New D4 and brake/accelerator modulation

[pjvdl]

Firstly, thanks to all for this great forum. I posted many months ago when I was deciding between a D4 and some other vehicles and, based in part on the advice I received here, ordered a D4 last december. It finally arrived in early May, but I must confess I didn't feel I could announce it until I had some photos that would do it justice

My wife an I are new to this 4WD business and so decided to find ourselves a 4WD training course to get a feel for what we and the vehicle are capable of. We found one that appealled and finally managed to take the D4 off road yesterday for a 4WD training day at Saunders Gorge. We had a great day and the location was fantastic; and now I can add pictures of other than a very clean looking car (although I must confess it didn't get anywhere near as dirty as I was hoping)!

But, I have a question.

One of the techniques our trainer was attempting to teach us was what he called brake/accelerator modulation. He explained that this technique involves braking with your left foot while accelerating with your right foot to ensure that the automatic torque convertor remains locked, an that we should use this technique together with feather braking to provide additional braking on hills when the engine braking is not sufficient. This makes sense to me, so all good so far. But, while practicing this, I noticed that when the brake pedal was touched, the engine revs would sometimes drop back to idle speed (800 rpm) and no amount of right foot persuasion would increase the revs until the brake pedal was released. The behaviour seemed more noticeable when the engine was under less load (i.e. near coasting), but it still occurred once or twice on reasonably strenous hill climbs when the engine was definitely working a little harder.

Our trainer seemed quite knowledgable in LR response and LR systems, and I have done a bit of reading as well and so have some theoretical knowledge, but we couldn't get it to stop exhibiting this behaviour. At first we thought it was the DSC, so we turned the DSC off. This made no difference. We also tried different terrain response modes, turning DSC on and off in each mode. The behaviour seemed less noticeable in mud and ruts mode (and possibly rock crawl - I can't quite remember), but it still occurred. No matter what we did, we couldn't get it to stop!

So, my questions are; does anyone know why the car is doing this? Is it some sort of fuel saving system? And, is this technique even appropriate in this car or should I be doing something different?

Oh, mustn't forget the photos

Shiny, new D4 on delivery day ...



Off-road at Saunders Gorge. Still not really dirty, but hey, we had fun. And those angles somehow never seem as bad in the photos as when you are in the car ...

[Neil P]

Our trainer seemed quite knowledgable

Who is this person and what did he charge you for is opinions ?

[rmp]

That is a well-known technique which is effective but not because of the torque convertor locking, even it did under those circumstances which is unlikely. The advatnages of the technique are that it forces the wheels to turn very slowly as opposed to braking them. Thus, if a a wheel hits a slippery spot it still turns and doesn't lock. In addition, you can use it in rocky terrain to stop a wheel "falling off" the back of a rock. LR themselves have adopted this by use of the Rock Crawl pre-charge system. It is a useful extra trick in the bag to complement normal braking, engine braking and HDC.

As to why your D4 responded that way -- I don't know. The technique certainly does work with D4 3.0 TDV6s, I've done it myself in rocks. If you are going to use it disengage HDC which will automatically engage in most TR settings. Was HDC off and the vehicle in first or reverse low?

[pjvdl]

The advatnages of the technique are that it forces the wheels to turn very slowly as opposed to braking them. Thus, if a a wheel hits a slippery spot it still turns and doesn't lock

Yes I agree. Looking back at my post I didn't make myself very clear, but he did explain the use of engine braking as a means of reduce skidding on steep slope and he certainly made it clear that this is the primary purpose of the technique.

Being very new to this, I may be getting this completely wrong, but my understanding from his explanation is that keeping the torque converter engaged provides a secondary advantage. That is, keeping revs slightly above idle (seemed to be abour 1200 rpm on the D4 3.0) removes that momentary period of free wheeling down a steep hill from idle, before the torque converter engages to provide engine breaking.

Again, I am certainly no expert, but he had us try the following two approaches as a comparison and it certainly seemed to make a difference:

1. First, deliberately stop on a steep slope. Brake and leave the car idling (no accelerator) in 1st gear low range. Take your foot off the brake without any acceleration. Using this approach, the car tended jolt forward momentarily before the torque converter engaged to provide engine braking to slow the car down.
2. Second, try the same thing, but with some acceleration. Deliberately stop on a steep slope. Brake (with left foot) and accelerate to about 1200 rpm in 1st gear low range (make sure you are braking enough to prevent movement while you are doing this!). Take your foot off the brake without any acceleration. Using this approach, the car tended jolt forward momentarily before the torque converter engaged to provide engine braking to slow the car down.

If you are going to use it disengage HDC which will automatically engage in most TR settings. Was HDC off and the vehicle in first or reverse low?

Yes, we generally had HDC off all day, as he was keen for us to learn how to do hill descents and ascents manually (well as manually as this car allows) before we tried the HDC, which we only did at the end of the day.

Your point on the gear we were in is an interesting one. I don't remember the problem happening when in 1st or reverse. We were only using 1st on steep descents, as he recommended using 2nd on all but the steepest ascents. It was only when we were practicing technical work (crossed axles - is that the right term?, ramp overs and negotiating tight "obstacle courses"), on relatively flat ground that required fine braking and acceleration control that it was happening. When doing this, I think we were always in at least 2nd gear low range.

I'm confused

[rmp]

You get engine braking in an auto whether the convertor is locked up or not. It's just better if it is which is why some people have switches to manually lock it. The Disco is pretty good at engine braking anyway.

Momentary period of idle...????

if you are on a hill and need to start off then you reduce brake pressure till the car moves, not let it go and then wait for the engine to catch it. Or, engage HDC, or drive thru the brakes (as you were taught). Whichever one seems best for the situation.

Hope he showed you how the HDC speed could be varied using the cruise controls.

Re the gears. The left-foot-brake technique is only for the gnarliest and steepest terrain - think really rocky descent where you dare not go above snails pace and clearance a constant worry (try HDC in that situation!) -- so by definition you're in first low anyway. It can be used as a poor man's LSD but never in a car with even basic traction control and the Disco is one of the best, so I wouldn't be using it over much if at all for just cross-axling situations on flat ground. Really tuff rocks, probably yes.

For the sitations I think you were in I'd be in second low and just letting the torque and traction control do its thing and it does it very well. Sometimes people that left foot brake apply the technique in places it's not necessarily needed, especially with modern vehicles.

Agree re ascents you want the highest gear you can which often in D3s and especially D4 3.0s is third low not second. I definitely wouldn't be left foot braking on an ascent in a Disco.

[ryrrek]

The D4 is programmed to reduce engine revs to idle speed when the brake pedal is touched. I experienced this when driving a demonstrator back in December, and again when I collected my D4 two weeks ago. I thought initially that it was a fault in the vehicle, but the dealer suggested that I was lightly touching the brake pedal when moving off, and this turned out to be correct.

[warren9981]

The torque converter does not lockup at such low revs/vehicle speed.
This technique has been discussed on here previously (I think?). It works as follows, I believe.

When preparing to descend a slope from flat ground at the top. Stop before the drop off, apply enough accelerator to just begin to drive the vehicle forward. Lock the accelerator foot at this position and do not reduce the revs. As you go over the edge and down the slope the drive through the torque converter provides engine braking. You can still provide left foot braking as required while still maintaining drive.

I think the aim is that when sitting at idle the torque converter is not full of transmission fluid. When the revs are increased fluid is pumped into the converter and fills it to provide drive. This is what provides the drive for engine braking.

I believe Landrover have put the HDC and other functions in to "dumb down the system" to allow for inexperienced drivers. The above method would thus not be required on the new vehicles.

Hope this was not too confusing.

[rmp]

If you are going to drive through the brakes then you must constantly modulate throttle and brake pressure dependent on the slope. Neither are fixed. You need to significantly increase brake pressure as you crest a gradient for descent. The idea is you apply sufficient braking power for the vehicle to be stopped, then apply revs to move it. You know if it's right when you lift off the accelerator and the vehicle comes to a nice slow halt.

HDC is one option for descending a hill. It is IMHO too fast for really rocky, steep descents where DTB comes into its own regardless of how new the vehicle is. Which method one uses is depednent on the situation and vehicle, they are all complementary to some degree. Most of the time I just brake normally and gently in first low, the simplest method of all.

Again, the point of DTB is to force wheels to turn slowly, not to lock up torque convertors.

[gghaggis]

As someone pointed out earlier, the D4 in low range on a steep hill won't allow the revs to increase if you have your foot on the brake (unless you stomp it). But note that this technique is not as useful in the D4 - you have gradient descent release to allow smooth take-up if the HDC is being used. As RMP said, this is sometimes still too fast (it is programmed at it's lowest speed if you're in rock-crawl + low 1st), in which case gently touch the brakes - in this situation the wheels won't lock up adversely (as they would on normal 4WD's) due to the rock-crawl TR setting. A D3 might lock up a little easier as its rock-crawl TR isn't quite the same.

Cheers,

Gordon

[pjvdl]

if you are on a hill and need to start off then you reduce brake pressure till the car moves, not let it go and then wait for the engine to catch it. Or, engage HDC, or drive thru the brakes (as you were taught). Whichever one seems best for the situation

That's a good point. On one or two exercises we tried steep hill descents with only engine breaking in 1st low range, and the D4 easily held its speed, so I doubt we encountered any situations where we really needed to use the brake/acceleration modulation technique. Rather, we were just practicing the technique. And yes, the HDC worked well on all the descents that we tried as well, so that is certainly an option.

And to think I began this thinking that I just need to brake going downhill and accelerate going uphill. A little naive

and especially D4 3.0s is third low not second. I definitely wouldn't be left foot braking on an ascent in a Disco.

We tried 3rd low a few times and it didn't have any problems on less severe ascents.

I believe Landrover have put the HDC and other functions in to "dumb down the system" to allow for inexperienced drivers. The above method would thus not be required on the new vehicles.

Yes, I agree that the technique is not really necessary with HDC, but I always like to know how to deal with situations when automatic systems can't deal with a particular situation (although I suspect that if the HDC can't deal with a piece of terrain, I probably wont be able to either).

The D4 is programmed to reduce engine revs to idle speed when the brake pedal is touched. I experienced this when driving a demonstrator back in December, and again when I collected my D4 two weeks ago. I thought initially that it was a fault in the vehicle, but the dealer suggested that I was lightly touching the brake pedal when moving off, and this turned out to be correct.

I knew someone would know the answer! But where does that leave the use of brake/acceleration modulation as a technique in the D4? It seems that it can't be relied upon if the engine is going to drop power when the brake is touched?