What you need to know about 4x4 traction control

Designed on the back of brake safety system technology, electronic traction control (ETC) is one of the most significant advances in 4x4 traction technology since it first saw the light of day in a 4x4 off-roader back in 1992.

The basic, sometimes not-so-effective traction system back then has evolved, becoming the single biggest improvement to a factory-standard 4x4 SUV or ute to getting it out of a sticky situation.

Electronic traction control helps a 4x4 off-roader in slippery conditions by doing the exact opposite of what ABS brakes do. An ABS brake system releases brake pressure on a wheel that is about to lock up, so the driver maintains steering control and braking distances are reduced.

ETC instead applies brake pressure on a specific wheel that is turning faster than the others, which occurs with an open differential as traction is lost.

There are, of course, other forms of traction control, from mechanical differential locks to various other systems that uses multi-plate clutches to lock a driveshaft when one wheel begins to spin faster than on the opposite side. Here, we’re only looking at ETC.

You can thank the engineers who developed anti-locking (ABS) brakes for electronic traction control (and other driver assistance systems, such as stability control). These systems share their basic principles and hardware with ABS brakes.

Although more primitive two-wheel ABS systems were used before, the first four-wheel ABS was used in the 1978 W126 Mercedes-Benz S-Class.

What happened in a typical (non-ABS) system back then was the wheels locked up, especially in panic braking situations. This resulted in longer braking distances, less stability and (if the front wheels locked) much-reduced or no steering response.

With the ABS system, it was discovered you could apply as much brake pedal force as you possibly could and the brakes would not lock up, making for far safer, shorter braking distances.

The basic ingredients of an ABS brake system are a wheel rotation sensor at each wheel, a hydraulic valve block to relieve brake fluid pressure to each individual brake and a computer (electronic control unit, or ECU) to instruct the sensors what to do and when to do it.

The ABS ECU receives electronic signals from each wheel sensor that measures wheel rotation speed. When brakes are applied, the sensor output will tell the ABS ECU if a wheel has begun to slow more than the others.

The ECU then activates the hydraulic valve to release some brake fluid away from the brake line to the slowing wheel, slightly releasing brake pressure on that wheel.

Electronic traction control shares a part of the ABS system, using the same wheel sensors and hydraulic valve circuit.

But ETC adds an electric pump and an accumulator, and given the extra work it does, a more complex ECU.

Instead of easing off a brake like ABS does, the ETC system instead does the opposite. It automatically adds braking pressure to an individual wheel to stop it rotating faster than the other wheels.

The way it does that is by having a reserve of pressurised brake fluid to supply to the hydraulic modulator circuit so braking effort can be applied.

An electric pump builds up enough brake fluid pressure (and stores it in an accumulator), so the system is ready to go.

Electronic traction control can only really be integrated at the factory; there’s no aftermarket ETC system. It is relatively light and simple, using mostly existing hardware.

Unlike a mechanical axle diff lock (which locks the wheels on an axle so they can only turn at the same rate), the key benefit of ETC is that it is on all the time, ready to go (although you can manually disable it).

With a mechanical diff lock, you have to slow down or stop to get it to activate, so some anticipation is needed for using it. When you no longer need the diff lock, again, you have to remember to disengage it.

Another thing ETC is good at is not allowing wheels to spin up excessively. When you’re in low range with either open diffs or even locked diffs, it can be difficult in some off-road terrain to apply power without ending up with excessive wheelspin.

If you are on a side slope, for example, because both wheels on the axle are spinning it’s really easy to move off line, potentially into a rock or tree next to the track. In some circumstances, a locked diff can cause the wheels to dig in, helping to get the vehicle bogged.

As ETC works on individual wheels and attempts to reduce wheelspin, it simply won’t permit such a sudden grip to massive slip situation.

When you need the slow, steady progress to avoid damage on a technical climb, the ETC is working at its best, grabbing and releasing any wheel that is starting to spin.

This is a reactive system, meaning that a wheel has to be spinning (or losing traction) before it’ll work, which can mean you lose precious momentum. If you are using ETC solidly for a long time (more than a minute or so, depending on the individual system) the ETC accumulator can run out of pressure, effectively disabling ETC.

ETC is also often not very good in sand, where wheels are often spinning at different speeds and the ETC system can be too intrusive, losing crucial momentum. Some newer systems are better at this, as we’ll explain below.

Where a vehicle has both a locking diff and ETC (there are a few), when the locking diff is engaged for many (but not all) 4x4s, the ETC for the non-diff-locked axle is automatically disengaged.

The 1992 Range Rover was the first to use a (two-wheel) traction control, and even when it went to four-wheel ETC in 1998 with the first-generation Land Rover Freelander, it was a slow-reacting and pretty clunky system.

Today’s traction control system components – such as ABS, ETC, stability control and various sub-systems – are not only lighter and more compact, but are much more sophisticated with faster reaction time and with better computing power.

A part of ETC that has evolved over the years is Terrain Modes, which we saw for the first time in the 2005 Land Rover Discovery 3, and is widespread across various 4x4s and SUVs today.

The terrain mode system is a far more sophisticated ETC set-up, because instead of a one-size-fits-all approach, it has dedicated, driver-switchable mode selection for specific traction scenarios such as rocks, mud, sand, grass or snow.

The engineers have designed the system’s software to make ETC intervention more or less aggressive, depending on the individual terrain setting you’ve selected.

A further development of ETC is electronic centre diff control – linked to the ETC system and the engine management system, this allows all drive to be precisely channelled to one wheel if necessary in low-grip situations.

To avoid getting stuck when off-roading involves many factors, everything from the variety of off-road terrain you’re trying to get across to the vehicle and driver.

Grippy off-road all-terrain or mud-terrain tyres, driver experience and skill, progressive engine torque, good low-range gearing, wheel travel and clearance can all make a difference.

So ETC isn’t perfect and won’t fix every off-road traction problem, yet it’s hard to think of any other 4x4 off-road technology advances this century that offers such a simple, easy and effective way to avoid getting bogged.