Part 2

Dynamic Stability Control

Brake light switch
Together with the signal from the brake pressure sensor, braking operations are detected. Two Hall effect sensors as switches are installed in the brake light switch: the brake light switch and the brake light test switch (redundant for safety reasons). The signals indicate whether the brake pedal has been pressed. The data transfer is digital.







The two redundant signals from the brake light switch are read in by the Car Access System (CAS). The CAS relays the signals to systems such as the Dynamic Stability Control (DSC) and for the engine-management control unit via the bus system.
A signal from the brake-light switch is still hard-wired to the footwell module (FRM).

NOTICE: Observe the electrical connection.

Depending on the vehicle, the brake light switch from the Front Electronic Module (FEM) is read in. Observe the wiring diagrams in the diagnosis.

2 brake pad wear sensors
Via a calculation model, the DSC identifies the thickness of the brake pad in order to determine the mileage remaining until service is required for the Condition Based Service. The single-stage brake pad wear sensors (front left and rear right in the inner brake pads) serve as an additional monitor of the calculation of the thickness of the brake pad.
The brake pad wear sensors are of single-stage design. When the conductor path is worn through, approx. 5 % of the remaining brake pad thickness is reached. This corresponds to a remaining distance of approx. 2500 kilometers.







A critical brake pad thickness is indicated in the instrument panel by a Next Service indicator (Condition Based Service CBS) as well as the red general brake warning light.
The following further control units are involved in functional networking:

Instrument panel: Instrument panel
Check Control symbols in the liquid crystal display call attention to malfunctions and system failures in the DSC.
The Check Control symbols all have a certain meaning.
If there is a Check Control message, additional supplementary instructions can be displayed on the central information display (CID).

EMF: Electromechanical parking brake (only F25)
The EMF control unit receives a processed speed signal from the DSC. The EMF recognizes from the signal whether the vehicle is stationary or moving. The signal is produced in the DSC using the signals from the two rear axle wheel speed sensors.

System functions
The following graphic shows the functional networking: F25











The following graphic shows the functional networking: F2x, F3x











The following system functions are described for the DSC:
- DTC: Dynamic Traction Control
- Brake standby through application at an early stage of the brake pads when required
- Dry braking of the brake discs on wet roads
- Fading compensation
- Drive-off assistant
- Cruise control (calculated in the ICM) with braking function
- ABS: Antilock braking system
- EBV: Electronic brake force distribution
- CBC: Cornering brake control
- MSR: Engine drag torque control
- ASC: Automatic Stability Control
- DSC: Dynamic Stability Control
- DBC: Dynamic Braking Control
- HDC: Hill Descent Control
- xDrive: Four-wheel drive
- Performance Control
- ADB-X: Automatic differential brake on all-wheel drive vehicles
- Automatic Hold
- Dynamic emergency braking
- Trailer Stability Control

The following additional functions that do not belong to driving dynamics control are integrated:
- RPA: Run Flat Indicator
- CBS: Condition Based Service (only scope of maintenance work for brake pad wear)

DTC: Dynamic Traction Control
The function DTC corresponds to that of DSC with a slightly modified control strategy.
DTC is designed to provide optimal DSC control of drive torque under specific road-surface conditions. The dynamic traction control (DTC) provides improved traction with partially reduced driving stability and is thus only recommended for exceptional situations.
DTC can be activated by shutting off DSC (DTC button). Advantage: DTC provides higher traction and improved dynamics where driving safety is limited. Interventions to stabilize the vehicle (e.g. reduction in the engine performance) are carried out slightly later than in the case of DSC. In certain situations, the driver must intervene correctively to stabilize the vehicle.

In the following exceptional situations, it can be useful to activate DTC briefly:
- When rocking the vehicle or starting off in deep snow or on loose, low-traction surfaces.
- When driving on snow-covered slopes, slush or uncleared roadways.
- When driving with snow chains.

Brake standby through application at an early stage of the brake pads when required
Early application of the brake pads to the brake discs reduce the brakes' response time. With rapid closure of the throttle (angle of the accelerator pedal), the brake pads are applied immediately. The DSC creates a low brake pressure without perceptibly decelerating the vehicle. This compensates for the clearance between the brake pad and brake disc. If there is no braking within half a second, the created early brake pressure is taken back. Applying the brake pads at an early stage is active at a driving speed greater than 70 km/h.

Dry braking of the brake discs on wet roads
Dry braking removes the moisture deposited on the brake disc when driving on wet roadways or in the rain. Here, the brake pads are applied slightly. This function also shortens the brakes' response time. The DSC uses periodic, cyclical control to dial in lower brake pressures based on the speed of the windscreen wipers. The brake pads are cyclically applied. The brake discs are wiped regularly in the process. This creates no measurable deceleration on the vehicle. How often and for how long the brake pads are applied depends on:
- Intensity of rain, based on windscreen wiper speed, etc.
- Driving speed greater than 70 kph

Fading compensation
Fading means: Braking efficiency decreases as a result of high brake disc temperatures (in the 400°C range). As a response to detected fading, DSC increases the brake pressure beyond the brake pressure applied by the driver in order to implement the specified deceleration request.
The DSC detects fading as follows: The DSC compares the current vehicle deceleration with a setpoint value related to the current brake pressure. The DSC increases the brake pressure until the target deceleration is reached or until all the wheels are subject to ABS control operation. The process is ended if the value drops below a speed threshold or an additional temperature threshold.

Drive-off assistant
The braking pressure required to prevent the vehicle from moving is retained during the switch from brake to accelerator pedal when starting off on a hill (up to approximately 2 seconds). Here, the drive-off assistant prevents the vehicle from rolling away in the following situations:
- uphill in a forward gear
- uphill in reverse gear

Here, the brake pressure required to stop the vehicle is maintained. The gradient is picked up by the longitudinal acceleration sensor in the DSC control unit. The required engine torque is calculated based on the slope angle of the road gradient. After detection of the wish to drive off, the brake pressure is reduced as soon as the applied engine torque is sufficient to move the vehicle in the desired direction of travel. If the parking brake is operated, the drive-off assistant is disabled. If there is no wish to drive off within approx. 2 seconds after releasing the brake pedal, the drive-off assistant is also deactivated.

Cruise control (calculated in the ICM) with braking function
The cruise control is integrated functionally in the Integrated Chassis Management (ICM). The ICM transmits the brake interventions to the DSC via an interface.
The cruise control maintains the speed at the selected level, between 30 km/h and 250 km/h. In comparison with a conventional cruise control, it has the following additional functions:
- If the engine drag torque in coasting (overrun) mode is not sufficient to maintain the selected speed, the DSC enhances deceleration with controlled brake intervention.
- Depending on the lateral acceleration, the set speed is adjusted during cornering. At the end of the bend, the required speed is reset.
- The so-called "hand gas mode" enables the driver to use the drop arm on the steering column to continuously accelerate or decelerate in 2 stages. This means that the desired speed is reached more quickly.
- During controlled hill descent, the coasting cutout as well as shiftdown of the automatic gearbox are adapted. The DSC uses a temperature model to adjust the distribution of braking torque between the front and rear axle. The coasting cutout has a favorable effect on fuel consumption. Automatic shifting down and distribution of torque place a load on the wheel brake when driving downhill.

ABS: Antilock braking system
The Antilock Brake System (ABS) prevents the wheels from blocking on braking. Advantage: short stopping distance, the direction of the vehicle remains stable and the vehicle can be steered. The brake pressure on all wheels is regulated in such a way that each wheel runs in an optimized slip range. Here, the slip is regulated in such a way that the highest possible braking and cornering forces can be transferred.

EBV: Electronic brake force distribution
The electronic brake force distribution (EBV) is an element of the ABS. EBV regulates the braking force distribution between the front and rear axles depending on the vehicle load. Advantage: Independently of the vehicle load, optimized stopping distances with simultaneous high driving stability are achieved. The brake pads wear more evenly. If the ABS fails, the EBV function remains as long as possible. The EBV function requires the signals from at least 1 wheel speed sensors per axle.

CBC: Cornering brake control
The Cornering Brake Control (CBC) is an extension of the ABS. CBC increases driving stability when braking in curves ("curve logic"). Advantage: optimization of the driving stability with partial braking on cornering. The shift in wheel loads on cornering (even with light braking) can reduce driving stability. If required, CBC creates a stabilizing counter-torque outside of the ABS control range during light braking.

MSR: Engine drag torque control
The engine drag torque control (MSR) prevents the blocking tendency of the drive gear on smooth roadways. On downshifting or in the case of an abrupt load reversal (especially on a roadway with a low coefficient of friction), there is a danger that the drive gears block due to the engine drag torque. Via the wheel speed sensors, MSR detects the blocking tendency even as it is about to happen. MSR briefly reduces the engine drag torque by accelerating slightly. Advantage: The drive wheels retain their cornering force in coasting (overrun) mode.

ASC: Automatic Stability Control
The Automatic Stability Control (ASC) uses brake and engine interventions to prevent the wheels from spinning on accelerating. Advantage: More traction and better driving stability. If the wheels on the driven axle have different levels of adhesion, the wheel with the tendency to spin is braked (micro-split). If necessary, the engine performance is also reduced.

DSC: Dynamic Stability Control
The current driving condition of the vehicle is detected by the dynamic stability control (DSC) by evaluation of the sensor signal. This driving condition is compared with the setpoint values determined using a calculation model. Advantage: In this way, the system recognizes unstable driving conditions. In the case of deviations that are above a control threshold stored in the DSC control unit, the vehicle is stabilized. The stabilization (within physical boundaries) is achieved by reducing the engine performance and individual braking of wheels. DSC intervention override the functions ABS and ASC. The DSC function can be deactivated using a button.

DBC: Dynamic Braking Control
In emergency braking situations, it is frequently the case that the brake pedal is not pressed strongly enough. ABS control range is then not activated. The Dynamic Brake Control (DBC) provides support in emergency braking situations by automatically boosting the brake pressure. Advantage: Shortest possible stopping distances in emergency braking situations by achieving ABS control operation on all four wheels. The pump increases brake pressure and places the brakes in the ABS closed-loop control range under the following conditions:
- Rapid operation of the brake pedal with inadequate brake pedal force
- 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 will depend will depend upon the vehicle's load factor and the road surface's coefficient of friction. Example of a typical situation: The traffic situation slows, making light braking necessary at first, but then demands as short a stopping distance as possible.

HDC: Hill Descent Control
During steep desecnts on low-traction road surfaces (sand, gravel, snow and ice) the engine's drag torque is not enough to maintain vehicle speed at a low, constant rate. Owing to its relatively low speed and the low-traction road surface, if the brakes are applied the vehicle will tend to pull to one side, despite ABS.
HDC employs selective braking intervention to consistently maintain the preset speed (maximum of 25 km/h) and simultaneously stabilize the vehicle in a process mirroring closed-loop control from DSC.

xDrive: Four-wheel drive
The VTG control unit employs the servomotor and the multiple-disc clutch assembly in the VTG for infinitely-variable distribution of drive torque. This process distributes the drive torque between the front and rear axles according to their respective requirements. The rear axle is always powered. This means that all of the drive torque is applied to the rear axle when the multidisc clutch is separated. The drive torque is distributed as follows in normal driving with four-wheel drive:
- 40 % to the front axle
- 60 % to the rear axle

Dynamic Stability Control (DSC) specifies the setpoint value for xDrive all-wheel control. The setpoint value varies according to the vehicle's instantaneous tendency toward oversteer or understeer and the wheel-slip rates. The setpoint value is transmitted to the VTG control unit.
The VTG control unit regulates the lockup torque of the multiple-disk clutch assembly in the transfer case.

Performance Control
The Performance Control increases the agility of the vehicle during fast cornering by means of targeted brake intervention and engine intervention. This form of active driver support assumes particular significance when the vehicle's front wheels start to plough and fail to respond to steering input. DSC responds to these conditions by generating braking pressure at the wheels on the inside of the curve. While previous strategies relied on the engine-management system for simultaneous reductions in drive torque, now engine torque is increased, regardless of the accelerator pedal's position. This additional engine torque has an effect on the outer cornering wheels. The vehicle is stabilized in this process. The precision of steering and tracking is substantially enhanced, as is traction.
The setpoint value for Performance Control is calculated by the Integrated Chassis Management (ICM) and transmitted to the DSC.
Performance Control, which is particularly effective when applied together with xDrive, furnishes clear benefits in vehicle handling response and active safety, especially on slippery road surfaces.

ADB-X: Automatic differential brake on all-wheel drive vehicles
ADB-X offers the functionality of a differential lock. When a wheel displayed signs of incipient traction loss it is braked automatically, allowing the remaining wheel on the axle to continue delivering drive torque.

Automatic Hold
The automatic hold function provides support through automatic application and release of the brake, e. g. in stop and- go traffic. When at a standstill, the vehicle is automatically held in position. The vehicle is prevented from rolling backward on hills.

Dynamic emergency braking
Pulling the parking brake button upward while the vehicle is in motion initiates a defined sequence of dynamic emergency braking regulated by the Dynamic Stability Control (DSC). This function is intended only for emergencies when the driver can no longer decelerate using the brake pedal (for instance, if the master cylinder fails).

Other occupants are also able to bring the vehicle to a standstill in this manner, for example if the driver suddenly loses consciousness. During dynamic emergency braking, hydraulic brake pressure is built up at all four wheel brakes. The brake lamps are activated and the DSC functions assume full operational status. This represents a major advantage when compared against manual parking brakes.

Trailer Stability Control
The Trailer Stability Control detects snaking movements of a trailer around the vertical axis. The dynamic stability control (DSC) monitors the yawing motion of the vehicle with the help of the DSC sensors. The system regulates beyond a speed of approximately 65 km/h. If the limit value is exceeded for oscillating outfit, the engine performance is reduced. The DSC also automatically brakes all 4 wheels. With deactivated or faulty DSC, the Trailer Stability Control is also disabled.

RPA: Run Flat Indicator
The Run Flat Indicator (RPA) is not a function of the driving dynamics control. The RPA is integrated in the DSC control unit, as for this function the 4 wheel speed signals are required. By comparing the wheel speeds of all four wheels, the system detects differences in the tire rolling circumferences of the individual wheels. This enables a gradual tire pressure loss to be detected. The Run Flat Indicator can be initialized in the Central Information Display (CID).

CBS: Condition Based Service
CBS is not a function of the driving dynamics control. Condition Based Service means "demand-based service". Various scopes of maintenance work, e. g. brake pads, are integrated in the CBS. The remaining distances for the front and rear brake pads are calculated separately in the DSC control unit. When making the calculation, the signal from the single-stage brake pad wear sensors is also taken into account (1 data point at 4.2 mm).

Notes for Service department

General notes

IMPORTANT: Replace DSC control unit.

When replacing the DSC control unit, the repair instructions must be followed (special tools needed).

Diagnosis instructions
The following service functions are available in the diagnosis system:
- Brake bleeding procedure (after replacement of the DSC unit)
- Interchange test of brake lines (following repairs on the brake line or accident repairs)
- Adjustment of DSC control unit (after replacement of the DSC control unit for adaptation to the DSC hydraulic control unit)

Information on encoding and programming
After replacement, the DSC control unit must be encoded and the solenoid valves calibrated.

Switch-on conditions
After every engine start, the DSC is operational.
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.