Thread: Idle-Jack Dangerous in low range reverse Puma

Tact/Neil
Please don't feel stunned about me!
The concern raised in that post was why information about this feature/s was not presented in the drivers handbook (I have not yet found reference to it anyway) as this is new technology for many.
Do I have both idle-jack and anti stall, or just one of them,etc.
After sliding down 100m of clay at Mt Stanley on the weekend, I can assure you that any extra revs provided were not appreciated.
Rich.

Originally Posted by tact

Am still stunned you and others have had an issue with this. In my vehicle, after selecting low range, selecting reverse/first, letting out the clutch normally, the transition from around 800rpm (normal idle) to 1100 rpm (jacked up idle) is barely perceptible, gradual, controlled and smooth. Even when at 1100 rpm even the slowest garden snails leave me in their dust.

I should video it and share it....

If yours lurches off dangerously there has to be a fault somewhere and needs a warranty fix!

Hey Neil, in my experience it's not so much that it lurches in low reverse, but that when reversing down hill in low the idle jack acceleration can be dangerous because it makes low reverse faster than it should be and it strongly resists braking. I have found that in low reverse on a steep driveway, when I tried to slow the vehicle by applying the brakes, the idle jack fought the brakes, artificially keeping engine revs / speed relatively high. Obviously it would be better not to touch the brakes in this situation, but the vehicle speed was way too fast for a safe descent ...Now I never reverse downhill in low.

In my 300tdi, low reverse is entirely predictable, allows engine braking to hold the vehicle back and is also responsive to the foot brake. In the Tdci engine braking and foot brake are strongly counteracted by the idle jack.

Originally Posted by FV1601

Tact/Neil
Pleas don't feel stunned about me!
The concern raised in that post was why information about this feature/s was not presented in the drivers handbook (I have not yet found reference to it anyway) as this is new technology for many.
Do I have both idle-jack and anti stall, or just one of them,etc.
After sliding down 100m of clay at Mt Stanley on the weekend, I can assure you that any extra revs provided were not appreciated.
Rich.

It's not documented in the owner's manual or even the workshop manual as far as I can see.

My experience with the vehicle speed when idling along in low reverse or low first is that it is very slow! Slow enough for even the steepest slippery slopes you'd want to tackle. (And here in Malaysian rainforests/jungle we get some fantastic, looooong, steeeeeep, slippery slopes).

What I have found is that on the steepest long greasy descents your wheels have almost no traction at all and basically are sliding all the way down anyway. With the wheels turning at something approximating vehicle actual speed (as opposed to being locked up on brakes or turning way too slow) you do get enough traction to see steering inputs have an effect.

In fact many times I use a technique more often used to maneuver a boat - turn the steering where you want to go and give the throttle a bit of a blip, turn steering agin and blip throttle again... when the vehicle is pointing where you want give it a little longer blip of throttle. Back to idle. Wash/rinse/repeat to the bottom of the hill.

With wheels not rotating at all, or rotating way too slow and arguably not much difference to not rotating at all, of course you just slide uncontrolled down the entire hill.

Related to idle Jack and anti stall, though not getting as much attention, is that the throttle return after a blipping as described above is a very controlled and slow affair before getting back down to idle revs. This slowing of the throttle return to idle is GOLD in the slippery downhill described above. Imagine this:
- you are sliding downhill, blip throttle to get some tyre bite and steerage. If after the blip your engine instantly goes back to idle speed (huge compression) as would be the case in older diesels without electrickery - you instantly break the traction you had for a moment. Been there, done that (in toyotas). You need skills to blip throttle just right and then with your right foot slowly reduce revs again
- the TDCi does that last bit for you. Your right foot can hit/release throttle as roughly/fast as you like and the ECU ensures a nice gradual return to idles revs after every blip.

Who here has a TDCi and a TDI? Can you do a slow race for us as an interesting comparo?
Which vehicle is slower at idle in first low?
Which vehicle is slower at idle in reverse low?

One of the assertions made to explain the need for idle Jack on TDCis is that it compensates for lower gearing and brings the vehicle speed back UP to what older models would do in low range.

A "slow race" between two stock tyred/geared defenders (TDCi/TDI) would close the book empirically on whether TDCi speed at idle is too fast.

I used to have a Tdi Disco but it was an auto so not comparable to a manual. The auto was too fast downhill, even in low first.
All I can say is the idle jack/anti-stall on the TDCi hasn't been a problem for me. In fact, low first is so low that at times I've had to accelerate to keep moving downhill at the desired speed.
I haven't noticed a problem in reverse, even on a steep 4WD course near Lismore a while ago. The anti-stall certainly seems to do a lot to prevent it stalling on a steep climb, which makes backing downhill a less frequent problem.
Interested to hear others' experiences.

Originally Posted by tact

Who here has a TDCi and a TDI? Can you do a slow race for us as an interesting comparo?
Which vehicle is slower at idle in first low?
Which vehicle is slower at idle in reverse low?

One of the assertions made to explain the need for idle Jack on TDCis is that it compensates for lower gearing and brings the vehicle speed back UP to what older models would do in low range.

A "slow race" between two stock tyred/geared defenders (TDCi/TDI) would close the book empirically on whether TDCi speed at idle is too fast.

My post above describes exactly that comparison. 300tdi is definately slower and safer in low reverse than Tdci in my experience. First low forward in Tdci is excellent, slightly better than Tdi. I've never had an issue with either in low first forward.

Originally Posted by MrLandy

My post above describes exactly that comparison. 300tdi is definately slower and safer in low reverse than Tdci in my experience. First low forward in Tdci is excellent, slightly better than Tdi. I've never had an issue with either in low first forward.

Reverse gear is slightly taller than 1st in the TDCi gearbox. (Ratios below).
(Edit: as is the case for some R380 and LT95 boxes as fitted to some older landys - so a quick Google reveals)

Will be interesting to get a real measure how TDCi compares to the speed of a TDi in low reverse. (As there may be ratio differences elsewhere in the drivetrain)

Gearbox ratios:
1st 2nd 3rd 4th 5th 6th Rev
5.441 2.840 1.721 1.223 1.00 0.742 4.935

Originally Posted by tact

Who here has a TDCi and a TDI? Can you do a slow race for us as an interesting comparo?
Which vehicle is slower at idle in first low?
Which vehicle is slower at idle in reverse low?

One of the assertions made to explain the need for idle Jack on TDCis is that it compensates for lower gearing and brings the vehicle speed back UP to what older models would do in low range.

A "slow race" between two stock tyred/geared defenders (TDCi/TDI) would close the book empirically on whether TDCi speed at idle is too fast.

Defender "slow race" - TDCi vs TDi