A front differential is a drivetrain component in rear-wheel-drive and some all-wheel-drive vehicles. The differential is a gearbox located at or near the center of the front axle where it is attached to the driveshaft. There, a series of gears changes the direction of the rotational movement of the driveshaft, turning it ninety degrees and out toward the wheels.
A differential gets its name because it also allows each of the attached axles (and wheels) to turn at different rates of speed to accommodate your vehicle in turns. Without that ability, your tires would drag or skid rather than roll smoothly along the ground.
Front differential service is not very different from that of an oil change. While the procedure varies from one vehicle make and model to the next, the steps are similar. First, a drain plug is removed to allow the gear oil to escape. A technician collects the old oil for disposal or recycling. Once the old lubricant is fully drained, the drain plug is reinserted, and fresh gear oil is pumped into a different inspection/fill hole in the differential to the proper level.
Some differentials do not feature a drain plug. Instead, these models require that the front differential cover plate be removed in order to drain the old fluid. When this is the case, the service is more complex and often requires other parts to be purchased, such as a new gasket for the cover. On the other hand, this procedure also allows the technician to inspect the inside of the differential.