Anti-Lock Braking System (ABS) And All Other Sub Systems

Anti-lock Braking System (ABS)

ABS prevents the wheels from locking when the brakes are applied.

Advantage: Shorter stopping distances, the car retains its directional stability and remains steerable.

Brake pressure is regulated at all wheels to ensure that each wheel runs in the best possible slip range. when this happens, slip is controlled so that the maximum possible braking and lateral stability forces can be transmitted.

ABS alone is available for braking if a sensor for DSC fails or if a bus fault occurs (PT-CAN or chassis CAN). ABS is the surviving safety function in circumstances in which DSC control is no longer possible.

Electronic brake force distribution (EBV) EBV is a constituent function of ABS. EBV regulates the brake force distribution between the front and rear axles, depending on vehicle load.

Advantage: Regardless of the load state of the vehicle, the best possible braking distance is achieved while driving stability is maintained.

If ABS fails, the EBV function is sustained for as long as possible.

The signals from at least two wheel speed sensors are needed for the EBV function.

Cornering Brake Control (CBC)

CBC is an extension of ABS. CBC increases driving stability when the brakes are applied as the car corners (cornering logic).

Advantage: Optimum driving stability if brakes are partially applied when cornering.

The shift in wheel loads as the car corners (the onset of this phenomenon requires no more than light application of the brakes) can result in a reduction in handling stability. If required, CBC generates a ststabilizing load moment when the brakes are applied lightly outside the ABS intervention range.

Automatic Stability Control (ASC)

ASC prevents the wheels from spinning under acceleration by means of controlled brake-system and engine-output intervention.

Advantage: More traction and better vehicle stability.

If, for example, one of the wheels of the drive axle is on a high-grip surface and the other is on a
slippery surface, the wheel tending to spin is braked. If necessary, the engine~s power output is also reduced.

Dynamic Traction Control (DTC)

DTC offers better traction as a trade-off against a reduction in stability in some circumstances. Consequently, its use should be reserved for exceptional conditions (driving in deep snow, for example).

The DTC function approximates to that of DSC with a slightly modified control strategy. DTC can be activated by deactivating DSC (DSC button). DTC intervenes in the braking actions to imitate the function of a conventional differential lock.

Advantage: Higher traction is available with DTC.

Vehicle stabilization intervention (e.g. reduced power output) is made slightly later than with DSC. This enhances traction with a slight loss of driving stability. Occasionally, a compromise is needed between vehicle stability and traction. This is especially true when accelerating and driving uphill on loose surfaces or in deep snow (= friction values demanding increased slip). DTC allows DSC to provide a high degree of vehicle stability while retaining sufficient traction.

Engine drag torque control (MSR)

The engine drag torque control (MSR) counteracts the tendency of the wheels to lock on smooth surfaces. The engine~s drag torque generated by downshifts or abrupt load changes can lock the driven wheels (especially on surfaces with a low coefficient of friction).

The wheel speed sensors tell MSR as soon as the wheels are about to lock. MSR then briefly reduces the engine~s drag torque by opening the throttle slightly.

Advantage: The drive wheels retain their lateral stability in overrun mode.

Dynamic Brake Control (DBC)

DBC assists the driver in emergency-braking situations, by automatically boosting brake pressure. Advantage: Shortest possible stopping distances in emergency-braking situations, because the ABS control threshold is reached at all four wheels.

In emergency-braking situations, drivers often fail to apply sufficient force to the brake pedal. ABS regulation is then not activated.

In the following situations, the return pump increases the brake pressure until ABS regulation is activated:

- When the brake pedal is rapidly depressed with insufficient pedal pressure

- When the brake pedal is depressed slowly and the demand for deceleration is subsequently high, after one wheel reaches the ABS control threshold.

Which wheel locks first depends on load and coefficient of friction of the road surface.

Example of a typical situation:

The traffic slows, making light braking necessary at first, but then demands as short a stopping

distance as possible.

Electronically Controlled Deceleration (ECD)

ECO (Electrically Controlled Deceleration) reacts to a request from the Active Cruise Control system

(ACC).
when ACC requires deceleration, DSC responds by applying the disc brakes on all four wheels (maximum rate of deceleration 2.5 mIs2).

when the car is on a decent with the speed preset by the driver, ECO automatically applies the brakes in order to keep the cars speed constant at the preset value.

when the brakes are applied automatically in this way, the brake lights are activated in accordance with the requirements of road-safety legislation. The light module does not activate the brake lights unless the vehicle~s rate of deceleration is greater than 1 mIs2. This prevents the brake lights from flickering on and off.

Runflat indicator (RPA)

RPA is not a function of the dynamic stability control system.

RPA is integrated into the DSC control unit as the four wheel speed signals are required for this function.

By comparing the speed signals for all four wheels, the system detects differences in rolling circumference at the individual wheels. This enables the system to recognize a sudden loss of pressure in the tires.