Thread: Descending Hills - D2 Auto Lockup for braking

I use Low First in Manual mode with or without HDC.

When I first got the car I thought HDC was a crock, until I did the above, now I think it's tops.

Just need to turn on HDC before heading downhill or it takes a while to come on.

Originally Posted by Cannon

When I first got the car I thought HDC was a crock, until I did the above, now I think it's tops.

I'm definitely going to experiment with a few different approaches to descending now I have a better idea of how the systems interact. Maximising engine braking by allowing the TC to lock up looks to be a key part of the strategy.

Hi Paul.

Another opinion .... ?

In off-road low range crawling/downhill conditions I always put the Auto into Sport mode ... instead of moving the shift points and holding gears longer as it does in high range, Sport mode in low range makes the ZF Auto act like a (clutchless) manual transmission with torque converter lockup in ALL gears ... whatever gear you select is the gear the auto holds .... it will do a very short burst of 1st low as you pull away from a standstill then go straight to the gear you've selected, and will only drop the selected gear if the engine is approaching a stall condition ... so, being that the auto is now locked in 1 gear only, it prevents the runaway thing Pedro was alluding to, because as the (lets say) downhill speed starts to increase, instead of the auto then up-shifting as the ground speed increases and letting the vehicle run away, it holds the gear selected, the rear wheels are now driving the torque converter & gearbox (not the engine), which causes the engine revs to increase, and then the torque converter locks up, loosing the "slip" that causes an un-sport mode selected box to runaway ... you need to give the disco its head and let it start to run, and as the revs rise the torque converter will lockup and haul the ground speed down ... if it's too slow, shift to 2nd ... the torque converter will slip then lockup, hauling down your speed again. You can use this with or without HDC. Your choice. The speed the HDC tries to maintain is dependant on what gear is selected ... lower the gear, the lower the speed it tries to maintain.

Example - A day out with some other AULRO folks saw us negotiating some steep loose downhill descents. I was in low, sport, 1st gear, and with the torque converter locked up, walked all the way down without touching the brakes ... just using engine compression to slow my descent ... another Td5 Auto D2a infront of me hadn't selected sport mode, but was in low range (Drive?), and all the way down the hill I could see his brake lights going on/off rapidly as he tried to halt his descent (I think he had the stick at D). You could see him slipping & sliding everytime he braked.

In a manual, for downhill, I'd always select low 1st and use engine braking ... no different to an auto D2 Td5 ... you just have to set it up right and allow the torque converter to lockup. "Drive" is not a good choice, even in sport mode, as the box will hold 4th, and 4th only, or go 1-2-3-4 without sport mode selected, causing a runaway as the torque converter won't lockup until you have achieved way too much ground speed, whereas selecting sport (or at least selecting an appropriate gear like 1st or 2nd) in low range, will cause the torque converter to lockup much sooner and allow engine braking to do the work .... jabbing at the brakes causes the wheels to loose traction and slip ... remember, sport mode makes the box act like a manual gearbox, by holding only the gear you select.

Go and have a play ... for slow level off-road, try 3rd or 4th low sport. You'll feel it go 1st-4th, then it will stay in 4th, keeping the engine revs low and in the best torque range. Then try a steep hill, with/without sport mode with 1st or 2nd gear selected and give it it's head and you'll feel when the torque converter locks up.

Hi Kev,

I think we might be talking the about same thing

The mode button selects Sport in high range and Manual in Low Range. Manual holds the selected gear unless the engine is at risk of stalling, but the Torque Convertor only locks up with 0% throttle input (See the paste from RAVE regarding low range in my first post). This sounds like what you were doing on your day out.

Manual mode
Manual mode is selected by pressing the mode button whenever low range is selected. With manual mode selected a dashboard lamp will illuminate the letter ‘M’. The transmission will function in a similar way to a manual gearbox. In this condition the transmission will change into the selected gear as soon as possible, and then remain in that gear at all speeds, changing down only to prevent engine stall. For example, if the selector position is in “3” while driving in normal modes, gears 1,2 and 3 will be available However, in manual mode, if position “3” is selected, the transmission will select 3rd gear very shortly after pulling away from rest.

This mode is designed to give improved off road performance, and kickdown is disabled in this mode.

BTW You can check the state of the Torque Convertor with your Nanocom if it has the AutoBox Module. AutoBox > General Inputs > "Solenoid Valve 3" shows the state of the valve which controls Torque Convertor lock up. It will show "Off" when the TC is unlocked.

cheers
Paul

Originally Posted by Fluids

Hi Paul.

Another opinion .... ?

In off-road low range crawling/downhill conditions I always put the Auto into Sport mode ... instead of moving the shift points and holding gears longer as it does in high range, Sport mode in low range makes the ZF Auto act like a (clutchless) manual transmission with torque converter lockup in ALL gears ... whatever gear you select is the gear the auto holds .... it will do a very short burst of 1st low as you pull away from a standstill then go straight to the gear you've selected, and will only drop the selected gear if the engine is approaching a stall condition ... so, being that the auto is now locked in 1 gear only, it prevents the runaway thing Pedro was alluding to, because as the (lets say) downhill speed starts to increase, instead of the auto then up-shifting as the ground speed increases and letting the vehicle run away, it holds the gear selected, the rear wheels are now driving the torque converter & gearbox (not the engine), which causes the engine revs to increase, and then the torque converter locks up, loosing the "slip" that causes an un-sport mode selected box to runaway ... you need to give the disco its head and let it start to run, and as the revs rise the torque converter will lockup and haul the ground speed down ... if it's too slow, shift to 2nd ... the torque converter will slip then lockup, hauling down your speed again. You can use this with or without HDC. Your choice. The speed the HDC tries to maintain is dependant on what gear is selected ... lower the gear, the lower the speed it tries to maintain.

Example - A day out with some other AULRO folks saw us negotiating some steep loose downhill descents. I was in low, sport, 1st gear, and with the torque converter locked up, walked all the way down without touching the brakes ... just using engine compression to slow my descent ... another Td5 Auto D2a infront of me hadn't selected sport mode, but was in low range (Drive?), and all the way down the hill I could see his brake lights going on/off rapidly as he tried to halt his descent (I think he had the stick at D). You could see him slipping & sliding everytime he braked.

In a manual, for downhill, I'd always select low 1st and use engine braking ... no different to an auto D2 Td5 ... you just have to set it up right and allow the torque converter to lockup. "Drive" is not a good choice, even in sport mode, as the box will hold 4th, and 4th only, or go 1-2-3-4 without sport mode selected, causing a runaway as the torque converter won't lockup until you have achieved way too much ground speed, whereas selecting sport (or at least selecting an appropriate gear like 1st or 2nd) in low range, will cause the torque converter to lockup much sooner and allow engine braking to do the work .... jabbing at the brakes causes the wheels to loose traction and slip ... remember, sport mode makes the box act like a manual gearbox, by holding only the gear you select.

Go and have a play ... for slow level off-road, try 3rd or 4th low sport. You'll feel it go 1st-4th, then it will stay in 4th, keeping the engine revs low and in the best torque range. Then try a steep hill, with/without sport mode with 1st or 2nd gear selected and give it it's head and you'll feel when the torque converter locks up.

This method works surprisingly well with the V8 also.

G'Day.
What Fluids said^!
I've used HDC about 3 times since I bought the car, mainly to test the operation. I only have ATRs on and exclusively use 1st low Man mode. I haven't checked the operation on a nanocom, but if I can get bruised by my seatbelt on a downhill, I'm quietly confident the system is operating as above ; )
Downhill control was the big question mark in my mind when I opted to purchase the auto (I couldn't find a manual at the time). I was pleasantly surprised!
I only use HDC on very slippery, steep clay descents.
Cheers, Dave.

"The speed the HDC tries to maintain is dependant on what gear is selected ... lower the gear, the lower the speed it tries to maintain."

Hi Fluids,

I was under the impression that HDC will limit vehicle speed to around 7.5 kmh irrespective of what gear you are in. Way too fast but, that's another debate!

Adjustable HDC only became available on the D3 and works much better (is slower) than on a D2.

Cheers,

Franz

Hi Franz. I run up to a local lookout every 4-6 weeks, and do a downhill run in low range with HDC to cycle the ABS ... I've noted that the downhill speed the HDC maintains seems dependant on the gear selected ....

... placebo effect ? ... maybe ... I'll take closer notice next time I do a run.