Part II
Dynamic Stability Control
JBE: Junction Box Electronics
The power distribution box at the JBE supplies the DSC unit with voltage. Terminal 30 for the return pump and the solenoid valves. Terminal 30g for the DSC control unit.
System functions
Following for the E70, E71 new system functions are described for the DSC:
- DTC: Dynamic Stability Control
- Brake standby through application at an early stage of the brake pads when required
- Dry braking of the brake discs on wet roadways
- Fading Brake Support
- Drive-off assistant
- Cruise control with brake function
- electromechanical and hydraulic parking brake
Other known system functions for the DSC are:
- 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 Brake Control
- ADB-X: All-wheel control
- HDC: Hill Descent Control
- 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
DTC: Dynamic Traction Control
DTC is a drive-optimized version of DSC for certain road conditions. The dynamic traction control (DTC) provides improved traction with partially reduced driving stability and is thus only recommended for exceptional situations.
In the following exceptional situations, it can be useful to activate DTC briefly:
- When 'rocking out' or driving off in deep snow or on loose surfaces.
- When driving on snow-covered upward inclines, in slush as well as on uncleared roadways
- When driving with snow chains.
The function DTC corresponds to that of DSC with a slightly modified control strategy. DTC can be activated by shutting off DSC (DTC button). With brake intervention, DTC replicates the function of a conventional differential interlock. This effect of this is to increase the input torque on those wheels that are on a higher-friction surface. Advantage: Higher traction is available with DTC. Interventions to stabiles 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 stabiles the vehicle.
Brake standby through application at an early stage of the brake pads when required
Application of the brake pads at an early stage shortens the response time of the brakes. 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 roadways
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. DSC employs a cyclical control process to reduce braking pressure in response to rain sensor signals and the position of the wash/wipe switch. This creates no measurable deceleration on the vehicle. The brake pads are cyclically applied. The brake discs are wiped regularly in the process. How often and for how long the brake pads are applied depends on:
- The intensity of the rain, e.g. speed of the wiper
- Driving speed greater than 70 km/h
Fading Brake Support
Fading means: the braking effect diminishes due to high brake disc temperature. As a response to detected fading, DSC increases the brake pressure beyond the brake pressure applied by the driver. At very high brake disc temperature, the fading compensation leads to the following being displayed:
- Brake warning lamp in yellow
- Check Control symbol in the LC display in the instrument cluster in yellow
The DSC detects fading as follows: The DSC compares the current vehicle deceleration with a setpoint value relative 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 terminated when the brake pedal is no longer pressed or the speed drops below a speed threshold.
Drive-off assistant
When driving off on an incline, a change from the brake pedal to the accelerator pedal is required. 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 necessary braking torque or engine torque is calculated from the 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 actuated, the drive off assistant is deactivated. 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 with brake function
The functionality of the cruise control system with its braking function is integrated in DSC. 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 additionally decelerates by means of controlled brake intervention.
- Depending on the current lateral acceleration, the speed in longitudinal direction is transverse acceleration during controlled cornering. At the end of the bend, the set speed is readjusted.
- The so-called "hand gas mode" enables the driver to use the steering-column lever to continuously accelerate or decelerate in 2 stages. This means that the desired speed is reached more quickly.
- During controlled downhill driving, the overrun fuel cutoff as well as shift down of the automatic transmission are adapted. The DSC uses a substitute temperature model to regulate the distribution of torque between the front and rear axle. The overrun fuel cutoff has a favorable effect on fuel consumption. Automatic shift-down and distribution of torque relieve the load on the wheel brake during hill descents.
EMF: electromechanical and hydraulic parking brake
The EMF is an electromechanical parking brake. DSC generates hydraulic stopping power whenever the engine is running. EMF uses an actuator to provide mechanical retaining force when the engine is off.
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 2 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 wheels on smooth roadways. On shifting down 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 wheels 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
Automatic Stability Control (ASC) uses brake and engine intervention to prevent the wheels from spinning during acceleration. 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 slip is braked. 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. 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 Brake Control
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. 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. In the following situations, the return pump increases the brake pressure until ABS control range is activated:
- 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.
All-wheel control
The dynamic stability control (DSC) specifies the target value for 4-wheel drive control with xDrive. The DSC control unit calculates the locking moment for the multi-plate clutch in the transfer box. The nominal value depends on the tendency of the vehicle to oversteer or understeer and the wheel slip. The target value is sent on the PT-CAN to the VTG control unit. The VTG control unit reports the actual set locking moment back to the DSC control unit. The DSC control unit calculates the locking moment of the multidisc clutch as follows:
- Pre-activation = driver request
- Driving dynamics control
- Detection of different tire rolling circumferences
HDC: Hill Descent Control
Hill Descent Control (HDC) is an automatic cruise control for descending hills in vehicles equipped with 4-wheel drive. The HDC can be switched on and off at the centre console switch centre. HDC automatically reduces the driving speed. Braking intervention at all four wheels is employed to reduce the vehicle's velocity to just above walking speed. This allows stable downhill driving on steep and slippery surfaces. The HDC keeps this speed constant (the DSC functions all remain active). The vehicle speed can be adjusted through a continuous, stepless range lying between the following values using the accelerator pedal and the brake pedal or steering-column stalk. Ex works, the lower freely programmable button on the multifunction steering wheel is set for the HDC.
Trailer Stability Control
The Trailer Stability Control detects snaking movements of a trailer around the vertical axis. The system works as of a speed of approx. 65 kph with the electrical connector for trailer occupied. The dynamic stability control (DSC) monitors the yawing motion of the vehicle with the help of the DSC sensors. If the limit value is exceeded on the swinging trailer, 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 as follows:
- On vehicle with Car Communication Computer (CCC): in the Central Information Display (CID) in the menu "Setup"
- On vehicles without Car Communication Computer (CCC): in the instrument cluster using onboard computer functions
CBS: Condition Based Service
CBS is not a function of the driving dynamics control. Condition Based Service means "demand-based service". Various service items, such as engine oil, spark plugs and brake pads, are included 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 condition of the brake pad wear sensors is taken into account (data point at 6 mm and 4 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:
- Venting procedure
- Brake line mix-up test:
- Steering angle sensor adjustment
- Adjustment / commissioning of active steering (must be carried out after adjustment of the steering angle sensor)
- Adjustment of DSC sensor
Notes on encoding/programming
After replacement, the DSC control unit must be encoded.
Switch-on conditions
After every engine start, the DSC is operational.
Not responsible for printing errors, mistakes and technical changes.