How to use HDC
The Owners manual provides operating instructions for the HDC that are basic at best.
HDC can be engaged in high or low range but only operates in Low Range.
The green HDC light illuminates solid in Low, and flashes in High.
Vehicle must be travelling at less than 50kmh/31mph.
Clutch must not be pressed (manual vehicles only)
In the Driving Techniques sub-section of the Off-Road Driving section, the Descending steep slopes page contains additional information on how to use rather than simply enable HDC.
Stop at least one vehicle length before the slope.
Select 1st gear / D1.
Engage HDC (ie press button).
Unless necessary to stop do not touch brakes or clutch.
The manual also notes that if the vehicle begins to slide HDC will accelerate the vehicle to maintain directional stability, but will automatically slow the vehicle as soon as possible.
Refering to Tom Sheppard's The Land Rover Experience book (1994 edition), it's interesting to note that the HDC behaviour apparently matches the recommended driver action in the these circumstances.
Sheppard notes that If the ground is too slippery to provide the grip for the retardation of the throttle-off engine and you begin to slide, be ready to use the accelerator to help the wheels 'catch up' with the vehicle and eliminate any wheel slide.
He also comments that There may be occasions - typically long descents of loose ground or extremely slippery clay, steep initially - where low range 2nd gear will be better in order to preclude an initial sliding-wheel glissade.
My guess is that the claims that HDC "runs away" are based incidents where the Disco has started sliding due to loss of traction and HDC has accelerated in a attempt to regain control.
Autobox “Pro Tip”
For best engine braking in low range put the box into Manual mode by pressing the mode button immediately behind the shifter. As the Owners manual states “Manual mode will provide maximum vehicle control and engine braking - ideal for use in severe off-road conditions.“
What the manual doesn’t mention is that with 0% throttle the Auto will lock the torque convertor in 1 and 2 when revs exceed a specific value.
From testing I did a few years ago with a Nanocom 1 while watching the TC lockup (solenoid 3) it appears the lockup occurs at around 2000 rpm, and the Torque Convertor unlocks again when revs drop below this point. An AULRO user who is a big fan of this function says that it occurs at 1800 rpm and stays locked for much longer periods. As the Auto takes into consideration rpm, throttle, roadspeed and current engine torque plus preprogrammed shift maps so it’s quite possible that both data points are correct depending on conditions.
The low range, 1 or 2, 0% throttle lockup seems to correlate with a road speed slightly higher than the minimum target speed of the HDC. Even without the use of HDC this 0% throttle lockup noticably “pulls back” the speed. If you keep an eye on the Tacho you’ll see the revs spike when the lockup engages then drop back.
One of keys to understanding HDC is the concept of target speed. This is only mentioned in passing in the Owners manual but is well documented in RAVE and the New Discovery Technical Briefing pdfs.
The minimum target speed depends on the transmission type, gear selected, and the terrain being traversed. If the terrain is very rough, very slippery, or the vehicle is negotiating tight corners a “Reduced” mode of operation is triggered which lowers the minimum target speed. The table below gives the minimum speeds in kilometers per hour.