A transfer case
refers to one of a group of devices typically used in the drivetrain
or all wheel drive
vehicles. These devices are used to divide, redirect, and transmit/redirect torque
between a transmission
and the differential
Many 4x4s utilize a drivetrain composed of a transmission coupled to a front and rear differential. The torque must be divided and transmitted from the transmission (typically mounted lengthwise) to the front and rear.
The transfer case is connected via a driveshaft to the transmission and divides the power front to back (through more driveshafts). Some cases utilize different ratio gears to adjust the final drive ratio, some do not. Some cases are mechanically (using in-cabin levers) or electronically controllable to switch between 2-wheel and 4-wheel drive on the fly, effectively disconnecting one of its output paths.
Many all wheel drive vehicles utilize a transversely mounted engine and transaxle containing a differential that drives the front wheels. A transfer case containing a high-torque bevel gear is mounted to an output shaft on the transaxle to redirect the torque 90 degees via a driveshaft to the rear diff.
Transfer cases of many makes and designs are notorious for failure because of extreme forces and environment they are commonly subject to. In all wheel drive mode, typically the front and rear axles are connected together in the drivetrain so they act as a single unit; under very high loads, a transfer case can undergo severe shock loading that may fracture its internal gears or even its outer casing. Their seals may also begin to fail, allowing gear oil to seep out. Once this happens, the bearings and gears may sieze causing the drivetrain to lock. This has happened many times on all wheel drive Diamond Star Motors cars such as the Eclipse, Laser and Talon, and the Mitsubishi 3000GT VR-4 and Dodge Stealth, leading to transfer case recalls.
To keep your transfer case healthy, check it often for cracks, and/or obvious oil leaks, and replace the oil and seals as necessary.