Dynamic Stability Control
Dynamic Stability Control
NOTE: Structure of this document.
The most important diagnosis instructions are described at the start of this document. These are followed by the functional description.
Diagnosis instructions
Remedying faults and/or clearing fault memory
If problems occur on clearing the fault memory following a repair, use the following procedure:
1. Remedy fault with ignition off
2. Switch ignition ON, clear fault memory
3. Switch ignition OFF, wait 5 - 10 seconds, switch ignition on again
4. Clear fault memory again
IMPORTANT: In some cases, indicator and warning lamps for certain faults only go out after a short test drive.
Sequence for replacement of DSC unit or DSC control module
After replacing the DSC unit or DSC control module: The parameters for the Flat Tire Monitor (RPA) as well as for the cruise-control system must be encoded using Progman.
Perform following procedures:
1. Encoding DSC
2. Venting routine for DSC unit
3. Check for swapped brake lines
4. Initialize Flat Tire Monitor (see Owner's Manual)
Replacement of steering-angle sensor
The steering-angle sensor forms a unit with the steering column switching center. The steering angle sensor cannot be replaced on its own. The service function "Calibration steering-angle sensor", or in the case of vehicles with active steering, the service function "Commissioning/Calibration active steering", must be run after the following work:
- Replacement of the steering column switch cluster (SZL)
- Calibration work on the steering geometry
- Work on the steering and/or front axle
Introduction
Dynamic Stability Control
As series standard, the new BMW 7 Series will be equipped with Dynamic Stability Control (DSC). For Dynamic Stability Control, it goes without saying that co-ordination with the central driving dynamics control in the master control module Integrated Chassis Management (ICM) had to be taken into account. The newly introduced function "Driving Dynamics Control" also influences the operation of DSC. Depending on the selected stage (COMFORT, NORMAL, SPORT, SPORT+), the threshold values and characteristics of the DSC interventions also have to be adapted. Alongside the co-ordination, there are other changes to Dynamic Stability Control that concern the installation location, display functions, as well as diagnosis and repair.
Brief description of components
The following components for dynamic stability control are described:
DSC unit
The DSC unit consists of the DSC control module and a hydraulic unit. The DSC control module activates the hydraulic unit via valve coils.
The DSC unit contains a brake pressure sensor. This brake pressure sensor picks up the braking pressure applied via the brake pedal and the brake booster.
The brake pressure sensor has a measuring range of 0 to 250 bar. Its zero point is only calibrated while the vehicle is being driven.
The new concept of the DSC unit enables more exact control. 2 x 3 pump elements with 6.5 mm work in the pump motor. This concept results in significantly improved pressure dynamics. It means that the brake pedal pulses to lesser degree during ABS control. It also leads to higher control quality with Hill Descent Control (HDC).
The distribution box at the junction box electronics (JBE) supplies the DSC unit with voltage. Once terminal 30 for the return-flow pump, once terminal 30 for the solenoid valves. The DSC control module receives terminal 30g.
4 wheel-speed sensors
The active wheel-speed sensors are connected to the DSC control module. The complete signal processing takes place in the active wheel-speed sensors.
The active wheel-speed sensors also detect the direction of travel.
The wheel speed is measured at a magnetic impulse wheel. The magnetic impulse wheel is divided into 96 increments per rotation of the wheel. Each increment change is detected by the active wheel-speed sensor and converted into a defined signal protocol of a pulse-width modulated signal.
ICM: Control module for Integrated Chassis Management
New: Sensors that used to be installed separately in the DSC sensor are now fitted in the ICM control module. The ICM control module uses these sensors to calculate variables that are important for the dynamic driving vehicle status:
- Longitudinal acceleration and vehicle inclination in longitudinal direction
- Lateral acceleration and vehicle inclination in lateral direction
- Yaw rate
Initially, the sensor signals are related to the sensor housing. However, the driving dynamics systems need these variables related to the co-ordinate system of the vehicle. The ICM control module performs the necessary conversion. The correction values necessary for this are determined and stored by calibration during commissioning of the ICM control module.
Calibration of the integrated sensors is necessary after replacement of the ICM control module. Calibration must take place while the vehicle is on a subsurface that is horizontal in longitudinal direction and lateral direction. Terminal 15 must be switched on.
Steering angle sensor
The steering-angle sensor is installed in the steering column switch cluster (SZL). The steering angle sensor measures the steering-wheel angle optically and without contact. The steering angle sensor is fitted on the board with the electronic evaluation unit. The steering-angle sensor consists of the following components: encode disk and optical sensor.
The code disk is connected to the steering wheel via the buffer spring cassette. When the steering wheel is moved, the code disk moves within the optical sensor. There are various line patterns for evaluation on the code disk.
The SZL is connected across the FlexRay via the central gateway module (ZGM) with the DSC control module.
DTC button
The DTC button is in the center console operating unit.
The DTC button switches the operating mode of the chassis control system. The DTC button can be used to select 3 switching states:
- DTC button not pressed after terminal change: DSC in operation (default)
- Press DTC button briefly: DTC
- Press DTC button for longer period: DSC and ASC switched off, ABS and ABD-X still active
New is the display with selected DTC button. "TRACTION" can be seen in the revolution counter of the instrument cluster. The symbol of the indicator lamp has changed as a result of legal regulations: Skidding vehicle with the labeling "OFF".
The DSC receives additional input signals from the following components:
Parking brake button
The parking brake button is in the center console behind the gear selector switch (GWS). With the engine running or vehicle moving: The parking brake operates via the DSC hydraulically on the disk brakes of the front and rear axle.
With the engine off and the vehicle stationary:
the parking brake operates with the aid of the electromechanical actuating unit via bowden cables on the drum brakes of the rear axle. Driving off against the blocked brake pressure leads to another pressure increase and a Check Control message. Starting the engine leads to a change from electromechanical to hydraulic parking brake engagement.
The DSC receives additional input signals from the following components:
Brake-fluid level switch
A low brake fluid level is picked up (in the expansion tank via a reed contact) and notified to the DSC control module. With a low brake fluid level, DSC is disabled. Otherwise, there is a danger that air will be drawn into the brake system.
The brake-fluid level switch detects a shortfall in the required brake fluid level in the brake fluid reservoir. The DSC control module sends the signal of the bus system to the instrument cluster. The instrument cluster issues a Check Control message. Normal motion of the brake fluid in the brake fluid reservoir does not normally lead to a Check Control message.
There is a permanent magnet in the moving float; this triggers the reed contact in the brake-fluid level switch firmly fitted in the reservoir. Here, the reed contact converts the float movement into an electrical signal. If the brake fluid level is above the minimum level, the reed contact is closed.
Brake light switch
Together with the signal from the brake pressure sensor, braking operations are detected. The brake-lamp switch contains 2 Hall sensors as switches: The brake-lamp switch and the brake-lamp test switch (duplicated as a safety measure). The signals indicate whether the brake pedal has been pressed. The data interchange is digital.
The two redundant signals from the brake-light switch are read in by the Car Access System (CAS). The CAS forwards the signals on the bus system, e.g. for the engine management system.
2 brake pad wear sensors
The brake pad wear sensors (front left and rear right in the internal brake pads) provide additional information on the thickness of the brake pad. This additional information (2 reference points) is used to calibrate the calculated value of the DSC.
A critical brake lining thickness is indicated in the instrument cluster by a service requirement display (Condition Based Service CBS) as well as the general brake warning light in red.
The following additional control modules are involved in the dynamic stability control:
KOMBI: Instrument cluster
In the event of failure malfunction of the DSC, a Check Control symbol appears in the LC display. 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).
CAS: Car Access System
The CAS receives a processed speed signal from the DSC. The CAS recognizes from the signal whether the vehicle is stationary or moving.
JBE: junction box electronics
The distribution box at the junction box electronics supplies the DSC unit with voltage.
Terminal 30 for the return-flow pump as well as the solenoid valve.
Terminal 30g for the DSC control module.
System functions
The following illustration shows the functional networking.
The following 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 rotors on wet roads
- Fading compensation
- Drive-off assistant
- Cruise control with braking function
- electromechanical and hydraulic parking brake
- ABS: Anti-lock braking system
- EBV: Electronic charge-air pressure distribution
- CBC: Cornering brake control
- MSR: Engine drag-torque control
- ASC: Automatic Stability Control
- DSC: Dynamic Stability Control
- DBC: Dynamic Braking Control
- Trailer stabilization control
Other known system functions for the DSC are:
The following additional functions that do not belong to driving dynamics control are integrated:
- RPA: Flat Tire Monitor
- CBS: Condition Based Service
DTC: Dynamic Traction Control
DTC is an optimized version of DSC for certain road conditions on forward momentum. 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 free' or driving off in deep snow or on loose ground.
- When driving on snow-covered upward inclines, in slush as well as on uncleared roads
- 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 down DSC (DTC button). With brake intervention, DTC replicates the function of a conventional differential interlock. It increases the input torque to wheels on a road surface with a higher coefficient of friction.
Advantage: with DTC, greater traction is available. Interventions to stabilize the vehicle (e.g. reduction in the engine output) are carried out slightly later than in the case of DSC. In certain situations, the driver must intervene correctively to stabilize 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 rotor. 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 kph.
Dry braking of the brake rotors on wet roads
Dry braking removes the moisture deposited on the brake rotor when driving on wet roads or in the rain. Here, the brake pads are applied slightly. This function also shortens the response time of the brakes. Depending on the signal of the rain sensor or the position of the wiper switch, the DSC creates low brake pressure in cycles. This does not lead to any perceptible braking on the vehicle. The brake pads are applied in cycles. The brake rotors 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 window wiper
- Driving speed greater than 70 kph
Fading compensation
Fading means: the braking action diminishes due to high brake-disc temperature. As a reaction 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:
- General 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 target 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. The operation is terminated when the brake pedal is no longer pressed or a speed threshold is undershot.
Drive-off assistant
When driving off on an incline, a switch from the brake pedal to the accelerator pedal is required.
Here, the drive-off assistant prevents the vehicle from rolling in the following situations:
- uphill in a forward gear
- uphill in reverse gear
Here, the brake pressure required to hold the vehicle is maintained. The road surface inclination is picked up by the longitudinal acceleration sensor in the DSC control module. The necessary braking torque or engine torque is calculated from the road surface inclination. 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 disabled. If there is no wish to drive off within approx. 2 seconds after releasing the brake pedal, the drive-off assistant is also disabled.
Cruise control with braking function
The cruise control with braking function is functionally integrated in the DSC. The cruise control keeps the selected speed constant between 30 kph and 250 kph.
In comparison with the conventional cruise control, there are the following additional functions:
- If the engine drag torque on coasting 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 lateral acceleration during controlled cornering. At the end of the bend, the required speed is reset.
- The so-called "handgas 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 hill descent, the coasting cutout as well as shiftdown 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 coasting cutout has a favourable effect on fuel consumption. Automatic shiftdown and distribution of torque relieve the load on the wheel brake during hill descents.
EMF: electromechanical and hydraulic parking brake
The EMF is a parking brake.
With the engine running, the DSC applies the retaining force hydraulically.
With the engine off, the EMF applies the retaining force mechanically via the actuating unit.
ABS: Anti-lock braking system
The anti-lock braking system (ABS) prevents the wheels from blocking on braking.
Advantage:
short braking 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 charge-air pressure distribution
The electronic braking force distribution (EBV) is a component of the ABS. EBV regulates the braking force distribution between the front and rear axles depending on the load.
Advantage:
independently of the load, optimized braking distances with simultaneous high driving stability are achieved. The brake lining wear is better distributed. 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 expansion of the ABS. CBC increases driving stability on 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 feedback 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 downshifting or in the case of abrupt load changes (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 lateral stability in 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 slip is braked. If necessary, the engine output is also reduced.
DSC: Dynamic Stability Control
The current drive status of the vehicle is detected by the dynamic stability control (DSC) by evaluation of the sensor signal. This drive status is compared with the target values determined using a calculation model. This enable detection of unstable driving states even as they are about to happen. In the case of deviations that are above a control threshold stored in the DSC control module, the vehicle is stabilized. The stabilization (within physical boundaries) is achieved by reducing the engine output 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
The Dynamic Braking Control (DBC) provides support in emergency braking situations by automatically boosting the brake pressure.
Advantage:
the shortest possible braking distances in emergency braking situations by reaching the ABS feedback control range on all 4 wheels. In emergency braking situations, it is frequently the case that the brake pedal is not pressed strongly enough. This means that the ABS feedback control range is not reached. The return-flow pump brings the braking into the ABS feedback control range by increasing the brake pressure in the following situations:
- Rapid operation of the brake pedal with inadequate pedal force
- Slow operation of the brake pedal and subsequent high deceleration requirement if one wheel reaches the control threshold for ABS. Which wheel blocks first depends on the load and road surface coefficient of friction.
Such situations are typical:
if the traffic situation means that initial braking is light but the traffic situation then requires a braking distance that is as short as possible.
Trailer stabilization control
The trailer stabilization control detects seesaw motion 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 output is reduced.
The DSC also automatically brakes all 4 wheels. With disabled or defective DSC, the trailer stabilization control is also disabled.
RPA: Flat Tire Monitor
The Flat Tire Monitor (RPA) is not a function of the driving dynamics control. The RPA is integrated in the DSC control module, as for this function the 4 wheel-speed signals are required.
The system uses the 4 wheel speeds to compare deviations in the rolling circumferences of the individual wheels. This means that a creeping loss of tyre inflation pressure is detected. The Flat Tire Monitor 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 "requirement-oriented service". Various maintenance items are integrated in the CBS, e.g. engine oil, spark plugs and brake pads. The remaining distances for the front and rear brake pads are calculated separately in the DSC control module. The state of the brake pad wear sensors is also used for the calculation (reference point at 6 mm and 4 mm).
Notes for Service department
General information
IMPORTANT: Replacing the DSC control module.
On replacement of the DSC control module, the repair instructions must be observed (special tool necessary).
Diagnosis instructions
The following service functions are available in the BMW diagnosis system:
- Venting procedure
- Check for swapped brake lines
- Steering angle sensor calibration
- Calibration / commissioning of active steering
(must be carried out after calibration of the steering-angle sensor)
- Calibration of ICM sensor system
Notes on encoding / programming
After replacement, the DSC control module must be encoded.
Switch-on conditions
After every engine start, the DSC is operational.
No liability can be accepted for printing or other faults. Subject to changes of a technical nature