The 4 wheel drive system
The gearbox of the permanent 4x4 system drives the front and rear driveshafts, which are linked to the four halfshafts throughout the differentials in the front and rear axles. When turning corners, ouside wheels travel further than the inside wheels, the axle differentials on all cars enable this to happen while at the same time retaining drive to each wheel.
However, while cornering, the rear wheels travel less distance than the front ones. If the front and rear axles are linked solidly, as in many part time 4x4s when in 4-low, this causes excessive tire wear and lead to drivetrain wind up that can put a lot of stress in the system.
The center differential overcomes this in the same way as the axle differentials compensate for individual wheel speeds on the same axle. The problem is that, in some situations, the action of the center differential leaves only one wheel driving the vehicle.
The Discovery is equipped with the manual center locking differential to overcome the above problem. The center differential lock only locks the drive to the two axles, but doesn't have any effect on the individual axle differentials. Therefore, in a typical cross axled situation, where only one front and one rear wheel are in contact with the ground or getting sufficient grip to maintain traction, you can still lose all traction because the axle differentials are taking the drive to each axle to the point of least resistance. The wheels not in contact just spins, even with the center differential locked.
The center diff lock ensures that if both wheels on one axle have traction, they will continue to drive the vehicle even if traction is lost in the other axle. this means that use of the center diff lock will keep you going in many situations. But remember, if you lose grip in one front wheel and one rear wheel, the center diff lock can't help you.
The center differential lock can be operated on the move at any speed as long as there is no difference in speed between the wheels. so, make sure the wheels are all rotating at the same speed.
AVOID center differential lock on normal road surfaces.