What does a transfer case do? How 4WD and AWD split power

One input, two outputs

The transmission sends its output into the transfer case. Inside, a chain or a gear set drives a second output shaft. One shaft goes to the rear axle and the other to the front axle. Each axle still has its own differential between its two wheels.

Ford’s description of its part-time system is a clear example. In 2H, power goes to the rear wheels only. In 4H, the transfer case mechanically interlocks the front and rear driveshafts so that both axles receive power. Ford calls this a two-speed transfer case because it also offers a low range.

A full-time all-wheel-drive car uses a different arrangement. Subaru’s description of its system has power at both axles all the time, with no mode for the driver to select. The work of dividing power happens inside the unit.

Sources: Ford Motor Company ↗ · Subaru of America ↗

Why locked four-wheel drive is not for dry pavement

In a turn, the front wheels follow a wider path than the rear wheels. The front driveshaft must turn slightly faster than the rear. A part-time transfer case in 4H locks the two shafts together, so they cannot differ. Something must give.

On snow, sand, or mud, the tires slip a little and release the difference. On dry pavement, the tires grip, and the driveline winds up instead. Ford states that 4H is not intended for use on dry pavement. The reason is this binding, which stresses the transfer case, the axles, and the tires.

A center differential solves the problem the same way an axle differential does between two wheels. Eaton’s explanation of an open differential applies: it lets two outputs turn at different speeds while both receive torque. That is why a full-time system can run on dry roads.

Sources: Ford Motor Company ↗ · Eaton ↗

Low range multiplies torque

4L adds a second gear reduction inside the transfer case. Ford describes it as locked four-wheel drive with additional gearing for increased torque. The wheels turn more slowly for a given engine speed, and the turning force at the wheels rises in the same proportion. The gear-ratio arithmetic is the same as in the transmission.

Ford names the uses: deep sand, steep grades, and pulling heavy objects. It also describes the procedure. The transmission goes into Neutral before the shift to 4L. The shift must not happen while the rear wheels slip or while the vehicle accelerates. Low range is a tool for slow work, not a mode for the road.

Sources: Ford Motor Company ↗ · University of California San Diego ↗

Automatic modes and crossovers

Ford’s 4A mode is electronically controlled four-wheel drive. The on-demand transfer case uses sensor inputs to decide how much torque goes to the front wheels, and Ford describes it for varied road conditions. The driver does not need to choose when the surface changes.

Many crossovers and sedans with all-wheel drive follow the automatic idea without a selectable transfer case. Subaru’s system keeps both axles engaged all the time. Other designs send power to the second axle through a clutch when the system detects slip. The badge says AWD in both cases. The manual says how the system behaves, and whether it has any mode for the driver.

Sources: Ford Motor Company ↗ · Subaru of America ↗

Common misunderstandings

All-wheel drive and four-wheel drive are not two names for the same thing. A part-time 4WD system has modes and a low range and needs a slippery surface in 4H. A full-time AWD system runs on any surface without a selection.

Driven wheels help the car move. They do not help it stop. Braking and cornering depend on the tires and the brakes, which are the same four contact patches in any mode. A 4WD badge does not shorten a stopping distance on ice.

4L is not an extra-grip mode for a wet highway. It is a slow, high-torque mode for off-road work. Ford’s description reserves it for deep sand, steep grades, and pulling heavy objects.

Sources: Ford Motor Company ↗ · Subaru of America ↗ · National Highway Traffic Safety Administration ↗

Questions people ask

Is AWD the same as 4WD? No. AWD usually describes a full-time or automatic system with no modes. 4WD usually describes a part-time system with 2H, 4H, and 4L.

Can I use 4H on a wet highway? Ford says 4H is for off-road or winter conditions such as deep snow, sand, or mud, and not for dry pavement. For varied on-road conditions, Ford describes 4A.

When do I use 4L? For slow, high-torque work: deep sand, steep grades, pulling heavy objects. Shift with the transmission in Neutral and the vehicle moving slowly or stopped, as the manual describes.

Do I need 4WD in snow? It helps the car start moving on a slippery surface. Winter tires change what the car can do in every direction, including stopping. Decide on tires first, then on the drive system.

Sources: Ford Motor Company ↗ · National Highway Traffic Safety Administration ↗