4 wheel Electronic Traction System

The 4-wheel Electronic Traction System, badged 4ETS, is Mercedes-Benz's brake-based traction control developed for its 4MATIC all-wheel-drive models. Rather than relying on mechanical differential locks to deal with a wheel that has lost grip, it uses the vehicle's brakes and electronics to achieve the same outcome, allowing torque to be redirected to whichever wheels can still put it to the road. It was introduced at the end of the 1990s and became a defining feature of the brand's SUVs and saloons.

The principle is straightforward but effective. Open differentials, by their nature, send torque along the path of least resistance, so when one wheel slips on ice, mud or a kerb it spins freely and starves the wheel with grip of drive. 4ETS uses the wheel-speed sensors shared with the ABS to detect when a wheel is rotating faster than the others. It then applies the brake to that individual spinning wheel. Because a differential always balances torque across its outputs, braking the slipping wheel raises the reaction torque it can transmit, which in turn forces an equal amount of torque through the differential to the wheel on the opposite side that still has traction. Applied across all four corners through the centre, front and rear differentials, this mimics the effect of three locking differentials at once.

For the driver the result is a vehicle that pulls away cleanly and climbs slippery gradients with no levers to engage and no diff locks to remember. The system works automatically and continuously, intervening only when needed and releasing the moment grip is restored, which makes capable all-wheel-drive behaviour accessible without any change in driving technique.

The great packaging advantage of 4ETS is that it allowed Mercedes to fit comparatively light, simple open differentials in vehicles such as the M-Class and later 4MATIC models, saving weight and cost while still delivering strong traction. The hardware for locking differentials, with its actuators and additional mechanical complexity, could largely be dispensed with, the electronics standing in for it.

There are inherent limitations. Because it relies on friction braking, sustained heavy use, such as a prolonged climb out of deep mud, generates significant heat in the brakes and the system will eventually reduce its intervention to protect them. It also cannot, on its own, match a true mechanical locker for continuous high-load articulation off-road, where uninterrupted torque to a wheel is required. For the on-road and moderate off-road duty most owners encounter, however, it is more than adequate and entirely transparent.

4ETS sits within a family of electronic chassis aids and shares its sensor set and logic with the traction control system and electronic stability control, alongside which it operates. It is conceptually a close relative of the automatic braking differential offered by other manufacturers, all of these using selective braking to substitute for mechanical limited-slip or locking hardware in an all-wheel-drive context.