we can avoid having any broken parts potentially maiming other parts of our transfer case.
When it comes to NP231 upgrades, there's one shop that has it covered better than the rest-JB Conversions. The transfer case specialists and originators of the slip yoke eliminator (SYE) have everything you need to give 1-ton strength to your stock 231 transfer case. For our own personal T-case needs and wants, we were after a heavy-duty chain kit, a fixed yoke on the back of our case, and a stronger planetary gearset. We weren't concerned with any 4:1 low range options and instead wanted to keep the 2.72:1 factory low range since this Jeep was going to be an all-around off-road toy and daily driver. Keeping a 2.72:1 gear ratio allows us to still use low range in the sand to climb dunes with, while 4:1 would have been too low for higher-speed sand use.