How Transfer Cases Work

Four-wheelers are a diverse bunch. There are rockcrawlers, mud runners, trail hounds, and dune bashers. There are big-truck fans and small-SUV fans. Ford guys, Chevy guys, and Jeepers. Even a few Scout lovers.

But as diverse as we are, there is one thing that we have in common: If it weren’t for the invention of the transfer case, we’d have other hobbies, like needlepoint. Or, if the call of the wild were just too strong, we’d get stuck an awful lot.

So we owe a debt of gratitude to that big hunk of metal slung under the cab. But sadly, to some ’wheelers, their transfer cases are just that: a big metal case that somehow, magically, drives the front wheels and lowers gears for climbing rocks. If that’s as far as your T-case knowledge goes, give us a few minutes and we’ll fill you in on what that hunk of metal is doing. Or, if your T-case expertise only goes as far as the full-time box under your old Blazer, allow us to bring you up to speed on newer transfer case developments.

THE BASICS

No matter how simple or complex your transfer case is, this is what it does: It splits the engine torque coming from the transmission and delivers it to output shafts that connect to driveshafts leading to the front and rear axles. Most transfer cases designed for off-road use are able to mechanically lock the front and rear driveshafts together, so each end of the vehicle gets an equal amount of engine torque.

Depending on the age and design of your transfer case, torque is delivered to the front output shaft either through a set of gears or via a chain. Most present-day T-cases are chain-driven, as the chains have proven to be lighter and quieter than gears. Hard-core ’wheelers tend to prefer the old-school gear-driven cases, however, because they can withstand greater amounts of torque without breaking. And gears will never stretch like a chain might when it’s under extreme stress.

The age of your transfer case also has a lot to do with the material used to construct its housing. Older T-cases, like the NP203, NP205, and the Dana 300, were built with cast-iron housings. They’re heavy, but bullet-proof. Since the 1980s, the quest for fuel efficiency has caused manufacturers to turn to lighter materials, like aluminum and magnesium, for T-case housings. These lightweight materials don’t necessarily mean the cases are weaker, though. For example, Ford’s Super Duty pickups carry NVG271 transfer cases with magnesium housings, and even the venerable Hummer has an aluminum housing around its NVG242 HD. But certain aluminum housings—like the NP208 and NP231—have earned reputations for weakness and cracking under severe-duty conditions. So if you’re modifying your 4x4 with big tires or a torquey motor, make sure your stock T-case is up to the load, or consider a swap.

ONE SPEED OR TWO?

For most of us, our transfer cases share another common factor: They’re two-speed units, which means a set of gears in the case—almost like an auxiliary transmission—can be engaged to lower the gear ratios coming from the transmission. This action, which occurs when you engage low-range, multiplies the torque going to the driven wheels. For effective climbing and pulling power, you want as low a low-range ratio (as high a number) as you can get. Most current factory T-cases have low-range ratios of between 2.50:1 and 2.70:1, though you can go far lower for specialized applications. (See the Ultra Low sidebar for more on these transfer cases.)

Single-speed transfer cases, like those found in many cute-utes and all-wheel-drive cars, are designed to split engine power fore and aft without a direct mechanical link between the two ends of the vehicle, and without any provision for low-range gear reduction. They work fine on graded dirt roads and on wet pavement, but when the going gets tough, their abilities are limited.

PART-TIME OR FULL-TIME?

It used to be that almost all transfer cases fell into one of two categories: part-time or full-time. A part-time transfer case is one that can shift between two- and four-wheel drive, allowing the driver to select the mode that best suits road conditions. Wear on the front-drive components is reduced with a part-time system, since they’re not always engaged, and fuel economy is generally better for the same reason.

A full-time case is permanently in four-wheel drive, so no action on the driver’s part is required to engage the system. In order to use a full-time transfer case on hard-packed surfaces without binding the driveline, the case must be built to allow driveline slip between the front and rear wheels. Some (like the NP203) contain differential gears inside the case, while others use some sort of clutch or viscous coupling between the T-case and the front driveshaft. In both cases, there is typically a means to lock the slipping mechanism for a true 50/50 torque split when conditions warrant.

We say these “used to be” the only two classifications because 4x4 makers have, over the past decade, developed all sorts of part-/full-time hybrid transfer cases that have broadened the driver’s choices. Jeep was among the first with its NVG242 Quadra-Trac system, introduced on the Grand Cherokee, that enabled the vehicle to operate in two-wheel drive, full-time four-wheel drive (with the center diff open) and part-time four-wheel drive (with the center diff locked) in both high- and low-ranges. In 1999, Jeep further refined the system by mating a new NVG249 T-case with sophisticated locking diffs in the Dana 30 and 44 axles to create the Quadra-Drive system. General Motors also got into the hybrid T-case game in 1998, when it introduced the NVG246 Autotrac case, with a computer-controlled all-wheel-drive mode as well as the traditional part-time 4-High and 4-Low modes.

SWAPPING T-CASES

If you’re planning to make serious modifications to your 4x4’s wheel and tire size, or if you’re going to put a lot more horsepower through the driveline than the original maker intended, you should consider a transfer case upgrade as part of your vehicle’s buildup. As with any other component, T-cases are designed to handle a certain amount of torque loading and driveline stress. Chains will stretch or break and cases can crack if you exceed the T-case’s limits.

There are a lot of factors to consider when planning a T-case swap. You may lust after one of the big gear-driven cases, but are you sure it will fit under your truck? If it’s an older case from a salvage yard, is it going to need rebuilding? You also have to make sure the case will be compatible with your transmission, or that adapter kits are available, as mounting plates and output shafts vary in size and diameter. And because the transfer cases themselves vary in length, you may have to shorten or lengthen the front and/or rear driveshafts to get them to fit. Also, some T-cases are a mixed bag of good and not-so-good features. Your “ideal” swap candidate may have a weak output shaft, for example, or a case prone to cracking. Be sure to check with your local 4x4 shop or driveline expert while you’re doing your swap homework.

As with every other vehicle component, transfer case technology is advancing at a rapid clip. Computers, gerotor pumps, viscous couplings, and clutch plates are taking over the work that used to be done by square-cut gears, twin-stick levers, and the driver’s common sense. We’d guess this story, done 10 years from now, would be very different. But we’d also guess that, even 10 years from now, there’ll still be ’wheelers out there with a fondness for cast-iron cases with gears inside.