A part-time transfer case receives power from the transmission and transfers it to two or all four wheels of the vehicle. This enables the driver to place the vehicle from 2WD into 4WD (four-wheel drive) by moving of a gear selector, pressing a button or moving a slide. The case can be independent off the transmission or married, where it may even share the same case. The most common transfer case is independent. An independent transfer case attaches to the output shaft of the transmission and has a driveshaft for the front and rear axle.
They can either chain or gear driven. Many manufacturers today use chain-driven transfer cases because they're lighter and quieter than their gear-based counterparts. Gear-driven transfer cases have more strength, though, and are found on heavy applications. Transfer cases usually use ATF, but check with the manufacturer's specifications before servicing the fluid.
A vacuum controlled unit requires a specified vacuum from the engine (17-21 "Hg). If low vacuum is found at the case, check all the lines and reservoirs for cracks and wear. They're also controlled manually, hydraulically, or electrically. Manual control is typically a shifter found on the floor of the vehicle. An electronically controlled transfer case contains a control module and a series of buttons or slides found on or around the instrument panel. A hydraulically controlled transfer case is operated by a hydraulic pump and a clutch pack. An AWD (all-wheel drive) system's viscous clutch contains a thick viscous fluid.
AWD (all-wheel drive) vehicles remain in four-wheel-drive at all times. These are full-time four-wheel-drive systems that operate transparently to the driver. They're designed to handle bad weather or light terrain but are not considered off-road vehicles. The high range of an AWD vehicle is typically 1:1; for every turn of the output shaft, there is one turn at each of the axle shafts.