The drive train serves two functions: it transfers power evenly from the engine to all drive wheels and it varies the amount of torque. The drive-train may include the differential, a transfer case (in four-wheel and all-wheel drive cars) and the transmission.
The differential is the mechanical component that spins both the left and right wheels, but lets them rotate at different speeds so that the vehicle can make turns. When a vehicle makes a turn, the outer wheel has to turn faster than the inner wheel, due to the difference in the length of the paths they take. The differential is located between the two wheels, and is attached to each wheel by an axle shaft.
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Four-wheel drive vehicles have a separate differential for each pair of wheels, connected through a transfer case. Differential fluid provides lubrication to gears, bearings, shafts and other internal components. Heat, pressure and friction can slowly breakdown the additives in the differential fluid. In addition, small particles of metal and clutch material may come off as they wear and these particles can mix with the fluid. Fluids can also become contaminated with water.
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Transfer Case Service
Four-wheel drive and all-wheel drive vehicles have transfer case service. This gearbox divides the transmission power output between the rear differential and the front differential. Most transfer cases allow the front wheels to be disengaged and spin freely during normal driving conditions. When desired, power is transferred to all four wheels.
Transfer case fluid provides lubrication to gears, bearings, shafts, and other internal components. Heat, pressure and friction slowly breakdown the additives in the transfer case fluid. In addition, small particles of metal may come off the gears as they wear; these metal particles can mix with the fluid. Fluids can also become contaminated with water.