ABS prevents the wheels from locking up when braking. Thanks to this, the car remains steerable during braking and does not change direction.
The ASR system prevents the drive wheels from spinning when starting off, accelerating and during the journey, for example, in the event of different wheel traction, when cornering and during sudden manoeuvres. This ensures optimum traction.
ASR operates in two different planes, if one or both wheels slip, the corresponding wheel is braked. This automatically reduces engine power.
The components available in the ABS system are supplemented for the ASR system with hydraulics, electronics and an electronic gas pedal (E - Gas). When the electronic pedal is depressed, the throttle is actuated by an electronically controlled electric motor.
With the help of four sensors common to ABS and ASR, the device measures the speed of all four wheels and processes the information.
When slipping begins, the corresponding brake is pressurized and the wheel is automatically braked. If the brake pressure reaches a value that brakes the wheel. It is kept constant. If the number of revolutions approaches the optimum, then the pressure decreases. Thanks to this, the wheel operates in the optimal grip range. Braking the wheel while starting off acts as a differential lock. If the second wheel is also slipping, it is adjusted in the same way.
When starting off on a wheel standing on viscous ground, a large part of the drive torque can be transmitted, it is also possible to start off on a one-sided slippery road.
The same thing happens during a trip, when one or both wheels slip and due to automatic braking, the vehicle's directional stability is maintained.
If the rear wheel spin results in automatic braking, with the second wheel on the slip limit, the electronic system detects the wheel force as too much and in a matter of milliseconds, using the electronic pedal, gives less gas. This happens when the driver in an emergency gives full throttle. Thanks to electronic control, the engine torque is reduced, and it is transmitted to both wheels without change.
The ASR logic is such that when a wheel spins at low speeds, the wheel is first braked (brake circuit adjustment), and at high speeds, the engine power decreases first (drive torque adjustment).
At the same time, the system constantly checks whether, for example, the quality of traction of the wheels with the ground has improved and, if improved, transfers all the power from the engine to the wheels.
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 the ASR, both systems complement each other perfectly.
In deep snow and the use of wheel chains, as well as, for example, when driving on loose sand on all-terrain tires, the fine adjustment of wheel slip has a negative effect on driving conditions. In such cases, a special program for the operation of the control device is provided, which is selected by the driver using a switch "Schneekettenschalter" (snow chain switch). In this case, increased wheel slip is allowed so that the car can "gnaw through" in the above situations. Since this adjustment has a negative effect on driving stability at high speeds, it is only effective up to a speed of 38 km/h. When reaching a speed of 60 km / h "snow chain logic" turns off. When the mode is on, the sign on the switch lights up.
The ASR system also has a built-in engine slip torque control system (MSR). It prevents too much braking of the wheels during engine braking and spinning if, for example, downshifting occurs on a slippery road or the gas is abruptly released in a low gear. When slipping begins, the braking torque of the engine is reduced due to the automatic opening of the throttle valve. Thanks to this, the car is stabilized, which means the effectiveness of the rear-wheel drive.