Thread: Idle-Jack Dangerous in low range reverse Puma

Another drop dead cool feature of the TDCi (off-road) comes as a byproduct of the way the throttle never seems to snap quickly back to zero. Instead it gradually reduces.

Likely this is an anti pollution thing. Throttle positioners are even on petrol engines going back a few decades.

But on those looooong slippy slidey, wheels always slipping slopes where I described turning the steering and blipping the throttle to get some kind of steerage way... It's great to be able to blip throttle and know it will slowly droop back to lower revs itself. This is how you keep any little grip you caught and bring vehicle speed back down again.

In a vehicle without this kind of feature you have emulate this with right foot. If the throttle (and with it the road wheels) suddenly drop back to idle speed the sudden action gives away any grip you gained by the blipping of the throttle.

See it in action easily. You are cruising along and lift throttle. At first it's like you don't have much compression braking. But then gradually the compression braking increases. Blip the throttle and immediately it seems you don't have a lot of compression braking again... But it comes.

Originally Posted by alien

Having not read the RAVE cd I bought from Dave's shop I'm going to have guess.

The idle up would require a low range signal and use the clutch fluid preasure switch.
When in low range has anyone tried pulling a wire from the clutch switch under the bonnet?
This switch is used for the cruise control I fitted and works on gear changes but not on a light touch of the pedal which is why I'm making the suggestion.
If it works it would not be hard to rig a dash mounted inline switch to disable the switch when required.

Just a thought.

The idle Jack feature (not anti stall) requires all the following to be true:
- low range selected
- first or reverse gear selected
- clutch pedal close to fully up

There has to be switches/sensors on t'fer lever or box, main shift lever or box, and the clutch pedal to detect it's near top of movement.

Defeating any of those sensors may do the job.

Not sure which is the best to tackle. Likely stay away from the t'fer case/shift sensor/switch as this one is likely also used as ECU input for the changed throttle curves when in low range

(When low range is selected the throttle response curves are vastly different to when in high range)

You'd stay away from the main shift sensor too. I guess you'd want this to still work so reverse light and sensors all keep working.

So - I have learnt this when in carparks with speed humps - when you approach a (killer) speed hump you need to time the 'lift' of the throttle - when you get it right in 2nd it slows perfectly to get over the speed hump to then go again for the next one.

At first I thought I had no engine compression - then I learn't that it holds the rev's to a point - then it it drops and the engine compression shows up.

Perhaps I am nuts - but I enjoy this in the Defender!

Originally Posted by tact;2452798 (snipped)

Another drop dead cool feature of the TDCi (off-road) comes as a byproduct of the way the throttle never seems to snap quickly back to zero. Instead it gradually reduces.

See it in action easily. You are cruising along and lift throttle. At first it's like you don't have much compression braking. But then gradually the compression braking increases. .

Originally Posted by Col 110

At first I thought I had no engine compression - then I learn't that it holds the rev's to a point - then it it drops and the engine compression shows up.

Perhaps I am nuts - but I enjoy this in the Defender!

Not nuts at all. Enlightened. I enjoy it on road and love it off-road.

Originally Posted by tact

Not nuts at all. Enlightened. I enjoy it on road and love it off-road.

Handy offroad, but I would prefer it to drop a fair bit quicker on road, but there again I don't like using brakes

Just wondering when you are crawling along and the idle jack feature is being used why the brake pedal does not cut out this feature, at least back to low idle. Surley the abs and traction control would be able to sort it out instead of the engine providing a constant rpm or are the wheels turning too slowly to enable use of the supplementary systems.

Originally Posted by manofaus

Just wondering when you are crawling along and the idle jack feature is being used why the brake pedal does not cut out this feature, at least back to low idle. Surley the abs and traction control would be able to sort it out instead of the engine providing a constant rpm or are the wheels turning too slowly to enable use of the supplementary systems.

Apparently the LR thinking is that when in low 1st/rev the gear ratios in the TDCi result in the wheels turning too slowly for a safe descent in loose/slippery conditions - when engine is ticking over at normal idle speed (~800rpm).

LR solution: bump engine idle speed up to ~1000-1100 rpm to raise vehicle speed at idle to something similar to the earlier Defenders.

Just guessing: If the above is all true (paraphrased: 800rpm in low first/rev is "unsafe") then allowing provision to drop idle rpm back to 800rpm is unsafe.

Am not advocating or defending the thinking. Just regurgitating what I gleaned from several different reading sources.

Edit: certainly in that use case (steep slippery downhill) brakes are a bad thing to touch!
Our Blessed Lady of Acceleration is the patron saint for this use case.

Originally Posted by tact

Apparently the LR thinking is that when in low 1st/rev the gear ratios in the TDCi result in the wheels turning too slowly for a safe descent in loose/slippery conditions - when engine is ticking over at normal idle speed (~800rpm).

LR solution: bump engine idle speed up to ~1000-1100 rpm to raise vehicle speed at idle to something similar to the earlier Defenders.

Just guessing: If the above is all true (paraphrased: 800rpm in low first/rev is "unsafe") then allowing provision to drop idle rpm back to 800rpm is unsafe.

Am not advocating or defending the thinking. Just regurgitating what I gleaned from several different reading sources.

Edit: certainly in that use case (steep slippery downhill) brakes are a bad thing to touch!
Our Blessed Lady of Acceleration is the patron saint for this use case.

Not if the idle-jack is forcing you to go too fast in reverse down a very steep slippery hill and you're headed for a rock wall! Brake! Nothing! Clutch in Brake! Slide! **** that was close! Give me no idle-jack compression anytime.