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Q: Please could you explain the operation of the diff lock system on a Land Rover.
A:The diff lock (as used in coil sprung Land Rovers ) functions as follows.
This unit is a center diff lock, which means that it operates BETWEEN axles.
A permanent 4x4 vehicle (Coil sprung Land Rover i.e. Defender Rover/RR) supplies driving torque to both axles equally in hard surfaces. Due to the differing route followed by the front and rear axles during a corner, the system uses a center differential. This center diff prevents "wind-up" on hard surfaces i.e. it lets the axles turn at differing speeds.
When you get into a situation where less traction is available or one wheel (any wheel) is in the air the vehicle will loose traction entirely and stop moving. All power is "escaping" from the airborne wheel, this is the center diff doing it's job as per normal operation.
In order to get past this, a center diff lock has been designed in to the diff: When the center diff is locked the diff effectively no longer exists i.e. the front and rear axles are locked together and driving power is now sent equally and unconditionally to BOTH axles.
The net effect of the center diff being locked is that power is available to both axles and as such, at least one wheel on each axle must spin for the vehicle to loose traction.
Vehicles with selectable 2x4 and 4x4 like Leaf sprung Land Rovers, do not have a center diff lock because the do not have a center diff at all. When the vehicle is in 2wd only the rear axle is driven while the front is disabled. When 4wd is engaged, the front and rear axles are locked together and power is again sent equally and unconditionally to both axles, therefore you can see that a Defender or Range Rover with center diff locked is identical to a Leaf Sprung Land Rover on 4wd.
So, where do I lock the center diff and where do I unlock it ?
If you are going off tar where there is a possibility of loosing traction, engage the center diff lock. i.e. Up and down hills, mud, ice, snow, uneven surfaces.
Do not use any diff lock on tarred / concrete surfaces or any similar high traction surface, the difference in front to rear axle speeds will cause extreme torque build-up in the transmission to such an extent that transmission components will wear excessively and/or snap. This build-up is called "wind-up", Even a little hard surface driving in diff lock will cause the diff lock to not dis-engage due to transmission tension
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