NP231 Transfer Case Master Overhaul Kit for Jeep

This is the Crown Automotive 231MASKIT master overhaul kit for the New Process NP231 (NP231J, Command-Trac) transfer case, the part-time chain-drive unit Jeep hung behind the AX15, AX5, NV3550, AW4, 32RH and 30RH in the Wrangler YJ and TJ, Cherokee XJ, Grand Cherokee ZJ and Comanche MJ. If you have a Jeep with Command-Trac, this is the case under it.

The NP231 dies in predictable ways. The rear output seal weeps down the slip yoke until the case runs low and the chain starts slapping. The front and rear output bearings go dry and turn into a whine that changes with road speed, not engine speed. The case-half sealer gets old and the unit sweats at the seam. None of that is worth fixing one piece at a time, because every one of those repairs means pulling the case, splitting it, and setting it back up.

So when the case is open, you replace the wear set in one pass. Drain it, pull the case from the transmission adapter, split the halves, press out the old bearings and drive in the new ones, set the new input, front output and rear output seals, then reseal the halves with the RTV included in the kit and torque the case back together. Refill with ATF+4 and cycle the shift lever through 2H, 4H and 4L before you put it on the ground.

Crown part number 231MASKIT. This is a bearing, seal and gasket overhaul set - it does not include the drive chain, planetary, or shift fork, so inspect those during teardown and order them separately if the chain has stretched or the fork pads are worn through.