ABS prevents the wheels from locking when braking. Thanks to this, the car remains controllable and does not change direction when braking.
The ASR system prevents the drive wheels from slipping when starting off, accelerating and during the journey, for example with different wheel grip, when turning and during sudden manoeuvres with a change in direction. This ensures optimum traction.
ASR operates in two different planes: if one or both wheels slip, the corresponding wheel is braked. At the same time, engine power is automatically reduced.
The components available in the ABS system are supplemented for the ASR system by hydraulics, electronics and an electronic gas pedal (E – Gas). When the electronic pedal is pressed, the throttle valve is driven by an electronically controlled electric motor.
Using four sensors, the device, common to both ABS and ASR, measures the rotation speed of all four wheels and processes the information.
When slippage begins, the pressure in the corresponding brake increases and the wheel is automatically braked. If the braking pressure reaches a value that brakes the wheel. It is maintained constant. If the speed approaches the optimum, the pressure decreases. Thanks to this, the wheel operates in the optimum grip range. Braking the wheel when starting off acts as a differential lock. If the second wheel also slips, it is regulated in the same way.
When starting off, a large part of the drive torque can be transferred to the wheel standing on sticky ground; starting off is also possible on a one-sided slippery road.
The same thing happens during a trip, when one or both wheels slip and the vehicle's directional stability is maintained by automatic braking.
If the rear wheel slips and the brakes are applied automatically, and the second wheel is on the slip limit, the electronic system detects the force on the wheels as too great and in a matter of milliseconds reduces the throttle with the help of the electronic pedal. This happens when the driver applies full throttle in an extreme situation. Thanks to the electronic control, the engine torque is reduced and transmitted to both wheels unchanged.
The logic of ASR is that when a wheel slips at low speeds, the wheel is first braked (brake circuit adjustment), and at high speeds, engine power first decreases (drive torque adjustment).
At the same time, the system constantly checks whether, for example, the quality of the wheels' grip on the ground has improved and, if it has, transfers all the power from the engine to the wheels.
Material borrowed from an online resource Mercedesman.ru
The ASR operation indicator informs the driver that the system is starting to adjust and that driving conditions are approaching the limit.
The operation of the ABS system is in no way dependent on ASR; both systems complement each other perfectly.
In deep snow and when using tire chains, as well as, for example, when driving on loose sand with all-terrain tires, fine wheel slip adjustment has a negative effect on driving conditions. In such cases, a special control program is provided, selected by the driver using the switch "Schneekettenschalter" (anti-skid chain switch). In this case, increased wheel slip is allowed so that the car can "gnaw through" in the above situations. Since such an adjustment has a negative effect on driving stability at high speeds, it is only effective up to a speed of 38 km/h. When the speed reaches 60 km/h, the "anti-skid chain logic" is switched off. When the mode is on, the sign on the switch lights up.
The ASR system also includes a slip torque control system (MSR). It prevents excessive braking of the wheels during engine braking and slippage if, for example, a downshift occurs on a slippery road or the accelerator is suddenly released in a low gear. When slippage begins, the engine braking torque is reduced by automatically opening the throttle valve. This stabilizes the vehicle, which means that the rear-wheel drive is effective.