What Is a Transfer Case?

Transfer cases are what make 4wd vehicles able to gain traction in the rain or while crawling over rocks. Essentially, the transfer case takes the power from your transmission and divide it between your front and rear differentials and respective axles.

How do Transfer Cases Work?

The transfer case is a part of the drivetrain. So, the engine generates power, which is funneled to the transmission. The transmission turns the input shaft into the transfer case. When 4WD is not engaged, that power is used to rotate the rear driveshaft, which turns the gears in the rear differential and subsequently the axle.

When the transfer case is engaged, the power is split between the rear driveshaft and the front driveshaft.

Types of Transfer Case

There are many different types of transfer cases. While the most obvious division is between part-time and full-time units, there are many other distinctions.

Chain-Driven vs Gear-Driven

Transfer cases generally use either chains or gears to engage. While most modern transfer cases are chain-driven, gear-driven transfer cases still are popular. Especially with old-school off-roaders who value their reliability and torque capacity.

Gear-driven transfer cases are heavier, but their added robustness helps them hold up on the trails.

Divorced vs Married Transfer Cases

Most transfer cases are bolted directly to the transmission. This is a “married” transmission. A divorced transfer case is what happens when the transmission and transfer case are separated by a small driveshaft.

Divorced transfer cases are more common in classic vehicles, but many still swear by them today. There are some major advantages.

• You can use 2WD transmissions that might not have 4WD equivalents.
• Better driveline angles for extreme rock crawling.
• Can work on the transmission or transfer case without affecting the other.

But there are also some disadvantages. The ground clearance is worse, and there are multiple u-joints that can go south.

Most people who have been happy with divorced transfer cases upgraded their u-joints to accommodate for the additional wear and tear.

Part-Time vs Full-Time

The difference between full-time and part-time transfer cases should be fairly self-explanatory. Full-time transfer cases power both axles at the same time, all of the time, regardless of terrain. While this means it’s always on when you need it, fuel economy takes a nosedive with full-time systems. They also require either a differential or a viscous coupling.

Part-time systems are designed to only be turned on when the going gets rough, literally. Because they don’t have a differential or viscous coupling, the front and rear axles must move at the same speed. Driveline bind is released via tire scrub. If you have a part-time system and leave it on constantly, even while driving on dry pavement, you’ll eventually have some handling issues and driveline damage.

Transfer Case Gears

Generally, most transfer cases will have a high gear and a low gear setting. Low gears are sometimes called crawler gears or granny gears. Essentially, these gears provide more torque at slower speeds. For intense off-roading, low gears are essential.

These two speeds will be referred to as 4-high (4H) and 4-low (4L). A typical transfer case will show something like 2H, 4H, N, and 4L. This is a way of saying that while you can usually shift into 4H without stopping, the same isn’t true for 4L.

When Transfer Cases Go Bad

With how simple the internal workings are on a transfer case, they generally will outlast your engine and transmission. But if they do fall apart, they’re pretty easy to swap. Many people opt to do a transfer case swap on their own with great success. This is only true for 4WD vehicles though. AWD systems can be more difficult to work on yourself.

Transfer Case Maintenance

The number one thing to do to keep your transfer case working perfectly is to stick to the maintenance outlined in your owner's manual. Transfer cases require fluid to run smoothly. What type of fluid will depend on your specific transfer case. Common lubricants are automatic transmission fluid or oil.

Getting More from Your 4WD

The transfer case is the “how” behind the magic of 4WD. Without it, crawling up super steep inclines or mudding would be essentially impossible.

Despite all of the things it makes possible, the transfer case itself is a rugged and long-lasting piece of equipment. For people who only use their 4WD as intended, the transfer case can easily outlive the engine and transmission.

Understanding this component and giving it the maintenance it needs are easy ways to get the most from your transfer case.

Source: 4WD Transfer Case Basics, Fordification | How Transfer Cases Work, MotorTrend | How It Works: The Drivetrain, Popular Mechanics