The ABS/ASC 5 system is a further development of the ABS/ASC and ABS/ASC+T systems as already used for a long time on series E32 and E34 vehicles.
The elementary ABS and ASC functions of the previous version have been retained:
ABS prevents the wheels locking when braking and thus retains the steerability of the vehicle.
ABS prevents the drive wheels from slipping and therefore also increases the driving stability and traction of the vehicle on slippery road surfaces.
The ABS safety lamp and the ASC multifunction lamp light for approx. 2 seconds (lamp check) as from ignition lock position 2 and then go out if no faults are found in the ABS/ASC system.
If the ABS safety circuit detects a fault in the ABS subsystem, the ABS and ASC systems are deactivated and the ABS safety lamp as well as the ASC multifunction lamp light up.
If the fault no longer applies during the next start procedure, the lamps will go out after the lamp check.
If the ASC safety circuit detects a fault in the subsystem ASC, depending on the type of fault, either both subsystems or only the ASC subsystem are deactivated and the corresponding lamps light up.
If the ASC system is switched on manually in passive mode, this situation is indicated to the driver by the steady light of the ASC multifunction lamp.
The driver's attention is drawn to an ASC control function by the ASC multifunction lamp flashing (3 Hz).
Note
The ABS safety lamp and the multifunction lamp light permanently during diagnosis (communication with the tester).
The DME main relay provides the positive supply of the control unit as of ignition lock position 2. The voltage is retained for approx. 5 seconds after "ignition OFF". (DME holding circuit)
Ground connection is realized via three ground lines.
The ABS/ASC control unit acquires the wheel speed signals via the speed sensors, processes the signals and then makes them available for other users such as the EGS, IKE (speedometer signal) or the EDC.
The ABS/ASC control unit outputs the speed output signals as square-wave signals with constant amplitude (low level <1 V / high level > 11 V). The frequency of the square-wave signal is dependent on the relevant wheel speed in the range from 0 to 1700 Hz (0 to 250 km/h).
The brake light switch signal serves as a redundant signal for recognizing an initiated braking procedure. This enables improvements in control comfort.
Faults in the brake light switch circuit are stored in the fault code memory, however, the ABS safety lamp is not activated when such faults occur.
The handbrake switch signals informs the ABS/ASC control unit that it is possible that the car is being driven with the handbrake applied.
This causes the control unit to set a higher threshold value for the engine drag torque control (idle speed increase).
The ASC system can be switched off and on while driving with the aid of the ASC button. The ASC safety circuit is always active. The ASC system is active after every new start (as from ignition lock position 2).
The terminal 61 signal is a redundant signal and serves the purpose of detecting whether the engine is running. At an alternator voltage below 8 V, the ASC system switches off and the multifunction lamp lights up. However, no fault is stored in the defect code memory.
In contrast to the previous versions, the wheel speed signals are provided by four "active" speed sensors.
The ABS/ASC control unit supplies the positive voltage to the speed sensors (7.6...8.4 V direct voltage) as from ignition lock position 2.
The speed sensor outputs the speed signal as a square-wave signal with constant amplitude (low level = 0.75 V / high level = 2.5 V) to the ABS/ASC control unit. The frequency of the square-wave signal is dependent on the relevant wheel speed in the range from 0 to 1700 Hz (0 to 250 km/h).
The brake pressure is controlled during ABS operation by means of four pairs of solenoid valves, one inlet valve and one outlet valve per wheel brake, in the hydraulic unit.
The valve relay provides the positive supply for the solenoid valve as of ignition lock position 2.
The solenoid valves are switched by the ABS/ASC control unit applying ground.
Depending on the driving situation which is determined by the wheel speed signals, the following operating statuses can be represented by activation of the solenoid valves:
Operating status |
Solenoid valve / status |
|---|---|
Pressure build-up |
Inlet valve open (no power applied) |
|
Outlet valve closed (no power applied) |
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) |
The front wheels are controlled individually and the rear wheels jointly.
The solenoid valves can be switched 5 to 12 times per second during controlled braking.
The ABS/ASC pump is activated by the ABS/ASC control unit during ABS operation.
No power is applied to the input and output valves for the front wheel brakes during ASC operation. The ASC changeover valve is closed (power applied) and the ASC intake valve is open (power applied). The input and output valves for the rear wheel brakes are opened or closed depending on the control situation.
The ABS/ASC pump is activated by the ABS/ASC control unit during ABS operation.
The pressure for brake intervention during an ASC control phase is applied directly from the ABS/ASC pump to the corresponding wheel brake.
The positive supply for the ABS/ASC pump is controlled by the ABS/ASC control unit via the motor relay. The negative supply is provided by a separate ground cable.
On automatic transmission vehicles, the ABS/ASC control unit can output a digital signal (0 V / 12 V) to the EGS control unit during ASC activation. This signal causes the EGS not to implement any pending gearshifts so as not to increase the torque at the drive axle.
One way of reducing the drive torque during ASC activation is to close the choke. For this purpose, the control unit activates an electric motor which operates the choke with the aid of a cable assembly. The throttle actuator is activated by clocking the positive and negative supply.
The power output stage in the ABS/ASC control unit for operating the throttle actuator is powered by a separate line (terminal 30).
The current position of the choke is detected by a throttle potentiometer and signalled back to the control unit.
The ABS/ASC control unit can determine the current load status of the engine with the aid of the input signals engine speed and current throttle angle. These factors serve as the basic data for possible engine intervention during ASC activation.
The ABS/ASC control unit can initiate the following engine interventions by outputting digital signals (0 V / 12 V) on three lines to the DME:
Engine intervention |
Effect |
Type of control |
|---|---|---|
Ignition timing adjustment |
Smooth reduction of drive torque |
ASC |
Blanking out ignition |
Fast reduction of drive torque |
ASC |
Increase in idle speed |
Reduction in engine drag torque on vehicles with manual transmission |
MSR |
The above-specified engine interventions are supplemented as required by control of the choke and influencing the individual rear wheel brakes.