NP231 Transfer Case Oil Pump Assembly, 38 Spline

The oil pump in an NP231 sits on the back of the mainshaft and is the only thing moving fluid inside the case. It picks up ATF from the bottom of the housing and pushes it through the drilled passage in the mainshaft so the drive chain, both sprockets, the rear bushing and the front output bearing get fed while the case is turning. When the pump quits or loses volume, the chain and bushings run on whatever splash they can catch, and that is how a case that was quiet at 100k ends up whining and dumping metal in the pan.

This is the complete gerotor pump assembly, part number ZTNP16209, with a 38 spline drive. It fits the New Process NP231, NP241, NP242 and NP243 cases, which covers Jeep Wrangler and Cherokee, Dodge Ram, and GM full size trucks and SUVs depending on year and drivetrain. Match your existing pump to this one before ordering, since some late GM applications use a different pump housing and pickup.

Two reasons to replace the pump rather than reuse it during a rebuild. First, the gerotor gears wear and the pump simply stops making pressure, which is the failure behind most chain and bushing damage. Second, the well known pump rub problem: the pump housing walks against the inside of the rear case half and grinds through the aluminum until the case leaks fluid onto the driveway. If you are already in the case for a chain or a bearing kit, a new pump is cheap insurance against pulling the transfer case back out.

Bench notes: the pump goes on the mainshaft behind the rear bearing, and the pickup screen and tube must be seated and clear before the case halves go together. Prime the pump with clean ATF, check that the case halves close with no gap at the sealing surface, and refill with the fluid the application calls for. Torque the case bolts in a crossing pattern so the sealer sets evenly.