Q. I recently bought a 1988 Ninety and would like to know a bit more about the diff lock. When it’s engaged the warning light on the dashboard doesn’t light and I’m not sure if the diff lock is working. Can you explain how the locking diff works and how I can check it, please?
Kevin Tennent Hindhead, Surrey
A. Your Land Rover has three diffs; one in each axle, one in the transfer box. It’s only this last one that locks: if you want to lock the others you can buy an aftermarket diff lock, ARB and KAM being the most common types.
If one wheel on any axle isn’t making contact with the ground, it’ll spin, taking all the torque from the opposite wheel – so you’ll have no drive from that axle. If this happens on, say, a rear wheel when the centre diff is engaged, you’ll still have drive to the front axle. That’ll pull the vehicle forward, assuming both front wheels are in contact with the ground.
The centre diff allows varying torque to be transmitted to the front and rear axles. This is essential because, when the vehicle is turning left or right on tarmac, even on a slight curve, the front wheels travel further than the rear, so the diff allows the front and rear propshafts to rotate at slightly different speeds. Without it, steering would be difficult and the tyres would scrub.
This is a disadvantage off-road, as explained above, hence the option to lock the diff to maintain drive. The tyres tend not to scrub when off-road because the loose surface allows the front/rear tyres to skip, relative to each other, although you’d rarely notice. You should always disengage centre diff lock before going back on to a firm surface, such as on public roads.
To check your diff is working, jack up one front wheel. You should be able to turn it by hand. Engage diff lock, which locks the propshafts together. You shouldn’t be able to turn it now (you may have to turn it a bit to allow the gear teeth to mesh).
If this works, it’s okay. If not, it’s probably the gearlever-to-transfer box linkage (it seizes and sometimes needs adjusting). Remove the right-hand centre floor panels to reveal it.
If the light doesn’t work, it could be the bulb or the switch, which is on top of the transfer box – more often than not under a big lump of mud.