NP203 Transfer Case

The full-time four-wheel-drive NP203 transfer case is one of the most standard items found in ’70s fullsize 4x4s. Big, heavy, and relatively strong despite its chaindrive, this box can be converted to part-time with a simple gear or shaft swap inside the case. But few people understand how the insides tick on this case, so we thought we’d clue you in to some of the innards and theory without a complete rebuild or overhaul.

The full-time aspect of the 203 is all in the rear section of the unit. The case has an internal differential that selectively splits the power to the front and rear axles when the shifter is not in one of the locked positions. When shifted into Hi-Loc or Lo-Loc, the differential locks to provide equal power to both driveshafts. If you removed either driveshaft and tried to drive the truck with the case in the nonlocked position, the internal diff would send all the power to the yoke with the missing shaft. It’s just like an open rear axle differential, where all the power goes to the tire with the least traction. However, if the case was shifted into a locked position, power would be equally distributed to the front and rear, so you’d be able to drive the rig as long as one driveshaft was still in place.

Many of you with 203s are running around without a front driveshaft and with the shifter in Hi-Loc since that’s the free method of overriding the full-time 4x4 feature. But after a while you’ll probably notice a clunk emanating from the case. Whether your rig is a Dodge, a Chevy, or a Ford, the NP203 will develop internal wear when subjected to this type of use. Minimal driving isn’t a problem, but continual driving in the Hi-Loc position without the front shaft installed causes the internal coupler on the differential to develop a lot of slop, eventually causing a banging noise and then destruction.

The differential carrier is also an item that can shred with this type of abuse. Many 203 cases use a pressed-steel cage for the differential gears instead of the two-piece heavy-duty style, and Jason Bunch at Tri-County Gear says the pressed-steel unit wears out quickly if you do a quasi conversion this way. The best bet is to install a conversion kit in the 203 case and add locking front hubs to eliminate wear and drag on the front axle internal components. Even the ’74-’79 Dodge trucks and Ramchargers with the live-axle front end can be converted to use hubs with a MileMarker kit.

Such was the situation on our subject vehicle, a ’74 Dodge Ramcharger with a couple thousand miles in Hi-Loc—and a terrible clunk. The noise from the T-case got so bad it made us think the whole shebang was going to be scattered across the freeway during our rush-hour commute. Fortunately, it made it to Tri-County Gear, where Bunch diagnosed and inspected the dead Dodge case. After a thorough evaluation, the good Dr. Bunch recommended installing the MileMarker part-time kit into a rebuilt NP203 from Boyce Equipment—something that had been rolling around in the back of the Ramcharger for a while, adding ballast and just waiting for this opportunity.

Once finished, the new parts had eliminated the drivetrain clunk and provided extra peace of mind. Because of the conversion, the Dodge now has a two-wheel-drive low-range position, although we’re going to use it sparingly to prevent overstressing the rear axle. The case we got from Boyce Equipment was also a near-direct replacement for the Dodge, but many of the internals will also fit Fords and Chevys.