How To Use 4Wd?
Posted 21 December 2010 - 03:03 PM
recently I started to explore more about the function of a 4wd system. As mentioned in other thread, I own a toyota prado TZ 3.0 and after searching from the internet, I learn that this is a full time 4WD vehicle (4wd come with differential lock).
The 4WD has 4 shifts: H, HL, N, LL and comes with standard auto transmission system (P, R, N, D, 2, L)
My daily use setting is engaged on the "H" and just put on "D" when driving.
When is the time to engage HL, N, LL?
I did test every 4WD shifts and learned that when shifted to HL, the rear wheel tended to skid. When shift to "N" and "LL" the car never move at all although I put my auto trans to "D-Drive". I tested it on the pavement road but with very slow speed (worry if spoil the trans case).
When I park my car (in "P" mode) and shift the 4WD to "LL", I noticed that the "A/T P" light is on. What does it mean?
When the mechanic warnned me not to try 4WD on the pavement road, I felt scared and never try it after that. But I dont feel satisfied untill I get to know how to use the 4WD.
Can any guru dont mind to teach a word or two?
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Posted 23 December 2010 - 09:56 AM
The H or L refers to High or Low ratio on the transfer box. H is for driving at normal speeds, L is for driving at slower speeds - mainly offroad. N is neutral - no drive at all.
Before moving onto the next stage, you need to understand that your car has three differentials. A differential allows a wheel to travel at a different speed to another wheel. You have a front diff (so the two front wheels can travel at different speeds to each other), a rear diff (same for the rears) and a centre diff - the centre diff allows the front axle to travel at a different speed to the rear axle - ie. the three diffs together allow each of the 4 wheels to travel at different speeds. This is essential when you're going round corners!
The second letter - (LL or HL) refers to the centre differential being locked. Normally its 'open' - with an open centre diff you can drive on road in 4x4 mode. all 4 wheels are receiving power. Unfortunately it only takes loss of traction on one wheel to stop the car from being able to move forward. this is because the centre diff sends power to the fastest axle and the axle diff sends power to the fastest wheel. this is what you call "full time" 4x4.
With a centre diff locked the front and read axles are forced to turn at the same speed. it now takes a loss of traction on 2 wheels to stop us moving forward - one at the front and one at the back. This is exactly what you need on a slippery surface, but if you try to drive in this mode on road you risk something called "windup". because tyres have high grip on tarmac surfaces it is more likely that the speed difference between the front and back axles around a corner are not going to result in loss of traction. this can result in enormous stress on your transmission and can damage or even break it. only use a locked centre diff on loose surfaces!
Your prado may or may not have a lockable rear diff (on a separate switch). This forces both rear wheels to travel at the same speed - so if you lock the centre and rear diffs you force three of your 4 wheels to move at the same speed - even better for getting you moving when you're stuck.
Posted 31 December 2010 - 06:18 AM
Posted 20 February 2011 - 06:51 PM
I didn't expect a case of wine, but a 'thank you' would have been welcome.
The most clear and succinct explanation of 4WD I have ever come across.
Posted 16 January 2012 - 05:29 PM
Many thanks in advance.
Posted 17 January 2012 - 06:42 AM
Posted 21 January 2012 - 10:04 PM
Posted 30 May 2012 - 05:47 AM
I own a Prado TZ Auto (1996) and am very satisfied customer..Was looking for How?When? to engage 4x4 googled key words and stumbled upon the thread.....Great work Sir..
Posted 01 September 2012 - 04:52 PM
If you are going in a perfectly straight line then differentials don't matter. Once you turn a corner then they do. Imagine turning in a tight circle on a muddy surface, so you can see the wheel tracks. The outer wheels have to travel further that the wheels closer to the centre of the circle in the same time, so obviously need to travel faster than the inner wheels. To allow this, there is a differential on each axle to allow each wheel on the axle to travel at a different speed, yet both be powered by the engine.
Now think of the rear wheels versus the front wheels. Because the rear wheels don't steer, they don't follow in the tracks of the front wheels, but 'cut the corner' so to speak. So the rear wheels travel more slowly when going around a corner than the front wheels do. The front inner and rear outer are not too different (depending on wheelbase) but there is a big difference between the front outer and the rear inner. For a normal car this doesn't matter, because only one axle is driven anyway, but LandCruisers have full-time 4x4, so there is a differential between the two axles as well. This is why you don't engage the diff locks when on a solid surface, the wheels need to go at different speeds but you won't let them, which puts a lot of strain on the axles, and will also force some of the wheels to lose traction. If you find you can't disengage the diff lock after using it on soft ground (often because you came onto hard ground with the steering wheel not straight), it means there is torque holding the diff lock in place, and not letting it drop out. So you straighten the wheel and go backwards and forwards a few dozen yards to release the torque.
Marlot mentioned that you engage centre diff-lock (HL, or LL for lower ratio gears) if one of the wheels loses traction, and rear diff as well if you lose traction with a wheel on each axle. In a non 4x4, one axle doesn't matter, you just have to lose traction on the driven axle, but in a 4x4 they are all driven, so losing traction on any wheel is enough. So why have full time 4x4 (the Mitsubishi Shogun doesn't, eg)? In order to move the weight of your vehicle forward, you need to apply a force to the ground through the tyres. If you are powering only two wheels, the entire force is split between those two, but with 4x4 you spread the force over four wheels. So although 4x4 means you have to engage diff lock if any wheel loses traction, you are less likely to lose traction in the first place, because each wheel requires less traction to move you forward. Also, whether off or on-road, it is still true that if you turn the vehicle the wheels need to travel at different rates, and if you lock the differential you stop that, so forcing one or more wheels to lose traction. Therefore, even off-road, you don't engage diff lock UNLESS you have lost traction, and only until you get to ground where you have traction again.
Finally, a practical example. I live at the top of a steep winding track in scotland. During winter, that track is covered in an inch of ice with snow on top. The LC has no problem with this. At the top, I go into LL 1st gear and lock rear diff as well. Then point downhill, and keep my feet off the pedals! In low range first it goes down with engine braking at walking pace, and with all diffs locked, one of the wheels losing traction doesn't mean I suddenly lurch forward. As soon as I'm at the bottom, disengage all locks, go into high range, and drive off. Works like a charm.
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