Ok, here is the 231 FAQ. I will discuss how to identify the different variations along with many of the options available to upgrade the existing transfer case.
The History:Let’s first discuss the company who makes the transfer case. New Venture Gear was a joint venture of General Motors Corp. and Chrysler Corp. The company specializes in turning out transmissions, transaxles, and transfer cases. New Process Gear is a division of New Venture Gear.
Models:The first NV231-J was seen in the second half of 1987 on the Wrangler platform. The previous transfer case was a NP207-J. The 'J' designation stands for Jeep. There are other applications such as 'D' for Dodge and 'C' for Chevy. There is also 'HD' for Heavy Duty. The transfer cases are easily identified by their metal ID badge located on the rear half of the housing. When the TJ came out in 1996 as a 1997 model, the transfer case wore a new name, New Venture Gear. All are geared to 2.73:1 in 4LO.
Chains:Now there are two different chains used on the 231's. The standard chain is 1" wide. These are found on the NP/NVG 231-J and NVG 231-C. The NVG 231-D and the NVG 231-HD use a 1 1/4" chain and sprocket set. This is done so that the Dodge Ram 1500 could use the transfer case with the 318/5.2L Magnum V8.
Planetary Gears:The standard gear set is a three grear planetary. This is found on the NP/NVG 231-J and the NVG231-C. The NVG 231-D and NVG 231-HD used a heavy duty 6 gear planetary. These are also found on the NVG 241's.
There are also gear reduction planetary kits. The first kits were developed under Mepco 4x4 then switched over to their sister company TeraFlex. The 4to1 replaced the front half of the transfer case with a larger unit that houses a 6 gear planetary and a 4:1 ratio. Newer units use a 5 gear planetary.
Mepco also designed an external housing that helds a secondary planetary. With a cable actuated shifter it was able to be used independantly or in conjunction with the existing tranfser case. Giving the user more gear combinations. This is not longer being sold but don't fret.
Klune V 'Extreme Underdrive' has picked up the torch. They have two different version of the Klune V. 'Goliath' with it's 2.7:1 and the 'David' with it's 4:1 ratio. The Klune V is able to use a remote cable or a stick shifter.
Input Shaft:Now, because the same transfer case is used with the different automatic and manual transmissions (AX-5, AX-15, 30RH, 32RH, 42RLE NV3550, NV1500, etc...) different input shafts have to be used.
Pre-'91 AW4, AX4, AX5 use a 21 spline short input shaft. While the pre-'95 AX-15, and Post-'90 AW4 uses a 23 spline short input shaft.
Peugeot BA-10/5 used a 21 spline long input shaft. The Post '94 AX-15, 30RH, 32RH used a 23 spline long input shaft.
ZJ's from '93-'95 all used a Medium 23 spline medium input shaft.
Now to make things a bit more confusing, the bearing width and gear cut on the shafts changed between early (pre '95) and late (post-'97) models. So in essence there are 7 different variations of the input shaft.
Shift Forks:The standard 231 uses 2HI/N/4HI/4LO shift pattern. Companies likes JB Conversions and Teraflex have come up with a new shift fork that allows the use of 2LO. These aren't cheap but if you want more choices in gears, it's worth it.
Speaking of shifting, the pre-'96 NP231-J's do not have a true Neutral gear. For the 1997 Production, NVG corrected this and gave the 231 true neutral. This is important for flat towing. The 2Lo will also give those with older 231's a true Neutral.
Output Shaft:The first conversions were seen in the early 90's by a company called Mechanically Inclined Technicians (M.I.T.). Later, Currie Enterprises jumped aboard. They would replace the stock shaft with a modified shaft. The kits were fairly expensive until Rubicon Express came around. They sold a Do-It-Yourself (D.I.Y.) kit that sold for $99. What you got was a flange and a back plate for the transfer case. You would have to cut, drill and tap the shaft. Complete disassembly was not required though.
Soon after, JB Conversion started selling Heavy Duty shafts. These were NVG 241 output shafts. They were not only larger but were shorter then the stock 231 shaft. They also required a extra support bearing and rear housing. These were the preferred kits.
That is until JB Conversion raised the bar and came out with a Super Short Conversion kit. This kit knocks off a few more inches by removing the mechanical speedometer sensor and replacing it with a compact tone ring and pick-up sensor. If you want the longest driveshaft possible then this is the only way to go.
All these are known as Slip-Yoke Conversion or Slip-Yoke eliminators (S.Y.E.). These are for the rear output shaft only. There have been some variations on the front output shaft that should be noted. Early models use a standard U-joint yoke. Cherokees and Grand Cherokees sometimes used what is known as a Double-Cardon yoke. (*will be discussed later.)
Drive Shaft:Technically this isn't a part of the transfer case but I'll address it here. The first driveshaft used with the NP 231-J was what is known as a 'reverse slip' drive shaft. These drive shafts use a slip yoke at one end that slides onto the spline of the output shaft in the transfer case. To adjust for compression and expansion with the suspension, the driveshaft will slide on the spline. This is considered a weak point since the driveshaft can and does slip off the spline. If torque is applied with minimal spline contact it might result in twisted or broken spline. Another problem is what to do afterwards. With the shaft removed, the transfer case is left with a large hole that will leak transfer case oil. To solve this, a yoke conversion is recommended. (*see above)
If angles are too extreme for the use of a single U-joint a double-cardon setup is required. This is when two standard U-joints are used together to share the angle load. Double-Cardon's are also known as Constant Velocity or CV but aren't really CV joints since they do not use a ball and socket setup. CV set-up's are weaker then a single U-joint but with upgraded U-joints it's neglectable.
