From stationary, I was going to go down a steep, narrow tarmac road. In high range, I manually selected 1st gear to limit my speed. This was not an off road situation where HDC would be prudent and not sufficiently steep to need the brakes as well as 1st gear. I just wanted to limit my speed to about 10mph.

From a stationary start, there was a slow increase in road speed with the engine staying at about 750rpm until at around 10mph there was a sudden rapid increase in engine speed up to about 2000rpm with a rapid increase in road speed as well. After a few seconds the revs returned to 750 and the road speed dropped back as engine braking was re-established. All this with no input from me - except a surprised look.

Repeating the exercise since has shown that this is repeatable in my 07 TdV8 and also in a dealer's 09 TdV8. If the hill is too shallow the critical speed is not reached and no uncommanded speed increase occurs. The same thing happens if manual 2nd. gear is selected but at a higher speed.

My dealer treated it as a feature of the emissions control system but I don't believe this and as far as I am concerned it is a fault which, if it is common, even only to TdV8s, could catch someone out and needs dealing with. If you fancy having a go to find out if yours does it (and it will help me make a case with LR) try this:


Requires a hill with no traffic which is sufficiently steep and long to allow speeds from rest up to about 20mph - say about 20% (1 in 5) and 500m or longer.

1. Vehicle stationary, engine idling, normal height, no special terrain response program selected, gearbox in high ratio, hill descent control not selected, foot brake on, park brake off.

2. Select 1st gear manually using command shift, (revs should show about 750)

3. Foot off brake and do NOT touch accelerator

4. Allow car to roll down hill while keeping an eye on revs – hill needs to be sufficiently steep to allow speed to increase to about 10mph

What happens? - (without touching accelerator or brakes)

(a) Do the revs stay low and engine braking holds speed down?

(b) Once speed has reached about 10mph do the revs suddenly rise to about 2000 with a rapid increase in road speed and loss of engine braking?

(c) If the revs do rise significantly, then after 3 or 4 seconds do they drop back to 750 and engine braking returns (If hill is too steep this may not happen)

Interesting. 2005 Zambezi TDV6 - Gone but not forgotten
2009 Alaska TDV8 - Gone and much missed.



WINNER - 2009 �Idler Of The Year� Award
Runner Up - 2009 �Just Doing What It�s Designed To Do� Award


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Yes my TDV8 does something similar.

When I leave home the road goes down hill, the speed limit is 30 Km/h, so I just leave the gearbox in auto and gas pedal off, after a while the RPMs goes up, speeds increase a bit then RPMs drop back after 1 or 2 secs.

Never really lloked at the actual RPMs but should be similar to your experience. '08 RRS TDV8
I converted my diesel RRS to run on an environmentally friendly mixture of caribou fat and baby seals oil

Thanks Flydive
I had tried the test using the same hill with but auto D selected instead of manual 1st. The result was no real engine braking and rapid speed increase - revs rose until auto 2nd gear engaged then revs dropped but road speed rose quickly - as you would expect from an auto.

I also tried the manual 1st gear test in our old auto Freelander. This behaved as I expected with strong engine braking and no sign of the sudden increase in revs and speed that the TdV8 showed.

Anyone fancy trying the hill test for me this weekend - I appreciate most people don't live near steep hills without traffic

Bob,

Tried this..

I found non of the above. The engine revs rise quickly temporarily but the vehicle speed does not rise suddenly as described its obviously controlled to do this so not a fault. Personally i dont think i've ever manually selected first for going down a hill before doing this, mine does a fantastic job of keeping straight and controlled in auto on the steepest of slopes. I have selected 1st before going up a small ridge to stop it changing gear on the breakover but thats about it. I'd be interested in what situation you've found benefit from this (i'm interested not questioning?)

Cheers Trust me, i'm an Engineer!

Hi micrometer

Thanks for trying it, looks as if your supercharged doesn't behave in the same way as my TdV8.

I find 1st or 2nd most useful for slow controlled descents of very steep, very narrow country lanes with poor views ahead, particularly those with hairpin bends - yes you could just use the brakes, which is OK until you hit the cow muck, mud or loose stones washed down onto the road halfway round a hairpin. Having said that it's not an everyday experience.

Still I don't believe the designers intended the vehicle to have uncommanded acceleration and it could catch someone out.