Hi
Can anyone tell me how the Hill Descent Control works? It is very impressive especially in snow and feels as though it may be operating two brake bands simultaneously in the transmission, but I would like to know how it actually works.

Thank you
Adam..

Don't have in-depth knowledge but it uses ABS: https://live.landrover.co.uk/technology/la...technology

It is great as you say. Current: Volvo XC90 T8 Inscription Pro PHEV with some optionals on top
Gone: MY16 RRS2 3.0 SDV6 Autobiography Dynamic, Carpathian Grey & Santorini Black Sliding Pano/Ebony & Ivory 2 Tone
Gone: MY13 RRS 3.0 SDV6 HSE Black Edition, Orkney Grey/Ivory
Gone: MY09 RRS 2.7 TDV6 HSE, Santorini Black/Almond

Very similar to cruise control really. It's controlled by the Vehicle Dynamic Module (AKA ABS module ) and uses the vehicles normal brakes.  Visiting from DISCO3.CO.UK
2006 Discovery 3 TDV6 Auto HSE Zambezi Silver

Member RRSTDV8 posted this useful document (which covers terrain response controls) a while back ... if you are interested. Current: Volvo XC90 T8 Inscription Pro PHEV with some optionals on top
Gone: MY16 RRS2 3.0 SDV6 Autobiography Dynamic, Carpathian Grey & Santorini Black Sliding Pano/Ebony & Ivory 2 Tone
Gone: MY13 RRS 3.0 SDV6 HSE Black Edition, Orkney Grey/Ivory
Gone: MY09 RRS 2.7 TDV6 HSE, Santorini Black/Almond

HDC uses the brakes to control the vehicle's speed. It controls individual wheels and makes use of the ABS wheel speed sensors to monitor what is going on at each corner. It's part of a suite of systems that make up the Terrain Response system.

Three useful things to remember:
1. It lights the brake lights when in operation - this is why a blown bulb causes HDC errors. This is hardwired via the HDC relay in the battery junction box.
2. You can control the speed using the cruise control speed button. This allows you to fine tune the descent speed. Very useful.
3. HDC works in reverse as well as in forward gears. Very useful if you have a refusal during a climb. Also useful if you have to back up on a slippery slope. Engage low range and HDC and you can reverse in total control on icy roads - helpful when getting close to someone to tow them up the hill...

Oh, and it is normal for HDC to make a chattering noise when its in operation - it's the same as you get when ABS kicks in; that's because it uses the ABS system to control the speed. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

Thank you all for such brilliant information. I am going to read the Graz paper on the way home (on the train, not whilst driving !!).

The chattering sound of the HDC, coupled with the not very smooth application of brakes, made me think it was all happening in the transmission. It is astonishing how much braking effort it can find on packed snow with out sliding.

So a supplementary question, to straighten out my train of thought. If I have two cross axle diffs and a centre diff, why do individual wheels not rotate when jacked clear of the floor? Presumably there must be some form of limited slip in each of the differentials? I understand that there are electronic diff locks on the centre diff at least (rear is optional?), but with engine off, should that diff be free to slip?

Thanks
Adam..

Do you mean "Why can't a single wheel be turned when only it is off the ground?" by that?

When in "Park", the transmission is locked into place by a pawl, so this is not free to move. A differential is a torque divider and all torque must still be transferred somewhere:

Say the input shaft, that drives the crowheel, is called "A"; the output shafts are called "B" and "C". The matematical equation for a standard open differential would be "A"="B"+"C". With "A" fixed in position, and "B" fixed as the wheel is on the ground, you get 0=0+"C". "C" can therefore only equal 0. With the opposite wheel off the ground, you can move the wheel as the other can turn in the opposite direction, i.e., 0=-1+1.

What I haven't tried, is with only one wheel on the ground. In theory, the opposite wheel to it would also turn by moving the other two (or one of them) but there would be a large amout of friction and momentum to overcome in the drivetrain too.  Visiting from DISCO3.CO.UK
2006 Discovery 3 TDV6 Auto HSE Zambezi Silver

Doh !! I wasn`t thinking about it being in park. Will try that out when I next have the car on the jack.

Do you have the gearbox in park when you jack up one corner? That will prevent the wheel rotating. Have it in neutral (with the grounded diagonally-opposite wheel chocked to prevent rolling if necessary) and you should be able to rotate an individual wheel when its lifted.

The front diff is open, the rear diff is open unless the optional e-diff is fitted. The transfer box centre diff is open normally but can be "locked" with a multi-plate clutch. This is not the same as the traditional mechanically locked centre diff e.g. on the Defender. The RRS's centre diff system allows variable locking as required by the Terrain Response system and is capable of sending 100% of torque to either axle (but defaults to 50/50).

The traction control system uses wheel braking (and power reduction in extremis) to maximise traction. The system requires some slip before it works - common to all such systems. A common mistake is to come off the throttle when the traction control kicks in. You should maintain a steady, light throttle application to allow the system to work out what is happening and then selectively brake a slipping wheel. You find that the vehicle seems to pause and then move on again in such situations. It's quite impressive. As ever, the tyres are the limiting factor (assuming ground clearance is adequate). 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed