As an accredited 4WD trainer, I offer customised courses based on the requirements of my students, whether they be 4WD hobbyists or groups from the resource sector. These courses are split into two sections.
The first section is practical driving, which participants are always champing at the bit to try out. Meanwhile, the other portion of theory is often met with disdain. Because of that I always try to start it off with an interesting task. I point towards a parking lot full of cars and ask the students to identify which vehicles they think are true 4WDs.
More often than not, this amuses everyone, but at the same time it’s met with expressionless blank looks or volleys of different interpretations. Discounting suggestions that large tyres or height distinguishes 4WDs, I ask, “What about that state city bus? Is that a 4WD?”
I never give in and inevitably, after a bit of tooing and froing, the consensus among the group comes to this: a true 4WD is fitted with low range capability, or rather, a transfer case (TC). The presence of a transfer case is undeniable proof you are dealing with a true 4WD, as opposed to a fake wannabe plastered with 4WD badges as a marketing ploy.
WHAT IS A TRANSFER CASE?
As you already know all vehicles have their engine paired with either a manual or automatic gear box commonly known as a transmission. A transmission is made up of a set of gears and gear trains that convert controlled power from an engine to provide speed and torque which turns another device, namely the wheels. It enables a vehicle to efficiently handle all types of road conditions, such as straight roads, corners, hills and varying loads. It even accounts for challenges such as the road surface, obstacles or strong head winds, while maintaining the driver’s desired performance.
So what happens when ‘road conditions’ become challenging? Enter the TC. A transfer case is basically another gear box that’s paired with the normal transmission. It has two main functions. The first is to connect (provide power) to the front wheels. In a part-time 4WD, such as a Nissan Patrol, Jeep Wrangler, Toyota Hilux or Isuzu MU-X, it is only the rear wheels that are normally powered for normal high traction conditions. Whereas a full-time 4WD, such as a Jeep Grand Cherokee or Toyota Prado or LandCruiser, defaults to 4WD all the time, regardless of surface, to provide ultimate traction.
As a side note, before I get onto the second main function of a transfer case, I will point out that over the years many manufacturers have modified, introduced or even removed 4WD functions. For example, lockable centre diff. Things can get really confusing, like with Mitsubishi’s Super Select, which has a centre diff that can be locked by changing from 4H to 4HLC, where LC is “Locked Centre Diff”. Toyota Prado has a centre diff option that can lock the centre diff in low and high range, allowing it to be opened in low range for better manoeuvrability for certain circumstances.
Of course, with some 4WDs like the Toyota LandCruiser 70 series, there’s a further step when wishing to use 4WD – that is, manually engaging the front hubs (free-wheeling hubs) by turning the dial from free to lock, or else you’re not going anywhere if stuck.
It would be very difficult to cover every scenario within the confines of this column, especially looking back over the last fifty or so years of 4WD evolution.
Now, where were we? The second function of the transfer case is its role, in both part-time and constant 4WDs, of engaging low range gear ratios. Low range provides increased torque to all four wheels at a lower and more controlled speed.
ENGAGING THE TRANSFER CASE
As mentioned, manufacturers are always exploring avenues to improve their 4WD’s capability for both on and off road. Modern 4WDs, besides having low range capability, are now embracing computer technology to go even further. Manufacturers like Land Rover and Jeep, for example, have developed systems to enhance drivability in certain offroad conditions. Land Rover has its Terrain Response System and an infinitely-variable locking centre diff. Jeep has its Selec-Terrain which further tunes the 4WD system for conditions such as rocks, sand and snow. Many others have their slant on improving their own model’s drivability, like Toyota’s Crawl Control, which is an amazing piece of technology.
The transfer case is brought into play, using a variety of systems, depending on 4WD make, model and technology. It is usually selected manually by the customary stubby lever or a dial on the dash, or even buttons, with even this simple operation evolving with new technology and computer participation.
In most part-time 4WDs, you’ll still find the options of 2H (2 wheel high range), 4H (4 wheel high range), N (neutral) and 4L (4 wheel low range), with the occasional Auto selection which allows a part-time 4WD to be a constant 4WD. Many 4WDs, like LandCruiser, Prado and Jeep, have full-time 4WD systems – where variable power is constantly available to all four wheels to enhance safety, handling and more on hard surfaces – and do not have a 2H selection (that I know of).
WHEN TO USE EACH SELECTION
In part-time 4WDs, 2H is set for normal everyday driving on hard surfaces, as it’s only providing power to the rear wheels like a conventional 2WD vehicle. The vehicle can be driven like any other vehicle you’d drive when road traction and conditions are good, such as on dry bitumen.
A constant 4WD will usually show 4H on the TC selector and is able to be driven in the same manner because of its fitted centre diff, which allows for different wheel rotation front to back. Of course, if you never turned, you’d not need a centre diff, but the reality is making a turn results in all four wheels rotating at various revolutions – hence the need for a centre diff.
In a part-time 4WD, selecting 4H now connects the front shafts along with the rear drive shafts inside the TC, which supplies power to all wheels, similarly to a constant 4WD. A successful engagement shows up on the dash with symbols like ‘4WD’, ‘4x4’ or similar. It doesn’t change the overall gearing ratios but is chosen when requiring more traction or where some slippage may be inevitable at higher speed. You might want to choose 4H on loose gravel roads, easy dirt tracks, and on snow or sand – in which cases it will enhance safety while maintaining forward and controlled progress. I’d recommend to keeping speeds below 90kph when in 4H.
With many newer 4WDs, selecting 4H in a part-timer may adjust a vehicle’s systems, such as stability control, braking and others features to further enhance traction, reliability and safety. When travelling on terrain like sand in 4H, momentum is paramount, so it’s best to disable the vehicle’s traction control (where fitted) to stop the ABS system kicking in. This can usually be achieved by prolonged pressing of the ESC button confirmed by illumination on the dash. It should be noted, depending on make/model, that ABS is not always fully disengaged and can in fact be ‘adapted’ by the vehicle’s systems.
4L like 4H locks in the front drive shafts and provides power to all wheels to turn at the same time, but more importantly it engages lower gear ratios within the TC. Engaging 4L provides even more traction and maximises torque, disables or alters ESC, and further ‘adapts’ some of a modern vehicles systems to maximise the vehicle’s capability.
When using 4L, keep the speed below 25kph, or else you’re simply in the wrong range and should select 4H. Prolonged 4L at high speed can cause irreparable damage to a vehicle’s components.
Low-range has many benefits. It delivers more torque, plus better safety and control, enabling you to drive tricky obstacles, such as very loose surfaces, mud and water crossings. Plus, it assists with controlled negotiation of rocks, ruts and very steep terrain.
An added benefit of LR is that it takes advantage of the engine’s compression combined with lower gears, such as first gear, for descending steep hills, and thereby saves the brakes from overheating or locking up. It can even be used on a slippery boat ramp, to slow a decent or ascent, thus minimising any chance of sliding. A big environmental benefit of a slow controlled drive in 4L is that it reduces damage to tracks, especially when wet.
It’s not always rosy. Using 4H and, even more so, 4L for too long or in the wrong conditions can impede handling and steering, negatively impact turning circle, and increase fuel economy.
WHAT RANGE FOR PERFORMING RECOVERY?
Depending on the type of vehicles and the situation at the time, I suggest selecting 4L when performing most recoveries including snatch strap scenarios.
I also suggest that both 4WDs involved engage and drive in low-range 2nd to lessen driveline damage throughout the undertaking.
While not always controllable, try to avoid spinning tyres excessively when performing a difficult recovery, as abrupt stops or times when traction is suddenly gained could result in broken CVs or uni-joints or even worse.
WHEN IT’S TIME TO HEAD HOME
For both part-time and constant 4WDs, it’s important to disengage 4L once the surface returns to high traction (eg. bitumen) and, for part-timers, to not use 4H either. Additionally, vehicles fitted with a centre diff should not use this feature on high tractions roads; doing so would place excessive strain on the driveline components. This is commonly referred to as transmission wind-up and can possibly result in expensive damage or long waits for the tow truck. On the topic of tow trucks, if there’s not one available and you need to be towed instead, select neutral on the TC and leave automatic transmission in park or in gear for manual 4WDs.