All-wheel drive vehicles are known, and have the advantage that, under difficult operating conditions, such as snow, ice, sand or the like, the traction available for the vehicle is substantially improved. In order to avoid starting difficulties of the vehicle if the wheels slip or are difficult to turn, it has been customary to provide lock-up elements for one or all of the differentials; the rear wheels, particularly, were often supplied with locking-type differentials in which the respective wheels are securely connected together so that spin of one wheel with respect to another is eliminated. Non-slip or low-slip differentials or lock-up differentials may be used in the position of any of the differentials in the drive system.
Lock-up differentials, or low-slip or non-slip differentials have a disadvantage; critical operating conditions may occur upon application of braking, so that the control of the vehicle can be impaired. This is particularly so if, in addition to the four-wheel drive and the no-slip or locking differentials, the vehicle has an automatic brake control system (ABS) in which the braking pressure, typically hydraulic pressure, applied to the respective wheels is controlled in accordance with slip of the wheels. Such ABSs are well known.