Transfer Cases & Components

The transfer case transfers engine power from the engine to both the front and rear axles on Four-Wheel Drive (4WD) and All-Wheel Drive (AWD) vehicles. It is attached to the transmission and transmits power to the differentials via driveshafts. There are part time and full time transfer cases, and AWD vehicles can be equipped with various devices such as center differentials and viscous couplings that allow the front and rear driveshafts to turn at different speeds.

Part time transfer cases have been around for many years and are the type most commonly found on vehicles used off-road. They allow the driver to select 2WD for dry road use, and 4WD for off-roading or ice and snow. When 4WD is selected, power is transferred to the front driveshaft by a chain or a set of gears in the transfer case. The transfer case also has a High and a Low range. Low range gears are used for extreme situations like slow speed rock crawling, where additional torque is needed. The traditional method of shifting between 2WD and 4WD and from High to Low range was by a lever connected to the transfer case with linkage, however many newer vehicles have a dash mounted selector switch and an electric transfer case motor for gear engagement. 4WD High can be selected while the vehicle is moving at low speeds, but 4WD Low can usually only be selected when the vehicle is stopped.

Full time 4WD transfer cases enable the vehicle to be used in 4WD on dry pavement, and depending on the vehicle, 2WD operation may not be possible. Vehicles with part time transfer cases cannot be driven on dry roads when in 4WD because, while the front and rear driveshafts turn at the same speed, the front and rear wheels need to turn at different speeds when turning a corner. Using 4WD on dry pavement with a part time transfer case can result in “transmission wind-up”, which can cause understeer and tire skid, and in extreme cases, driveline damage. A full time transfer case has a center differential, which allows the front and rear driveshafts to turn at different speeds.

There are too many variants of AWD for an in-depth discussion here. Many AWD systems are based on a Front-Wheel Drive (FWD) platform. Often these systems are configured to provide all or most of the power to the front wheels and only provide a percentage of power to the rear wheels when there is wheel slippage and traction is needed. Like full-time 4WD, AWD systems have a center differential, viscous coupling, or other method to provide power to both axles while avoiding understeer and driveline binding.