Transfer case is a power take off unit that sends power to both the
front and rear axle assemblies on a multi-wheel drive vehicle. They are
mounted on the transmission. Transfer cases drive the front output shaft
with either a gear train or a flat Hyvo-type chain. Transfer cases
designed for four-wheel-drive vehicles offer a low-range drive mode,
where all-wheel-drive vehicles typically have a single-speed transfer
case. The transfer case in an all-wheel-drive system contains a device
that allows for speed difference between the front and the rear wheels.
This could be a vicious coupling, center differential or other type of
gear set. The transfer case on a part-time four wheel-drive system locks the front-axle driveshaft to the rear axle drive shaft, so the wheels are forced to spin at the same speed. This requires that the tires slip when the car goes around a turn. On dry concrete, it is not easy for the tires to slip, so the four-wheel drive should be disengaged in order to avoid jerky turns and extra wear on the tires and drivetrain.
Some transfer cases also contain an additional set of gears that give the vehicle a low range. This extra gear ratio give the vehicle extra torque and a super-slow output speed.
Purpose
Transfer case routes torque from the transmission to both the front and rear axles. Depending on the design, the transfer case may provide equal amounts of torque to the front and rear axles. They have different modes and ranges of operation.
Maintenance
Transfer case should be checked at every oil change to ensure that it has enough lubricant. Many require periodic changes of oil or fluid to maintain peak performance. The common problems of transfer case include no four wheel drive operations, four-wheel drive operation only in some modes or the inability to switch modes.