The story
History: facts, each with the kind of source it rests on
Full-time systems drive both axles at all times through a center, or interaxle, differential, which lets the front and rear axles turn at different speeds. A part-time system has none, so its tires must slip a little in turns, and using it engaged on a hard surface winds up the driveline and in time damages it. A published reference
An open differential cannot limit the power sent to a slipping wheel, so the wheel that has grip gets little or none. Locking a differential joins its output shafts so they turn together, and this is generally done at the center differential. Some cars, such as the AMC Eagle and the Lexus RX300, used a viscous coupling at the center. A published reference