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Kenny57
Member Since: 12 Apr 2020
Location: Essex
Posts: 171
  
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Hi,
was out yesterday on a big off-road course having a practise on some serious descents on loose soil.
On my first attempt at a big descent I started on the brakes then part way down lifted off to let the HDC take control. The moment I lifted off there was a heart-stopping moment when the car suddenly accelerated before the HDC attempted to take control. There was then an equally unnerving scrabble to regain control and slow the vehicle. It did do this successfully and everything was back under control by the bottom of the slope, but it wasn't fun by any means. I wondered if this was because of my starting the descent on the brakes so, summoning up my courage, I tried again this time rolling over the precipice with the HDC engaged and feet firmly off the pedals. Again there was a noticeable lag before the HDC kicked in causing excessive speed to be gained before it eventually gained control. Lastly I tried the descent using just the brakes. This lead to a much more stable descent under full control at all times. All of these attempts were made with in Low Ratio, Auto Selected, and terrain response in "Normal"
Discussing this with other drivers, all thought in more slippery/wet conditions the sudden acceleration part way through the first attempt and the initial speed gained before the HDC kicked in on the second would have lead to a total loss of control. At first I thought the lag in response may be a function of the automatic box and the torque converter unwinding/winding up, but a fellow driver in an '05 Manual D3 was finding the same issue and having an equally squeaky-bum time testing its abilities.
We were all surprised at the poor performance of the HDC so I've been having a read up and found this under ABS control on page 362 of the D3 Workshop Manual...
"HDC is automatically switched on or off and target speeds are adjusted in response to the Terrain Response program selected. The responsiveness of the HDC function is also increased where required.
Automatic operation of HDC aims to assist the driver by switching the system on or off when it is of most benefit. Target speeds for HDC operation are also adjusted according the vehicle operating conditions."
So, my question is: Does anyone have any information on how the HDC target speed is adjusted by each of the Terrain Response programs and by the "vehicle operating conditions"?
Many thanks in advance.
Ken
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 22nd Aug 2021 11:46 pm |
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galwaygreen
Member Since: 30 Oct 2011
Location: plymouth
Posts: 6525
 
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think on a d4 its the cruise control buttons you adjust...minus and plus signs,,,think touching the brakes cuts the hdc out...long time since we used it...........there are benefits in having a LRE half day....learn what the car can do and someone to answer questions
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| 23rd Aug 2021 12:16 am |
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Pete K
Member Since: 15 Jan 2016
Location: GL
Posts: 10574
 
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D3 also uses + cruise buttons
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| 23rd Aug 2021 1:29 am |
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Gareth
Site Moderator
Member Since: 07 Dec 2004
Location: Bramhall
Posts: 26754
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Yes, CC speed + and - let you control the descent. The Terrain Response setting also affects the way it works. For a slippery slope I would have it in Grass Gravel and Snow setting.
It’s an outstanding feature and is really very good when used properly. It cannot however, alter the laws of physics and get you down an impossibly slippery slope. If it is so steep and slippery to be impossible to slow or stop the descent, it should still keep the wheels turning to allow you to steer.
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| 23rd Aug 2021 7:03 am |
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Kenny57
Member Since: 12 Apr 2020
Location: Essex
Posts: 171
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Thanks for the replies guys.
Sorry, should have included that someone at the site mentioned the cruise control buttons did this on his D4. We did try it on both D3's and couldn't seem get any reaction. I tried all the buttons in various orders but couldn't detect any change in response (we did some smaller descents to see what happened). This may of course be because there's a specific way of making the adjustment and we were just guessing.
Would love to do an LRE course one day, just hoping someone here knows something about it.
Something else I forgot to mention was that as the slope lessened at the bottom of the descent it was noticeable that the HDC suddenly got very aggressive and slowed the car so much that a dab of accelerator was needed to encourage it to the bottom. Is this what is might be meant by "The responsiveness of the HDC function is also increased where required."?
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| 23rd Aug 2021 7:10 am |
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Gareth
Site Moderator
Member Since: 07 Dec 2004
Location: Bramhall
Posts: 26754
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The other thing to be aware of, is the HDC needs the wheels turning before it can work. It uses the abs system to judge wheel speed.
That’s why it’s important to keep your feet off the pedals. Yes the accelerator will cut the hdc if you press it. It should return as soon as you let the pedal go. Dabbing the brakes can lock the wheels at slow speed and prevent the hdc from coming back. So maybe this is what you did ?
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| 23rd Aug 2021 7:39 am |
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Kenny57
Member Since: 12 Apr 2020
Location: Essex
Posts: 171
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Yes indeed, I quickly learned not to try and intervene with the brake once the HDC was doing its thing!
And thank you for reminding me the ABS needs a wheel (more than one wheel?) to slip before engaging. That may be the issue I was having, but the speed gain really felt a bit excessive before it tried to get everything under control, particularly when I started the descent purely relying on the HDC!
Hence my question about the actual "Target Speed" differences for different settings. I know for example that in normal drive and normal terrain response setting, on gentle slopes hdc has a much lower target speed with the gearbox in low ratio than in high ratio. I haven't measured it but roughly 5mph and 15mph at a guess. And yes, even 5mph would be a bit fast to start a steep descent and have a wheel slip to engage the HDC. Given what it says in the manual, I'm hoping someone has some actual numbers for the operation of the system. 
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| 23rd Aug 2021 8:00 am |
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Hardware
Member Since: 28 Jun 2016
Location: Hiding under the M60
Posts: 12889
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Wheels don't need to slip, just reach trigger speed. Try it on a near flat tarmac road ! .
Dean
====================================
2011 D4 XS - OBD port protection, RLD spare wheel protector, All LED interiors lights, Timed Climate enabled, iiD tool paired.
2011 D4 Landmark - Stolen from same dealer before I paid for it
2011 D4 GS - Stolen whilst at dealer  ... All LED interiors lights, DRLs, Spare Wheel protector.
1996 300Tdi - Eaten by tin worms
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| 23rd Aug 2021 8:15 am |
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Kenny57
Member Since: 12 Apr 2020
Location: Essex
Posts: 171
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Aha! Thank you Hardware, that's a much more logical way for it to work.
So I'm guessing somewhere there must exist a table of trigger speeds for each of the terrain response settings, and maybe even a description of how it adapts to other situations. For example, does it engage below the trigger speed if it detects slip on x number of wheels?
PS, I've accidentally engaged it on a near flat bit of tarmac road... its very effective isn't it! 
PPS, so I should have been able to work that out shouldn't I. Sorry. 
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| 23rd Aug 2021 8:30 am |
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Gareth
Site Moderator
Member Since: 07 Dec 2004
Location: Bramhall
Posts: 26754
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I think the minimum hdc speed is 4 kph max 12 kph. It is in the manual somewhere. I have sold my Discovery now so not got a manual.
As said earlier, if the slope was steep and slippy the vehicle may have slid, with the wheels turning at the target speed set, hence why you could still steer.
The system is very clever. When I did my LRE course, we had a very steep slope, with a strip of rollers down one side. We would go down on HDC, and then deliberately steer into the rollers with the left wheels, the system would handle it. We also let the car roll over the top in neutral and let it roll down the hill a few metres gathering speed before engaging the hdc, and it would take control without too much fuss. That was scary!
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| 23rd Aug 2021 8:35 am |
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G3Z
Member Since: 16 Dec 2018
Location: South Wales
Posts: 549
 
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I have a GS Manual D3, so no Cruise Control for me. My HDC works off the gears. I find low range 1st gear and hill decent works wonders down steep slopes. As others have said, the key is to not intervene with the system- switch it on before heading down the hill and let it work its magic. Defender 110 2.4 TDCi Utility Station Wagon.
2007 Disco 3 GS TDV6 Manual
200,000+ mile club 😎
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| 23rd Aug 2021 8:48 am |
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L319
Member Since: 14 Dec 2013
Location: Herefordshire
Posts: 2081
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Off road on steep slopes I don't use the brakes at all. I use a lower enough gear so I can control the general speed with the throttle and have the HDC engaged. This means I can also control the HDC intervention speed with the throttle as well , more throttle ,higher intervention speed, closed throttle and lowest default speed selected. I am not aware if there is a upper limit intervention speed on a D3 but I have used it up to 30 mph
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| 23rd Aug 2021 9:07 am |
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Kenny57
Member Since: 12 Apr 2020
Location: Essex
Posts: 171
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Thanks for the input guys, very interesting. I'll keep digging to see if there's an actual definitive description of how it reacts and controls. I'll post my results if I find any.
Cheers, Ken
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| 24th Aug 2021 7:21 pm |
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