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 ABS/ASC 5 system comprises the functions ABS and ASC+T. ABS stands for antilock brake system, ASC+T for automatic stability control + traction.
The antilock brake system (ABS) prevents the wheels locking even in the case of a full emergency brake on a slippery road surface, thus considerably increasing the driving stability and steerability of the vehicle.
The automatic stability control + traction (ASC+T) is in effect a reverse of the ABS system. It prevents the drive wheels from slipping during acceleration and therefore also increases the driving stability and traction of the vehicle.
The ABS/ASC control unit is ready for operation as of ignition lock position 2.
The negative supply is produced via two ground lines.
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/ASC control unit provides the voltage supply to the active sensors (7.6 ... 8.4 V direct voltage).
Since other control units also require the wheel speeds, such as the instrument cluster electronics (IKE) or electronic damper control (EDC), the speed signals are processed accordingly in the ABS/ASC control unit.
These 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 lies in the range between 0 ... 1700 Hz (0 ... 250 km/h).
The hydraulic unit consists of solenoid valves, a two-circuit return pump and an accumulator chamber per brake circuit.
The solenoid valves are switched by the ABS/ASC 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 changeover valve are necessary for ASC 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 solenoid valves are powered by the valve relay as of ignition lock position 2. The valve relay is activated by the ABS/ASC control unit.
The return pump is activated only during ABS or ASC control. It pumps the brake fluid, flowing from the wheel brake cylinders when the pressure is reduced, via the corresponding accumulator back into the relevant circuit of the master brake cylinder. The return pump is switched on and off by the engine relay, controlled by the ABS/ASC 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/ASC 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/ASC system is switched on automatically and then ready for operation every time the ignition is switched on. If the entire system is free of faults, both the ABS safety lamp as well as the multifunction lamp go out after approx. 2 seconds (lamp check).
If the safety circuit detects a fault in the subsystem ABS, the entire system, i.e. ABS and ASC, is switched off (ABS safety lamp and multifunction lamp light up). 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 subsystem ASC, either the entire system or the subsystem ASC is switched off depending on the type of fault (ABS safety lamp and/or multifunction lamp light up).
The ASC subsystem can be switched off with the ASC passive button under special driving conditions (e.g. driving off in deep snow). This is indicated by a steady light in the multifunction lamp.
The driver's attention is drawn to an ASC control function by the ASC multifunction lamp flashing while driving (frequency 3 Hz).
Note
During diagnosis (communication with the diagnosis and information system), the ABS safety lamp and the multifunction lamp light constantly.
The brake light switch informs the ABS/ASC control unit of when the brake pedal is operated. This results in immediate termination of any brake intervention during an ASC control function thus improving the control comfort.
Note
Faults in the brake light switch circuit are stored in the defect code memory, however, the ABS safety lamp does not light up in this case.
The handbrake switch informs the ABS/ASC 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 ASC subsystem can be switched off with the ASC passive button under special driving conditions (e.g. driving off in deep snow). This is indicated by a steady light in the multifunction lamp. The ABS subsystem remains operable without restriction.
The ASC system is reactivated after pressing the button once again.
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: Pressure retention, pressure reduction and pressure build-up.
If a speed sensor signals strong wheel deceleration, i.e. a tendency to lock, initially the brake pressure of this wheel is not increased, i.e. the pressure is retained at the level previously reached.
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) |
ASC reduces the high torques at the drive wheels by correspondingly influencing the engine management and by braking the drive wheels.
This is achieved by following control interventions:
The automatic transmission control is also influenced.
The most important interfaces of the ABS/ASC control unit are those for digital diesel electronics (DDE) and for adaptive transmission control (AGS).
The data exchange between DDE, AGS and ABS/ASC does not take place by the usual means of 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.
The ABS/ASC control unit can instruct the digital diesel electronics to carry out following engine interventions:
Intervention |
Effect |
|---|---|
Reduction in injection quantity |
Reduction in drive torque |
Increase in idle speed |
Reduction in engine drag torque on vehicles with manual transmission |
Vehicles equipped with an adaptive transmission control (AGS) can receive a signal from the ABS/ASC control unit during an ASC control phase, i.e. mainly on road surfaces with snow and ice, to the effect that imminent gearshifts, resulting in an increase in the torque in the drive axle are suppressed.