This LRO reader emailed LRO.firstname.lastname@example.org seeking an explanation on how to use low-ratio and diff lock in a Discovery 1 V8 automatic.
Can you tell me how and when to use the high-and low ratio gears and the diff lock in my Discovery 1 V8 auto?
Carl Neston Sandy, Bedforshire
When the lever is fully backward, the vehicle is in the high-ratio gears. This is the normal ratio for road use and is usually suitable for easy off-road tracks. Pushing the lever forward engages the low-ratio gears. You might want to use these for more challenging scenarios, eg to give precise, very-low-speed control when driving over rocks; to optimise traction when tackling steep climbs; and to give engine braking and control on steep descents. Basically, if first gear in high ratio is too high when off-road, you should be in low ratio.
The same lever controls the diff lock - this is the centre diff in the transfer box. Move the lever sideways to engage or disengage diff lock.
Whichever side the lever is on, it’s still using the same gear ratios. So, if the lever is moved to diff lock position, it can still move forward to engage high ratio and back to engage low ratio. Whichever ratio it’s in, you can move it sideways to disengage diff lock and you will still be in the same gear ratios.
It’s designed to be able to select between the same high - or low - ratio gears, regardless of whether diff lock is engaged or disengaged.
Diff lock should always be disengaged when driving on roads or any other firm surface. When disengaged, the centre diff is free to operate, meaning the front end and rear axles are still driven, but are free to rotate at different speeds. This is essential on the road becauase, when cornering, the back wheels tend to cut the corner, so the back wheels and axle rotate more slowly than the fronts. If they couldn’t do this, the tyres would rapidly become scrubbed and steering would be difficult.
But, when driving 0ff-road on a loose or slippery surface - with both rear wheels spinning in mud, say - you’d get no drive to the front wheels, and you’d therefore be stuck.
This is where diff lock comes in. By engaging the diff lock the transmission is locked mechanically, so both axles rotate and the front axle pulls you out - even if the back wheels have no grip. The idea, of course, is to engage diff lock before you get stuck, so engage diff lock if you know or suspect the track ahead is going to be loose or slippery.
When you turn a corner off-road on loose ground with diff lock engaged, the fact that the axles are locked and so can’t turn at different speeds for the bend doesn’t matter - the wheels will just force their own rotational speed by skidding a bit on gravel or mud. You won’t notice it and it won’t do any harm, but if the track firms up and becomes unyielding to the tyres, disengage the diff lock, as for road use.
You’ll sometimes find that, after disengaging diff lock, the diff lock warning lamp stays on. It usually goes out after a few yards once the axles have sorted themselves out. If the lamp doesn’t extinguish, stop and reverse a few yards - that’ll do the trick. It’s perfectly normal practice.
This workshop advice appeared in the December 2009 issue of LRO. Back issues are available to download on digital devices here.