Transfer case

A transfer case is a part of the drivetrain of four-wheel-drive, all-wheel-drive, and other multiple powered axle vehicles. The transfer case transfers power from the transmission to the front and rear axles by means of drive shafts. It also synchronizes the difference between the rotation of the front and rear wheels, and may contain one or more sets of low range gears for off-road use.
Types.
Transfer cases used on "part-time" four-wheel-drive off-road vehicles such as trucks, truggies, rock-crawling vehicles and some military vehicles generally allow the driver to select 2WD or 4WD, as well as high or low gear ranges. Those used in sports cars and performance sedans are usually "transparent" to the driver; there is no shifter or select lever.
Drive type.
Gear-driven.
There are two different types of internal power-transfer mechanism found in most transfer cases. Gear-driven transfer cases use sets of gears to drive either the front or both the front and rear driveshafts. These are generally strong, heavy units that are used in large trucks, but there are currently several gear drive cases in production for passenger cars.
Chain-driven.
Chain-driven transfer cases use a chain to drive most often only one axle but can drive both axles. Chain-driven transfer cases are quieter and lighter than gear-driven ones. They are used in vehicles such as compact trucks, full-size trucks, Jeeps, and SUVs. Some off-road driving enthusiasts modify their vehicles to use gear-driven transfer cases, accepting the additional weight and noise to gain the extra strength they generally provide.
Housing type.
Married.
Transfer cases are also classified as either "divorced"/independent or "married". Married transfer cases are bolted directly to the transmission, usually between the transmission's output shaft and the rear or main driveshaft. Sometimes a married transfer case is an integral part of the transmission and the two components share the same housing or "case", as is commonly found on recent Subaru products and some other all-wheel-drive cars.
Divorced/independent.
A divorced or independent transfer case is completely separate from the transmission. It is located further down the driveline than a married transfer case and connected to the transmission output shaft by a short driveshaft. Independent transfer cases are used on very long wheelbase vehicles, such as commercial trucks or military trucks. This setup is also optimal for modified 4x4 because it's easier to change engine and transmissions, preserving the original 4WD system.
Transfer case shift type.
M.S.O.F..
Manual Shift On-the-Fly transfer cases have a selector lever on the driver's side floor transmission hump and may also have either two sealed automatic front axle locking hubs or two manual front axle hub selectors of "LOCK" and "UNLOCK" or "FREE". To engage the four-wheel-drive system the vehicle must be moving at a low speed. The speed at which 4x4 can be engaged depends on the vehicle. This is only for the four-wheel-drive high setting. To engage the four-wheel-drive low setting, the vehicle must be stopped and the transmission must be shifted to neutral, then the four-wheel-drive low can be selected.
E.S.O.F..
Electronic Shift On-the-Fly (ESOF) transfer cases have a dash-mounted selector switch or buttons with front sealed automatic locking axle hubs or drive flanges. Unlike the manual transfer case, this system has a transfer case motor. To engage the four-wheel-drive system the vehicle must be moving at a slower speed. The speed at which 4x4 can be engaged depends on the vehicle. This is only for the four-wheel-drive high setting. To engage the four-wheel-drive low setting, the vehicle must be stopped and the transmission must be shifted to neutral, then the four-wheel-drive low can be selected.