The ABS/DSC control unit contains the ABS and DSC functions. ABS stands for antilock brake system, DSC for dynamic stability control.
The antilock brake system prevents the wheels locking and thus increases driving stability and steerability of the vehicle even under extreme braking conditions.
The dynamic stability control is a consistent further development of automatic stability control + traction. Automatic stability control + traction (ASC+T) prevents the drive wheels spinning during acceleration and thus increases driving stability and traction of the vehicle.
The traction control systems ABS and ASC+T always use the rotational slip, i.e. wheel speed differences between front and rear axle as the control variable. The dynamic stability control additionally uses the transversal slip, i.e. wheel speed differences between both wheels on the front axle, to control the drive power. In this way, the engine output can be reduced to such an extent that the vehicle can no longer be under-steered or over-steered during fast cornering.
The signals of the two front speed sensors and of the steering angle sensor are used as the basis for calculating the transversal slip.
This functional description is subdivided into following sections:
The control unit basically consists of following components:
Note
The ABS/DSC control unit is ready for operation as of ignition lock position 2. The power is applied via the DME main relay. The power supply is retained for approx. 5 ... 20 seconds after switching off the ignition (DME holding circuit).
The main safety feature in the ABS/DSC control unit is the parallel signal conditioning and processing in the two microcontrollers.
Since both microcontrollers work through the same program, the output variables must be identical when the input variables are identical. If this is not the case, a fault is detected and the entire ABS/DSC system is shut down. This internal fault monitoring facility is constantly in progress.
In addition, after exceeding a driving speed of 6 km/h at the beginning of every trip, the solenoid valves and return pump are temporarily activated and the feedback to the output stages is monitored. The entire system is also shut down if a fault occurs.
The speed sensors and speed sensor lines are monitored both when stationary as well as when driving by different methods such as voltage level monitoring of the speed sensor lines, comparison of wheel speeds with a reference speed, static slip monitoring etc.
The solenoid valve lines are monitored dependent on the drive status for a break and short to positive or ground.
The switch-on status and afterrunning of the return pump motor are also monitored electrically. If no voltage is applied at the pump motor for a certain time after activation or if the voltage is not below a specified minimum value within a certain period of time on completion of activation, a fault is detected and the entire system is shut down.
Note
After a safety shut-down, the normal service brake system as well as the engine control are still available without restrictions.
The wheel speed signals are made available by four "active" speed sensors.
These active speed sensors operate in accordance with the Hall principle, in which changes in the magnetic field strength produce alternating voltages with the frequency used as a measure for the wheel speed. In contrast to the passive speed sensors, the actual sensor element and electronics for signal conditioning are integrated in the active speed sensors.
Advantages of active sensors:
The speed signals are square-wave signals with constant amplitude (low level = 0.75 V, high level = 2.5 V), frequencies within the range from 0 ... approx. 1700 Hz are obtained for the speed range from 0 ... 250 km/h dependent on the tyre circumference.
The ABS/DSC control unit provides the voltage supply to the active sensors (7.6 ... 8.4 V direct voltage).
The wheel speeds are also required by other control units, for instance, the instrument cluster electronics (IKE) or electronic damper control (EDC). The speed signals are correspondingly processed in the ABS/DSC control unit for this purpose.
The speed output signals are square-wave signals with constant amplitude (low level < 1 V, high level > 11 V). The frequency is dependent on the wheel speeds and is within the range from 0 too 1700 Hz (0 to 250 km/h).
In addition to other measured variables, the ABS/DSC control unit also requires the steering angle to calculate the transverse dynamics.
The steering angle is recorded by a separate steering angle sensor. This steering angle sensor is designed as a 360 degree potentiometer. The measuring range is 2 x 180 degrees.
To ensure satisfactory measurement results, a double steering angle sensor is installed. The two slider contacts are offset by 90 degrees with respect to each other.
The steering angle sensor can, for example, be installed such that when the wheels are in the straight ahead position a very low voltage is applied at the slider.
If the steering wheel is now turned to the right or left, the voltage picked off from the slider increases in linear progression until the maximum value of approx. 5 Volt is reached at an angle of 180 degrees. When the steering wheel is turned further in the same direction, the voltage at the slider decreases in linear progression until the minimum value of approx. 0.5 V is reached.
The delta voltage formed in this way by the potentiometer is evaluated by the control unit and converted into steering angle.
The positive and negative supply of both potentiometers takes place via the ABS/DSC control unit. The supply voltage is 5 Volt.
Note
When replacing the steering angle sensor, it is not necessary to adjust the position of the sensor with respect to the steering wheel. After installation, however, steering angle adjustment with the diagnosis and information system must be carried out (Section "Service Functions", Subsection "Steering Angle Adjustment").
The hydraulic unit consists of the solenoid valves, an accumulator chamber per brake circuit and a two-circuit return pump.
The solenoid valves are switched by the ABS/DSC control unit. Depending on the switching status, they connect the wheel brake cylinders with the corresponding circuit of the master brake cylinder or of the return pump or close the wheel brake cylinder with respect to both.
Two solenoid valves, one inlet and one outlet valve, are necessary for each of the four control circuits. An additional intake valve and a switchover valve are necessary for DSC control. The intake valve facilitates the pressure build-up via the self-priming pump element in the return pump. The changeover valve prevents the brake fluid flowing back into the supply reservoir during control cycles.
The positive supply for the solenoid valves is routed via the valve relay. This relay is activated as of ignition lock position 2 by the ABS/DSC control unit.
The return pump pumps the brake fluid flowing from the wheel brake cylinders via the corresponding accumulator back into the relevant circuit of the master brake cylinder. The positive supply for the return pump is routed via the motor relay. This relay is activated as of ignition lock position 2 by the ABS/DSC control unit.
The accumulators in the hydraulic unit serve the purpose of temporarily taking up the brake fluid which accumulates suddenly during pressure reduction.
ABS/DSC system faults or system statuses are indicated in the instrument cluster with the aid of the ABS safety lamp and the multifunction lamp.
The ABS safety lamp and the multifunction lamp light as from ignition lock position 2. If the entire system is free of faults, both lamps go out after approx. 2 seconds (lamp check).
If the safety circuit detects a fault in the ABS subsystem, the entire system, i.e. the ABS and DSC subsystems, is switched off (ABS safety lamp and multifunction lamp light). If the fault no longer occurs during the next starting procedure both lamps go out again after the lamp check.
If the safety circuit detects a fault in the DSC subsystem, dependent on the type of fault, either the entire system or only the DSC subsystem is switched off (ABS safety lamp and/or multifunction lamp lights).
The multifunction lamp lights continuously if the DSC subsystem is switched off with the DSC passive button.
The multifunction lamp flashing while driving (frequency approx. 3 Hz) indicates to the driver that the DSC control is defective.
Note
The ABS safety lamp and the multifunction lamp light during diagnosis (communication with the diagnosis and information system).
The brake light switch informs the ABS/DSC control unit of when the brake pedal is operated. This results in immediate termination of any brake intervention during an DSC control function thus improving the control comfort.
Faults in the brake light switch circuit are stored in the defect code memory, however, the ABS safety lamp is not activated.
The handbrake switch informs the ABS/DSC control unit that the handbrake is applied. This results in an increase in the threshold for the engine drag torque control (MSR) thus improving the control comfort.
The DSC function can be switched off with the DSC passive button. This is indicated by a multifunction lamp lighting in the instrument cluster. The ABS control function, however, is still operable.
The DSC function is reactivated by pressing the button again as long as the difference between the maximum rear wheel speed and minimum front wheel speed is less than 20 km/h.
During the ABS control procedure, the front wheels are controlled individually while the rear wheels are controlled together. An ABS control cycle consists of the three phases retaining pressure, reducing pressure and building up pressure.
If a wheel sensor signals a distinct wheel deceleration, i.e. there is a tendency for the wheel to lock, initially the brake pressure of this wheel is not increased, i.e. the pressure is maintained at the level reached previously.
If the wheel deceleration continues to increase, the brake pressure is reduced so that the wheel is braked to a lesser extent.
The wheel rotates faster again due to the reduced brake pressure. The pressure is increased again when a certain speed limit is reached. Rotation of the wheel is slowed down again by this increase in pressure.
Depending on the condition of the road surface, approx. 5 to 12 control cycles take place per second. The return pump is permanently activated during the control procedure.
With no power applied, the inlet valves are open in their rest position while the outlet valves are closed in their rest position:
Pressure phase |
Solenoid valve / status |
|---|---|
Pressure retention |
Inlet valve / closed (power applied) |
|
Outlet valve / closed (no power applied) |
Pressure reduction |
Inlet valve / closed (power applied) |
|
Outlet valve / opened (power applied) |
Pressure build-up |
Inlet valve / opened (no power applied) |
|
Outlet valve / closed (no power applied) |
The ABS/DSC control unit reduces the drive torque if the drive wheels spin or if the vehicle tends to under-steer or over-steer.
This is achieved by following control interventions:
The control interventions in the engine management take place via the electronic throttle control system (EML) and the digital motor electronics (DME).
The brake intervention on the drive wheels takes place parallel to ABS control via the three activation phases, pressure built-up, pressure retention and pressure reduction. The pressure required for brake intervention during a DSC control procedure is applied via the intake valve and the self-priming pump element directly to the corresponding wheel brake. The changeover valve prevents the brake fluid flowing back into the supply reservoir during control cycles.
The system pressure is limited by a pressure relief valve to approx. 90 bar. The pressure relief valve is integrated in the changeover valve.
The ABS/DSC control unit features interfaces to the electronic throttle control (EML), digital motor electronics (DME) and to the adaptive transmission control (AGS).
The electronic throttle control system and digital motor electronics reduce the drive torque while the adaptive transmission control required information from the ABS/DSC control unit to detect various driving and ambient conditions.
Data exchange between these control units takes place not as in earlier models via a large number of individual wires but rather via a serial bus system, the so-called CAN-bus (Controller Area Network). This bus system consists of two data links (CAN_L and CAN_H) and a shield (CAN_S) to protect from external spurious signals.
All users in this bus system have equal rights, i.e. each control unit can both transmit as well as receive. In the event of one user failing, the bus still remains available for the remaining users.
Note
All control units connected to the bus must feature the same bus index. This index is indicated in the diagnosis program on the first page of the control unit identification.
On request by the ABS/DSC control unit, the electronic throttle control system reduces or increases the torque at the power axle by changing the throttle position corresponding to the request. This intervention is the first yet relatively slow measure in reducing the drive torque in a DSC control procedure.
The electronic throttle control replaces the mechanical link between the accelerator pedal and throttle valve. The position of the accelerator pedal is converted into an electrical signal by a pedal position sensor and then converted in the EML control unit into a control voltage for an electrical actuator motor which in turn operates the throttle valve.
On request by the ABS/DSC control unit, the digital motor electronics varies the ignition timing (ignition angle) or blanks out the ignition as injection pulses:
Intervention |
Effect |
|---|---|
Ignition timing set towards "retard" |
Fast reduction of drive torque |
Blanking out ignition/injection pulses |
Fast reduction of drive torque |
The adaptive transmission control permanently receives the four wheel speed signals as well as the message "DSC passive" or "DSC active".
The AGS control unit detects cornering and winter driving by way of the wheel speed signals.
The "DSC passive" message prevents selection of the winter driving program.