Replaces the mainshaft oil pump in an NP231 transfer case so the chain, sprockets and bushings keep getting fluid instead of running dry.
If your Jeep Wrangler, Cherokee, Dodge Ram or GM 4x4 has a transfer case that has started to whine, run hot, or shed fine metal onto the drain plug magnet, the oil pump is one of the first internal parts to suspect. This is a new gerotor oil pump assembly, part number ZTNP16209, with a 38-spline bore, built for the New Process NP231 family of chain-drive cases and the NP241, NP242 and NP243 units that share its pump design.
An NP231 has no external pump and no pressure circuit like an automatic transmission. Its only lubrication system is this small gerotor pump, which rides on the rear of the mainshaft and turns whenever the rear driveshaft turns. It draws fluid from the sump through a pickup tube and screen, then sends it forward through the drilled center of the mainshaft. That oil is what feeds the drive sprocket bushing, the planetary gear set, the range and mode hubs and the chain. Splash from the chain helps, but the bushings and the planetary depend on the pump flow, especially at highway speed in two-wheel drive when the chain is barely moving fluid around.
Because the pump is driven by the mainshaft, it only works when the vehicle is rolling, and its output rises and falls with road speed. That is why a worn pump tends to show up first as heat and noise on long drives rather than as an immediate failure.
Start with the fluid. Pull the fill plug, check the level, then drain a sample and look at the magnet. A noise that changes with road speed in both two-wheel and four-wheel drive points to the transfer case or the output bearings rather than the transmission; our guide to separating whine and clunk noises behind a 4L60E walks through how to isolate the source on a GM truck, and the 46RE noise guide does the same for a Ram or Jeep with a Chrysler automatic. If the case comes apart, look at the pump housing ears, the gerotor pockets and the matching pocket in the rear case half for wear marks. Scoring on the gerotor faces or a pump that spins with visible side play means it is no longer moving full volume.
This pump is listed for NP231, NP241, NP242 and NP243 transfer cases, and it interchanges with Mopar 4338936. In practice that covers many Jeep Wrangler, Cherokee and Grand Cherokee cases, Dodge Dakota, Durango and Ram applications, and GM trucks and SUVs that used these New Process units. The detail that matters is the spline count. These cases were built with more than one mainshaft spline count over the years, and this assembly is the 38-spline version. Count the internal splines on your old pump or the pump journal on the mainshaft before you order. If you are not sure which case you have, read the tag on the rear housing or send us the year and VIN and we will check it for you. You can also browse the rest of our transfer case and transmission hard parts.
A case that starved for oil rarely hurts only the pump. If you are in there, look hard at the chain, and consider a full NP231 master overhaul kit with bearings, seals and gaskets, plus a new NP231/NP233 drive chain if the old one has stretched.
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It is listed for NP231, NP241, NP242 and NP243 cases and cross-references to Mopar 4338936. The deciding detail is the spline count: this pump has a 38-spline bore. Count the splines on your old pump or the mainshaft before ordering, or send us your year and VIN.
Yes, the pump comes out once the rear case half is removed, so it can be done on its own. Since the case is already open, most people also replace the seals, check the chain for stretch and inspect the sprocket bushing for heat marks.
NP231 cases use automatic transmission fluid, not gear oil. Use the exact ATF listed in your owner's manual for the transfer case, and prime the new pump with that same fluid before you install it.
Low fluid, contaminated fluid and long highway miles are the usual causes. Metal from a worn chain or bushing circulates through the gerotor and scores it, and the pump then moves less oil, which speeds up the wear on everything else.
Inspect it closely. The pickup tube and screen must be clean, uncracked and fully seated in the pump. A damaged or loose pickup will let the new pump suck air and starve the case just like a worn pump.
