Permanent four-wheel drive

All current Land Rover products have permanent four-wheel drive transmission (known also as full-time). Range Rover was the founder vehicle with this system way back in 1970.

Land Rover first used it in 1980 with the launch of the Stage One V8 and three years later it became the standard transmission system for the 'new' 110, followed a year later by the 90. Owners of 101s will be quick to point out that these vehicles also used this system for the few years that they were produced for military orders.

The system incorporates a centre differential to prevent 'transmission wind-up'. This is the term given when any four-wheel drive vehicle, having a fully locked up centre transfer case between the front and rear propshafts, winds up when the vehicle is driven around a corner on tarmac or concrete. This occurs when the road wheels on the outside of the bend have to travel a lot further than the wheels on the inside of the bend.

Either pulling the button upon pre-1982 Range rovers should lock this centre differential, or pushing the low range lever over to the left on later models; likewise on Land Rover 90, 110,130 and Discovery, when traction is likely to be lost at one or more wheels.

Range Rovers made after 1988, or thereabouts, is the exception to the rule as they are fitted with a user-friendly automatic 'viscous control unit' (VCU). In conditions such as ice, snow, and mud or on other surfaces requiring maximum traction, uncontrolled spinning of wheels will be limited by the viscous control unit, which is combined within the third centre differential.

To make a point I have purposely not engaged the centre diff lock while cross axled ...

... but with it engaged, no problem - drive both axles

The action of the VCU is automatic and its effect is to distribute the driving torque from the transfer box to the axles in proportion to the available adhesion. In simple terms, when the VCU sense one propshaft revolving at a different speed to the other, the unit 'locks up' to ensure that a proportion of drive goes to the axle with the weight on it, for example, when climbing or descending hills.

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(c)Goodwinch Limited 2006