Diff lock question

Hello Gang

I have a question about the diff lock arrangement on a Defender. Please bear with me - this is my girlfriend's (recently acquired) car and I need to get my head around the actual mechanics of the diff lock as it doesn't appear to be working.

I felt something was wrong when driving through a boggy area of a field - forward motion was lost, and lots of mud being thrown up by wheels. Engaged diff lock and nothing changed (except that the light came on!). We managed to get out (eventually) by reversing.

I've just been outside and tried driving the car at low speed in 1st gear (fast tickover) with the steering locked hard over and difflock engaged. As it was a gravel surface, I'd expect there to be a poughed furrow left by the wheels on the inside of the turn, but nothing doing. The steering feels normal when driving (slowly) along a gravel track and turning from lock-to-lock regardless of whether the diff lock is enaged or not.

Checking out previous threads in this forum suggests that if the light comes on then the diff is definitely locked, and there is the expected delay in the light going off when diff lock is disengaged. According to the service manual and handbook the fact that the light comes on means that diff lock is definitely engaged !?!

Both the official service manual and the Haynes manual are very vague in this area. I can see how the transfer box can lock front and rear driveshafts, but I don't see how that mechanism can lock the front and rear differentials.

Am I completely misunderstanding how this should work?

Thanks for any pointers!

Right then it's not easy to explain but here goes :-

On a bog standard 90 you have three diffs one on each axle and one in the transfer box.

The ones in the axles enable the two wheels on its axle to rotate at different speeds.

The one in the transfer box enables the two axles to rotate at different speeds.

So if your front wheels loose traction and you havent got your transfer box diff locked all the power will be transfered to the front wheels which will spin and the back wheels won't do anything.

If you then lock the transfer box diff the power will go to each axle equally and the rear wheels will rotate.

If with the transfer box diff locked you loose traction to one wheel on each axle then the axle diffs will divert the power to those wheels and they will spin leaving the wheels with traction stationary.

The diff lock does not lock the front and rear diffs, it only locks the centre.

Because the mechanism that actually engages the difflock is spring loaded the fact that the difflock light comes on doesn't necessarily mean that it has been engaged.

One way to check if your diff lock is working is to jack up a wheel and engage the diff lock. Very carefully slowly release the clutch if the vehicle tries to move forward then it's working and engaged. If the jacked up wheel just spins then it's not. Just be very careful as you don't want it to jump off the jack.

Or jack the wheels on one axle of the ground put the vehicle in gear and difflock on, rotate both wheels in the same direction if they lock up 'cos your trying to turn the engine then its working. A bit safer than trying to drive it off the jack but it takes two people unless you've got long legs!