A differential consists of a ring gear, pinion gear, side gears, spider gears, and bearings. All of these components may be encased in an axle housing or they may be located inside an automatic or manual transmission/transaxle. Positive-lock differentials, which may go by names such as Positraction or Traction-lock, have the ability to shift torque from the wheel that is slipping to the wheel that is not. Four-wheel-drive vehicles have a separate differential for each pair of wheels.
The differential transfers torque from the drive shaft or transmission output to the differential's drive axles. The spider gears and side gears allow the axles to turn at different rates, which is necessary when the car makes a turn. The outer wheel must turn faster than the inner wheel, creating a speed differential (which is how it got its name).
Have the axle lube level checked with every oil change. On most front-wheel-drive vehicles, the differential is part of the manual or automatic transaxle, and therefore does not require a separate differential lube check. If you have a rear-wheel-drive or four-wheel-drive vehicle, check the owner's manual to find out the recommended interval for differential lube changes.
Positive-lock differentials may require a different lube or a lube additive. Many manufacturers claim their differentials to be "lubed for life", unless it has been submerged in water. If you have a pickup or SUV, this may happen quite often, especially if you pull a boat. If this is the case, have the axle lube changed at least once a year.
In cold climates, you may want to consider changing the standard axle lube to a synthetic type. Synthetic lubes flow easier in cold weather, improving lubrication. With a 4x4, this is doubly the case because of the two axles. Always use a lubricant that meets or exceeds the manufacturer's lube specifications. When cared for properly, differentials provide many thousands of miles of trouble-free operation. When a differential problem does occur, symptoms may include: a high-pitch sound when accelerating or decelerating, clunking when accelerating or when shifting between reverse and drive, or a howling sound. Differential problems should be checked out as soon as possible by a qualified service professional.