New Process 231
In 1987 Jeep began the gradual introduction of the New Process 231 transfer case. The Np231 was initially introduced in Cherokees, followed shortly thereafter into the Wrangler. Jeep enthusiasts initially received the transfer case with mixed opinions. Many jeepers hoping for the grand old days of the gear driven Dana 300 were obviously a little disappointed. Skeptics aside, the medium duty NP231 proved itself reliable and capable for most applications. The transfer case is by no means perfect, but coupled with aftermarket upgrades; the NP231 transfer case meets the expectations of most jeep consumers.
Identification and Features
The chain driven NP231 was offered with a driver side front driveshaft shaft assembly only. The rear output shaft offer a centered slip yoke design. The slip yoke feature caused the transfer case to be somewhat long at 21.5”. The Np231 continued to utilize the round circular six bolt pattern as seen on the previous NP207 and Dana 300 transfer cases. The Np231 case was a die cast aluminum 2 piece design. The standard 231 weighs approximately 70 lbs.
One of the easiest ways to identify the Np231 is the metal tag located on the back of the transfer case. This red and silver tag offers some valuable information concerning the transfer case. This tag indicates from top to bottom: the model type, production build #, date, and lastly 2:72; identifying the standard low range for the Np231.
The input shafts were available in either 21 or 23 splines. These inputs were further offered with a short or long design. These variations were determined by year, engine size, and transmission options. The chart below gives a general breakdown of not only length and spline count but also includes a pitch difference found on the internal planetary gear portion of the various input shafts.
The Np231 operates as 4 basic functions: 2H, 4H, N and 4 low. Since the transfer case is a chain driven design, a true twin stick is not functionally available. There are internal aftermarket kits available which allows for a 2wd rear low only function.
The speedometer feature of the NP231 can be broken down by three different styles. The speedometer was always located in the rear nose cone of the NP231. The 1987-1991 used a standard mechanical drive which basically turned a cable. In 1992, The Np231 featured a similar mechanical drive however the spinning gear was converted to a digital square wave signal. The interchangeable plastic gears were the same as previous mechanical units. These replaceable gears allow the consumer to quickly and easily recalibrate their speedometer when doing common tire and ring and pinion changes. The TJ introduced a similar vehicle speed sensor (VSS) however the speedometer drive used a new shorter gear assembly.
Strength and Upgrades
The NP231 has long been the topic of many campfire debates as well as internet forum arguments. There has been much discussion as to the strength of this transfer case. In some corners, the transfer case has been proven durable and dependable; capable of handling V8 power, larger tires and moderate off-roading. For others, they view the transfer case as problematic with questionable manufacturing design choices. Both ranges of opinions hold some validity. Luckily with the support of the aftermarket, numerous fixes and upgrades are available. These include wider chains, heavy duty planetaries, low range gears, and fixed yoke kits. The most common upgrade to the NP231 by far is the “fixed yoke kit” also known as an SYE (slip yoke eliminator). More information can be found here PNNP231-SYE concerning the NP231 fixed yoke kit.
These upgrades eliminate certain pitfalls and clearly strengthen the transfer case. For some off roaders, the cost of upgrades with limited results just doesn’t fill the void compared to a gear driven transfer case. The consumer should heavily weigh his driving style, build choices, and most importantly: his options. We have seen many Wrangler owners who have installed numerous upgrades including lower gearing to the tune of $1600-$1800 only to replace the transfer case. Aftermarket transfer cases such as the Atlas have brought back dependable gear driven options to the market.