NP203
Problems & Part-Time Conversion

The New Process 203 is the full-time four-wheel-drive case that came in a lot of mid-1970s full-size trucks and SUVs: Chevrolet and GMC K-series pickups, Blazers, Jimmys and Suburbans, Dodge W-series and Ramchargers, and Ford F-series and Broncos. It is a big, heavy case built in two sections. The rear section is chain-driven and houses a center differential. The front section is a separate range box that provides a 2.0:1 low range.

Because of the center differential, the NP203 drives both axles all the time. There is no 2WD position. The shifter has High, High Lock, Neutral, Low Lock, and on some versions Low. The "Lock" positions lock the center differential so the front and rear turn together, which is what you need off-road. In the unlocked positions the differential lets the axles turn at different speeds on pavement.

That design was the factory answer to "people forget to shift into 4WD." Fifty years later, it is mostly the reason these trucks get poor fuel mileage and wear out their front ends.

Common NP203 Problems

Center differential wear

The center differential is a planetary unit that is always working, even on a straight highway, because front and rear tires never turn at exactly the same speed. Worn pinions and side gears show up as a clunk on acceleration and deceleration, a whine under light load, and eventually metal in the oil. Mismatched tire sizes front to rear speed this up badly, because the differential is slipping every mile.

Chain stretch

The NP203 chain is wide and strong, but most of these cases have a lot of miles on them. A stretched chain slaps the case, clunks on throttle changes, and in bad cases grooves the housing. See chain stretch symptoms for what to look and listen for.

Leaks between sections

The range box, the chain case, and the differential section all bolt together, and every joint has a gasket. Old NP203s leak from almost every one of them, plus the output seals and the shift rail seals. Because the range box and the main case can share oil in some versions and not others, check both fill plugs, not just one.

Will not lock, or will not unlock

Many trucks had a vacuum-operated indicator switch and lockout. A dead lock light is usually a vacuum line or switch problem, not internal. A case that binds and hops in turns on dry pavement is almost always stuck in a Lock position, either from linkage or a stuck shift rail. Driving it that way hammers the front axle. Our 4WD binding in turns guide explains why.

Fuel economy and front end wear

Turning the front driveshaft, front differential, and front axle shafts all the time costs fuel and wears the front axle U-joints, wheel bearings, and steering knuckle parts. That is not a failure, it is how the case was designed, and it is the main reason people convert.

How the Part-Time Conversion Works

A part-time conversion kit replaces the NP203's center differential with a solid coupling. Once it is installed, the front and rear outputs are locked together at all times, exactly like the case is permanently in High Lock. You no longer need the differential, and you no longer need the Lock positions.

That means the truck must have locking hubs on the front axle. With the hubs unlocked, the front wheels spin freely on their spindles and the truck drives as a 2WD. When you need four-wheel drive, you lock the hubs and you are in 4WD High. The range box still works, so you also keep the 2.0:1 low range.

A few things to know before you buy:

• The front driveshaft will still turn. The kit locks the outputs together, so the front shaft and front differential spin with the rear. The hubs disconnect the wheels, not the driveshaft. You still save a lot, because the axle shafts, U-joints, and hubs are no longer loaded, but it is not the same as a true part-time case.
• Do not drive on dry pavement with the hubs locked. With the differential gone, the case behaves like any part-time case: front and rear are locked together and will bind in turns.
• Check your front axle first. If your truck came full-time, it may have drive flanges instead of hubs. You will need hubs that fit your axle and spindle.
• Do the conversion during a reseal. The case has to be split to swap the differential, so it is the right time to replace the chain, bearings, and every gasket.

The NP203 locking hub conversion kit is the part that does the job. Plan on a long afternoon to a full day with the case on the bench.

Convert, Rebuild, or Swap?

For a numbers-matching restoration, keep the full-time setup and just reseal it. For a truck that gets driven, the conversion is the cheapest path to better mileage and less front-end wear. For a truck running big tires and seeing hard trail use, many builders skip both and swap to an NP205, which is gear-driven and far lighter on maintenance. Read the NP205 guide if you are leaning that way.

Fluid and Maintenance

The NP203 has more oil capacity than most cases and the level matters. Check both the main case and the range box. Look for metal on the drain plug magnet; a fine gray paste is normal wear, but chips or flakes mean the differential or a bearing is failing. A case that has been run low will often make noise in the range box first. Use the fluid listed in your service manual for your year and make; requirements changed during production and between brands. For general guidance on picking a transfer case fluid, see our transfer case fluid buyers guide.

FAQ

Is the NP203 a full-time transfer case?

Yes. The NP203 uses a center differential so it drives both axles all the time. The Lock positions lock that differential for off-road use. There is no 2WD position in stock form.

What does an NP203 part-time conversion kit do?

It replaces the center differential with a solid coupling, locking the front and rear outputs together. You then use locking hubs to disconnect the front wheels for 2WD driving.

Do I need locking hubs after converting an NP203?

Yes. Without locking hubs the front axle would be locked to the rear at all times and the truck would bind in every turn on pavement.

Will an NP203 conversion improve fuel mileage?

Usually, yes. With the hubs unlocked the front axle shafts and wheels are no longer driven. The front driveshaft still turns, so the gain is real but smaller than a true part-time case.

Why does my NP203 bind in turns?

It is likely stuck in High Lock or Low Lock, either from linkage, a stuck shift rail, or a bad indicator switch misleading you. Driving locked on dry pavement damages the front axle.