 | Home > Puma (Tdci) > "idle jack" |
 
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| Dave Bellis Member Since: 21 May 2014 Location: Shrewsbury Posts: 133   
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Forgive my ignorance but how does "idle jack" work on a hill descent, what stops the engine running away? I've used it a few times and in my humble opinion its a great feature.....
Dave |
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18th Jan 2016 7:43pm |
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| Caterham Member Since: 06 Nov 2008 Location: Birmingham Posts: 6298  
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There's hill decent which I suspec is what you're referring to and i'm not sure how that works other than low gears and high compression engine prevent the engine from running away. The abs might have something to do with it too.
My understanding is idle jack just increases the the tick over rate to try and minimise transmission shunt when using very low ratios ie 1st a reverse in lo range. |
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18th Jan 2016 8:08pm |
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| grafty99 Member Since: 15 Aug 2012 Location: North Devon Posts: 4785 
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As Caterham says it is to help in low range 1st, 2nd and reverse.
HDC on modern landrovers uses the brakes. On a defender its just the super low gearing that keeps the vehicle speed and revs low on a decent. There isn't enough weight in a defender to overcome the inertia of the engine in low range  2002 90 Td5 Station Wagon
1990 Vogue SE Triumph Tiger Explorer 1200 Td5 90 Thread http://www.defender2.net/forum/topic50767.html Tdi 110 Thread https://www.defender2.net/forum/topic69562.html RRC Thread http://www.defender2.net/forum/topic54492.html Instagram http://www.instagram.com/george_grafton |
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18th Jan 2016 9:10pm |
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| grafty99 Member Since: 15 Aug 2012 Location: North Devon Posts: 4785
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The only way to disable it is to disconnect the hi/lo range switch on the transfer box....
....you could always fit a isolating switch inline as an idle jack disable switch. DISCLAIMER It changes the throttle map and power delivery too so remember to only use it for low speed and low throttle activities or something might break! 2002 90 Td5 Station Wagon 1990 Vogue SE Triumph Tiger Explorer 1200 Td5 90 Thread http://www.defender2.net/forum/topic50767.html Tdi 110 Thread https://www.defender2.net/forum/topic69562.html RRC Thread http://www.defender2.net/forum/topic54492.html Instagram http://www.instagram.com/george_grafton |
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18th Jan 2016 9:55pm |
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| Tomcat 106 Member Since: 08 Jun 2015 Location: Essex, just. Posts: 124 
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Many Thanks Grafty
MY15.5 110XS CSW
1956 88" S1 2004 Tomcat 106 Comp safari |
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18th Jan 2016 9:58pm |
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| Dave Bellis Member Since: 21 May 2014 Location: Shrewsbury Posts: 133
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Thanks for the replies, still intrigued, it holds the revs at 1200 whether there is a decent descent/descent decent of 10 degrees or 45 degrees no matter the weight is inside....It apparently only happens in low one and reverse oh I don't have ABS so no braking involved....
I find it pretty useful reversing with a trailer.....surely the clutches aren't that fragile.......it is a Defender!!!! Dave |
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18th Jan 2016 10:35pm |
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| AndrewS Member Since: 10 Apr 2007 Location: Hereford Posts: 3707 
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Idle jack is a feature that land rover HAD to install into the Defender Puma engine software.
The reason for this is the engine has small diameter pistons but a long stroke making the engine tall. (this is why there is a bulge in the Puma bonnet) When an engine is designed with the above criteria it creates lots and lots of engine braking, too much for controlled 4x4 descent in poor conditions. As a result of the masses of engine braking when descending a steep slope in 1st low range at tick over (say 800 rpm) the wheels would lose traction and the vehicle would run/slide away. Experienced off road drivers in this situation would simply increase revs to get the wheels to rotate faster to match the ground speed thus re-gaining traction and control. So by increasing the revs to 1200 rpm this creates less engine braking resulting in less resistance between the tyres and driving surface thus providing grip and control. So its a safety bodge or 'get around' to make an engine work in a situation that was never designed to be used in, thanks to Ford 
As a result of the 1200 rpm idle jack off road driving techniques have had to be adapted to suit. 130's have feeling's as well you know  |
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18th Jan 2016 11:25pm |
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| nickhodgson Member Since: 08 Dec 2009 Location: Zambia Posts: 174 
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Hi AndrewS
Interesting information about the "idle jack" - especially the bit about having to increase idle speed to improve traction. I would actually find it more convenient to have a lower idle speed to make the car go even slower. What is your source for this information as a link would be useful? Regards Nick 1995 300tdi Defender 90 P/UP 2011 Puma Defender 130 D/C 2000 300tdi Defender 110 P/UP 2015 Discovery 4 SDV6 HSE |
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19th Jan 2016 9:01am |
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| Dave Bellis Member Since: 21 May 2014 Location: Shrewsbury Posts: 133
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AndrewS,
thanks for the comprehensive reply, I now understand what is exactly going on, I knew what it was for but didn't understand the how, now all make sense.......I'm new to Defenders and on the small amount of off road "stuff" I have done found the feature really useful......I'm certainly not a skilled off roader although would like to be.... Thanks again Dave |
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19th Jan 2016 9:53am |
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| blackwolf Member Since: 03 Nov 2009 Location: South West England Posts: 17382 
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AndrewS is (of course!) completely correct.
The Idle Jack feature was a work-around due to the TDCi engine providing too much engine braking on steep descents as a result of the long-stroke engine design coupled with the unusually low first gear. This was well-documented at the time (Landrover making a 'feature' from a bug really) but I'd be hard pushed to point you to any specific documentation now. The effect of the excessive amount of engine braking was to cause the vehicle to lose traction under conditions where previous Defender/Landrover vehciles would not lose traction, and this was felt to be unacceptable. It is of course well known that on steep descents on loose material (which are by their nature very dangerous) the driver may well have to accelerate to keep directional control of the vehicle, and the idle jack was a reasonably effective solution to the 'problem'. It does not however obviate the need for the skillful driving, and does require some driving techniques to be modified. For reasons which I have never understood there has also always been confusion in some people's minds between the anti-stall system and the idle jack. They are two completely separate systems with completely different purposes. |
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19th Jan 2016 10:08am |
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| nickhodgson Member Since: 08 Dec 2009 Location: Zambia Posts: 174
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Hi Thanks for the answers...
I do understand that if you are using engine braking down a steep hill and you loose traction (the wheels actually turn too slowly and skid) that you have to accelerate a bit to regain traction and in turn be able to better control the car. But having driven off road a lot I would prefer to have an initial slower speed and be able to make the decision to accelerate rather than a factory engineer telling me what I need to do.......but it appears to be common practice more and more to not let the used take his own decisions!! Another gripe for bush driving is the anti-stall. Not in 1st gear but particularly in 3rd or 4th high range. Where in my 300tdi engined cars I can slowly crest a hill in 3rd for example and the engine will reduce its revs to a low level (below idle speed) and then speed up slowly on the other side and you don't have to change gear. On the Puma car I have you have to change gear as the anti-stall tries to keep the revs and consequently the speed up. Its a small issue but a difference in my opinion that was not an improvement. Thanks Nick 1995 300tdi Defender 90 P/UP 2011 Puma Defender 130 D/C 2000 300tdi Defender 110 P/UP 2015 Discovery 4 SDV6 HSE |
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19th Jan 2016 12:26pm |
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| blackwolf Member Since: 03 Nov 2009 Location: South West England Posts: 17382
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I agree about the removal of the common sense option, but this sadly reflects the huge problem caused by the fact that many people now have absolutley no common sense. Rather than keeping the roads safe by preventing stupid people from driving, we make smarter cars to enable people who are even more stupid to drive. It is getting worse as cars get more and more technology.
I have to say that I disagree with you about the anti-stall, though. I think that the Puma anti-stall is brilliant and exploit it to the max. I have never had the 'problem' you mention since I never rely on and engine to run at a speed lower than its idle speed, since it isn't exactly good for the engine. I am quite happy to crest a summit on tickover under control of the antistall though. THe ability to pull away regardless of load or gradient without any use of the accelerator pedal is great, and the only times anti-stall has unexpected effects are times when a lower gear is more appropriate anyway. It is also very satisfying when driving in traffic just to let the vehicle drive itself in first and smell the slipping clutches of the cars behind as they try to keep the speed down to match! |
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19th Jan 2016 12:45pm |
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| lohr500 Member Since: 14 Sep 2014 Location: Skipton Posts: 1316 
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I'm struggling to get my head round the anti stall.
In effect didn't the mechanical diesel pump/ fuel governor on good old fashioned diesels (like the old 2 1/2 series diesel for example) do the same thing? IE try to keep the revs at a constant speed, irrespective of a varying engine load? Is the anti stall simply replicating on an electronically controlled injection system what we had for years on traditional diesels with mechanical injector pumps? Or am I missing something? |
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19th Jan 2016 7:58pm |
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| blackwolf Member Since: 03 Nov 2009 Location: South West England Posts: 17382
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In essence yes, but the advantage of modern electronics is that it can be far more responsive. Couple that with the substantial low revs torque of the Puma engine, and anti-stall becomes very effective, more so that any other light vehicle I have driven.
Another advantage of the modern control systems is that two dissimilar engine management strategies can coexist. When there is no throttle pedal input, the anti-stall will fuel the engine to keep the revs constant at tickover irrespective of load. When there is throttle input the system switches seamlessly to a conventional demand-based strategy. Traditional diesels with governor based pumps are less responsive since the throttle adjusts the governor rather than directly adjusting the fuelling. |
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19th Jan 2016 10:48pm |
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