A transfer case is the center of the drivetrain of four-wheel drive and some all-wheel drive vehicles. Mounted to the back of the transmission, it splits engine power and sends it to the front and rear axles by means of front and rear drive shafts. It also synchronizes the difference in rotation of the front and rear wheels, and may contain one or more sets of low range gears for off-road use.

Low range gears in the transfer case allow the vehicle to drive at much slower speeds while still operating within the usable power band or RPM range of the engine. This also increases the torque available at the axles. Low-range gears are used during slow-speed or extreme off-road maneuvers, such as navigating dangerous roads, rock crawling, or when pulling a heavy load. This feature is often not present on all-wheel drive vehicles.

On some vehicles, such as four-wheel drive trucks or vehicles intended for off-road use, the transfer case is controlled by the driver. The driver can engage the transfer case into either two-wheel drive or four-wheel drive mode. This is sometimes accomplished by means of a shifter, similar to that in a manual transmission. On some vehicles, the transfer case may be electronically operated by a switch or button. Others have transfer cases that are not selectable and are permanently locked into all-wheel drive mode. This type of transfer case may also be called the center differential.

If you dissect a transfer case, you will find a few components that are common in all transfer cases, such as an input shaft. An input shaft is spun by the transmission and is connected to two output shafts: one that spins the front driveshaft, and one that spins the rear driveshaft. Most modern transfer cases also have a differential. The transfer case differential is just like the differential in the axles. It allows one output to spin at a different rate than the other to avoid driveline bind on hard surfaces.

The three basic types of transfer cases are part-time 4WD, full-time 4WD, and active 4WD.

Part-time 4WD is the most common type of transfer case. It allows you to operate the vehicle in two-wheel drive, four-wheel drive high-range (4Hi), and four-wheel drive low-range (4Lo). The 4Hi operation typically utilizes a differential to improve drivability. Part-time 4WD systems offer smoother operation on pavement and better fuel economy since the front driveshaft and axle can be cut off completely from the power. The strongest transfer cases are generally part-time systems because they are designed for real off-road use, often in truck and utility vehicle applications.

Full-time 4WD is the simplest type of transfer case. It sends power to the front and rear axles all the time. To eliminate, or at least diminish, driveline bind on hard surfaces, this type of transfer case also uses a differential in high range. Some offer a 4Hi lock position which locks the differential to improve traction on slippery surfaces, but will also cause binding when operated on dry pavement.

Active 4WD is the easiest type of transfer case to use because it does not require any input from the driver. A variety of full-time and part-time systems have been developed that use electronic, computerized, or mechanical means to adjust the amount of power delivered to the axles according to wheel slip. They have a variety of names and levels of performance, but they provide some of the benefits of a part-time system without the owner ever having to switch anything. Active 4WD was designed for smooth operation without any input from the driver and can be found on everything from trucks to luxury sports cars.

The primary difference between the transfer case in a 4WD vehicle and that of an AWD vehicle is that the latter does not offer an additional torque multiplying low gear ratio used for serious off-roading. In all wheel drive vehicles, there is a need to allow for driveline slip to avoid binding the driveline. This is accomplished by including differential gears inside the transfer case.

The transfer case is always working, whether you use the four-wheel drive on your vehicle or not. Your transfer case fluid level and condition should be inspected every time you get an oil change. Transfer cases may be filled with gear oil, automatic transmission fluid (ATF), or specialty lubricants. It is important to regularly inspect the transfer case for any damage, leaks, or other concerns. The fluid level and condition should also be inspected, as transfer case fluid may leak from the output shaft seals, input shaft seal, case gaskets, or fluid inspection and drain plug gaskets.

If you are considering purchasing a vehicle with a transfer case, have an ASE certified technician visually inspect the case exterior, the fluid level and condition, and test drive the vehicle to make sure the four-wheel or all-wheel drive system operates properly. Follow the vehicle manufacturer’s recommendation regarding transfer case fluid type and how often it should be replaced. If the transfer case oil has become contaminated, shows evidence of metal particles, or is black in color, it should be replaced right away.