auto transmission on hill descents

Submitted: Sunday, Aug 03, 2003 at 20:45
ThreadID: 6349 Views:1965 Replies:5 FollowUps:8
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Hi all, Does anybody have any comments on auto versus manuel transmission for hill descents?
I have a Auto Nissan Terrano diesel which is fine for most descents with minimum braking when the track is dry and reasonably smooth, but if the track is slippery or deeply rutted there is no way the engine will hold her back to a safe speed.
Is there perhaps a way to limit engine revs when descending?
I find the auto fantastic most of the time but the inability to keep the speed down is a disapointment.
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Reply By: Truckster (Vic) - Sunday, Aug 03, 2003 at 21:28

Sunday, Aug 03, 2003 at 21:28
the only way to do it is to either pull the handbrake on slightly, as shown in any driver training, or to use your brakes which isnt the best option.

Final option is some low ratio gears like Rockhoppers or Marks low range, but doubt they would be available for the Terrano.
AnswerID: 26711

Reply By: Matt (W.A.) - Monday, Aug 04, 2003 at 08:27

Monday, Aug 04, 2003 at 08:27
Try turning the aircon on this will/should slow the revs down alittle more, I also agree with truckster but not too much to burn the baby out!

HTHKeep It On The Rough Stuff

Matt (W.A.)
AnswerID: 26732

Follow Up By: yarda - Monday, Aug 04, 2003 at 12:05

Monday, Aug 04, 2003 at 12:05
Hi Matt, the only problem with doing the aircon trick is , yes it adds an extra load to the engine but on an injected petrol motor it also activates the idle air system allowing extra air past the throttle body to keep the revs up . can cause some interesting hiccups etc. Brad
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Follow Up By: Matt (W.A.) - Monday, Aug 04, 2003 at 12:11

Monday, Aug 04, 2003 at 12:11
Brad, good point but what if it's a Diesel? will this still happen?Keep It On The Rough Stuff

Matt (W.A.)
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FollowupID: 18276

Follow Up By: yarda - Monday, Aug 04, 2003 at 12:40

Monday, Aug 04, 2003 at 12:40
Matt, it would depend on the fuel system in the vehicle - whether it is electronically controlled/ mechanical injection / computer or not. You would have to check each vehicle independantly, just have the car idling without any accessories on and switch on the aircon, you should hear the compressor clutch activate, then check for a change in the idle speed, if the idle drops about 50-100 rpm and stays down you wont have a problem, however if it drops then recovers to a higher idle speed than the initial base speed you may have troubles. Brad
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FollowupID: 18277

Follow Up By: Matt (W.A.) - Monday, Aug 04, 2003 at 12:47

Monday, Aug 04, 2003 at 12:47
Cheers for that.Keep It On The Rough Stuff

Matt (W.A.)
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FollowupID: 18278

Follow Up By: Matt M - Monday, Aug 04, 2003 at 14:47

Monday, Aug 04, 2003 at 14:47
By turning the Aircon on the revs can drop to allow more engine braking but with the Terrano there IS an increase in the engine revs with the Air on!! With it idling turn the aircon on and then off after a few seconds to see if it increases, Im sure you will find that it will!!

If you wanted there is little to disconnect this feature at the injector pump but may seem a little daunting if you dont know what you are looking at! The idle up just makes it more pleasant idling at the lights with the air on!

Let me know if you want more info, otherwise I would be learning the driving with the footbrake on, as described furthur on in this post!

Goodluck!

Matt.
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FollowupID: 18285

Follow Up By: bruce.h (WA) - Monday, Aug 04, 2003 at 14:57

Monday, Aug 04, 2003 at 14:57
Gday matt
yes even in a deisel you will find that the revs will jump up as the air con cuts in & if that happens at the wrong time you can end up in trouble,
best way is to feather the breaks when needed this lessons the chance of break fade out that can happen using the hand break method,by using the foot break not only do you have faster control over the amount of break in case of wheel lock you do not have to break your concerntration off of the track to fumble around with the hand break to release it

the other problem that i have found with the hand break method is that very few 4wd hand breaks work properlyso in some cases 2 clicks of the handle does nothing & in others it almost puts the breaks on full

hope this helps
Regards Bruce
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FollowupID: 18287

Reply By: Outnabout David (SA) - Monday, Aug 04, 2003 at 13:31

Monday, Aug 04, 2003 at 13:31
Jimmy,

This was on the Overlander forum recently and has worked for me on the few occasions when I have needed a bit more "feel". It takes a bit of practice though but I prefer this method to useing the handbrake method but it is really personal choice. Either way you must never apply either method of braking to hard as you will get wheel lockup.

From the Overlander forum
“There is another very effective method for taking auto fourbies down steep hills (slippery or rock hopping), often referred to as "driving through the brakes". My Pathy had a lovely auto box that had very good engine braking in low first but it would still tend to go too fast especially if you were crawling over some rocks down hill.

Here's the gist of it, see if you can get the hang of it after a bit of practice.

Basically you select low first (or reverse) and apply modest, steady pressure to the brake with your LEFT foot. Enough to hold the car with the handbrake off,
then gently apply power and the car will move slowly in a very controlled way, in fact dead slow if you want.
If you lift off the gas and it gently stops then you have about the right brake pressure.
On muddy slopes you will need to keep on enough power to keep all the wheels turning slowly and steadily whilst keeping absolutely constant pressure on the brakes.

A few of things happen when you use this technique,
1. you tend to fool the open diffs into seeing very similar torque across the axle so you get a very even drive across axles, particularly true on the front axle where your brakes become the dominant load on the axles, not the tyre grip (or lack of it)
2. you get all four wheels receiving very close to the same drive torque and nice and slow. You can suddenly feel the car biting on all four wheels insead of slipping and sliding as it will do if you attempt to roll down the hill on brakes alone.
3. your torque converter is slipping heaps (you are forcing it to) but your engine rpm will be somewhere around 1000-1500 (not at idle) and that usually means the torque converter will be trying hard to connect engine & tranny .. this is one you would have to ask an expert about, but I have heard claims that this is one of the reasons this technique works so well.

I have crawled down slopes and rock ledges where I have got on three wheels momentarily but despite a little bit of slip the car quickly regains composure as you still have all four wheels (restrained by the brakes) being driven nice and steadily. The same piece of track tackled on brakes alone would have been pretty dicey.

I have found this technique very valuable whenever you want to crawl an auto fourbie through a hard bit and you can easily go as slow as a Tojo diesel in Low first. In fact using this technique I could often get through better than manual diesels with open diffs

My guess is that you ATF fluid heats up a bit doing this but you are only doing it a couple of minutes here and there, plus your front discs may makes some weird squealing and rattling noises as the calipers move about under the combined forces of braking and drive. Nothing too alarming though as they are still working

Some 4WD training instructors teach this technique.

Go out and practice it and soon you will be using it often in the hard stuff. When you are good at it, you just apply a bit of brakes with the left foot whilst maintaining or slightly increasing the power anytime you look like losing momentum UPHILL because it helps even out the cross axle torque (poor man's LSD).

BTW, don't forget the key factor of tyre pressures. Let them down to 25psi and that alone will make a big difference
We have so little time to enjoy our land
AnswerID: 26755

Follow Up By: Member - AndrewPatrol - Monday, Aug 04, 2003 at 18:40

Monday, Aug 04, 2003 at 18:40
David, I was taught this method in a manual car and it works so well that you wont go back to a manual. I now have an auto and theres no way I'd go back to a manual. With this "left foot braking" and the auto's advantages uphill you'll be going straight past the other guys, within the limits of your suspension, of course!!!!! BTW its fairly critical to get the balance of accellerator and brake pressure right but when you do its like sex (almost).
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FollowupID: 18304

Reply By: Jimmy - Monday, Aug 04, 2003 at 20:47

Monday, Aug 04, 2003 at 20:47
Wow thanks everybody I am very happy that there is so many solutions to a problem that I had thought was going to restrict my fun somewhat.
I just had a play with the air con in the terrano and the revs drop by about 120 when it is switched on, so with the windows down it shouldnt switch back off at any unfortunate moments. I reckon if I combine this with a couple of clicks on the hand brake and have a bit of practice on a nice safe slippery slope with the left foot brake method I should be on my way!
Thanks again
Jimmy
AnswerID: 26785

Follow Up By: Mick - Monday, Aug 04, 2003 at 22:49

Monday, Aug 04, 2003 at 22:49
I think you should consider a 4wd course Jimmy
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FollowupID: 18346

Reply By: Rob from Cairns Offroad Training & Tours - Wednesday, Aug 06, 2003 at 14:03

Wednesday, Aug 06, 2003 at 14:03
I don't recommend the use of the handbrake on a steep hill descent with autos. By applying the hand brake you are assuming your level of grip will remain constant it seldom does. If your level of grip deteriorates and with your forward weight distibution you can unexpectedly lock your rear wheels. Also if your car is set up correctly your revs should rise to compensate for your a\c compressor cutting in. I always turn my A\C of in tricky conditions. These techniques are all part of a Nationally Recognised 4wd Course. Cheers Rob Berrill
Cairns Offroad Training & Tours
www.4wdtraining.com
4wdtraining@cairns.net.au
www.cedarparkresort.com.au
Cairns Offroad Training & Tours
AnswerID: 26940

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