CENTER DIFF LOCK

Manually operated center differential locking can be confusing...

Q Although I have had a number of older Range Rovers and several Land Rovers, I have never been sure of the purpose of the center differential; why there’s a ‘diff lock’ lever and what is accomplished by locking it. On the odd occasion I have tried it, when having trouble in deep mud, it hasn’t seemed to stop the wheels from spinning, so what’s it all about?

A   A permanent four-wheel drive system was adopted on the Range Rover, as opposed to the selectable four-wheel drive arrangement on the then current Series III utility vehicles. Whereas the Series III was designed to be used in 4X4 mode only in conditions of bad traction, the Range Rover was 4X4 all the time…and this is able to cause several transmission problems.

The gearbox of the permanent 4X4 system drives the front and rear prop shafts, which are linked to the four half shafts through the differential in the front and rear axles.

When turning corners the outside wheels travel further that the inside; the axle diffs on all cars enable this to retaining equal drive to each wheel.

However, while cornering, the rear wheels travel less distance than the fronts, because of the way the rear of the vehicle ‘cuts’ the corner. If the front and rear axles are linked solidly, as in many selectable four-wheel drive vehicles, this causes tire wear and can lead to “transmission wind-up’, excess tension within the system.

The center differential overcomes this in the same way as axle differentials compensate for individual wheel speeds on the same axle. The problem is that, in some situations, the action of the center diff leaves only on wheel ‘driving’ the vehicle.

On Land Rovers this is resolved by ‘locking’ the center diff by moving the transfer box lever to the left. On early Range Rovers and Land Rover One Ten V8s it is locked with a separate switch.

However, using ‘diff lock’ only locks up the drive to the two axles, and doesn’t have any effect on the individual axle diffs; consequently, in a typical ‘crossaxle’ situation, with only one front and one rear wheel in contact with the ground, or getting sufficient grip to maintain traction, you can still lose all traction because the axle diffs are taking the drive for each axle to the point of least resistance- the two wheels not in contact just spin uselessly, even with the center diff lever in the locked position.

What the center diff lock does achieve is to ensure that if both wheels on one axle are gripping they will continue to drive the vehicle if traction is lost on the other axle. This means that use of the diff lock will keep you going in many situations; but remember – if you lose grip from one front wheel and one rear wheel the center diff lock cannot help you.