NP231 Transfer Case TechPosted in How To: Transmission Drivetrain on February 1, 1998 Comment (0)
Whether you're on the trail or just flat-towing your 4x4 down the road, exploding your truck's transfer case can really be a drag. The NP231 transfer case found in many late-model Jeep, Dodge, and GM vehicles has a fairly good reputation for a chaindriven, aluminum-cased unit, but improper towing procedures can cause it to explode, and some of the internals are iffy for big tires and lots of torque. These facts make the NP231 a good candidate for upgrades and modifications to make it a truly durable unit.
One of the first upgrades for Jeep owners should be the short-shaft kit, which converts the rear output yoke to a fixed style rather than a slip yoke, to allow removal of the driveshaft without fluid loss and fitment of a longer driveshaft for better driveline angles. The story "Instant Superhero" in our Dec. '96 issue showed a heavy-duty JB Conversions shaft installed in a '97 Jeep TJ, which is much stronger than stock and shortens the overall transfer case length. In the same story, we featured a 4-to-1 reduction unit that greatly increased the low-range ratio for outstanding off-road gearing without the highway drawback of lower axle gears.
But even these trick parts and pieces won't overcome a serious drawback of most NP231 cases-towing the vehicle without a true Neutral position. We talked to Jason Bunch at Tri-County Gear, who said that the standard NP231 such as that found in a Jeep YJ Wrangler connects the front and rear driveshaft together when it's shifted into Neutral, and drivetrain windup is relieved by the front-axle disconnect being disengaged. Unfortunately, when the engine is started and the case is put in Neutral, the vacuum-operated disconnect engages and stays engaged even after the engine is turned off. After a few hundred miles of towing, the resulting drivetrain windup can literally cause the transfer case to explode, just as though it were driven in four-wheel drive on the pavement.
The simple solution is not to start the engine before shifting the transfer case into Neutral for towing or to unhook the vacuum hoses to the front axle. But Bunch has some special parts he installs to provide a true Neutral inside the T-case for hassle-free towing, as well as some other beef-up parts for strength and longevity. Although some people believe that converting to an older-style cast-iron case is the only solution to the problem, the NP231 has proven to be a perfectly adequate unit. Follow along as Bunch shows how he modifies and beefs up the NP231 into a durable, towable transfer case that won't leave your rig with a bang.