Discovery 2 HDC

Friday, March 17, 2017 - 13:45

The D2 Hill Descent Control always seems to polarise opinion, with owners either loving it or hating it.

The problem with HDC seems to be that it looks pretty simple - put the TC in low and push the HDC button and off you go... The HDC will certainly operate this way but it's not necessarily going to be particularly effective!!

If you read the documentation covering HDC it becomes apparent that one of the factors driving it's adoption was the removal of the user lockable Centre Diff Lock. The concern was that loss of traction on a single wheel would render engine braking ineffective leading to loss of control of the vehicle. There are a couple of other safety concerns, such as neutral being selected - possibly due to gear lever being knocked out of gear during a rough descent?

All these concerns underscore the key point that HDC is intended to assist when engine braking alone is insufficient to control vehicle speed.

The premise therefore seems to be that HDC is a safety net which works in conjunction with engine braking rather than a replacement for engine braking.

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.
  • Engage Low.
  • 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.

Target Speed

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.

Gear Manual Normal Manual Reduced Auto Normal Auto Reduced
1 7.0 7.0 7.0 7.0
2 8.3 7.0 7.0 7.0
3 9.6 7.0 12.0 9.6
4 12.0 9.6 12.0 9.6
5 14.0 11.2 - -
Rev 5.6 5.6 5.6 5.6
Neutral 14.0 last used 7.0 7.0

These minimum target speeds are used when the throttle input is 0%.
Below the mimimim target speeds engine braking is used to control speed.

The target speed can be raised to a maximum of 50kmh by applying throttle. For example, if you are driving down a hill using HDC and the gradient eases you can increase the speed the HDC activates by pressing the accelerator.

The HDC is designed in such a way that you can leave it enabled and drive on the throttle only. It's something that works quite well on the Auto, but I'd imagine that it might not be so much fun when changing gears using a manual!

If you give it a try you'll find that HDC applies brakes very gently when the road speed relatively high - as the road speed gets closer to mimimim target speed the application of brakes gets far more aggressive.