An antilock brake system (ABS) prevents the wheels locking when braking and thus retains the steerability of the vehicle.
The new ABS 5 system is a further development of the ABS variants as already used for a long time on series E32 and E34 vehicles.
As was the case with the old systems, ABS 5 is also a "closed system" with brake circuit division into "black/white" (front axle/rear axle).
In contrast to the previous "passive" speed sensors, ABS 5 makes use of "active" speed sensors. The current-controlled 3/3-way valves have been replaced by non-regulated 2/2-way valves.
The ABS safety lamp lights for approx. 2 seconds (lamp check) as from ignition lock position 2 and then goes out if no faults are found in the ABS system.
If the ABS safety circuits detects a fault, the ABS systems is deactivated and the ABS safety lamp lights.
If the fault no longer applies during the next start procedure, the lamp will go out after the lamp check.
Note
The ABS safety lamp is permanently activated 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).
The ground supply is produced via two ground lines.
The ABS control unit acquires the wheel speed signals via the four speed sensors, processes the signals and then makes them available for other users such as the IKE (speedometer signal) or the EDC.
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 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.
In contrast to the previous versions, the speed signals are supplied by four "active" speed sensors.
The ABS control unit supplies the voltage to the speed sensors (7.6...8.4 V direct voltage) as from ignition lock position 2.
The speed signals are square-wave signals with constant amplitude (low level = 0.75 V, high level = 2.5 V). The frequency is dependent on the wheel speeds and is within the range from 0 to 1700 Hz (0 to 250 km/h).
The three phases: pressure build-up, pressure retention and pressure reduction are controlled in three independently operating control circuits:
Two solenoid valves, one inlet and one outlet valve, are necessary for each control circuit. The three pairs of solenoid valves are integrated in the hydraulic unit.
The valve relay provides the positive supply for the solenoid valves as of ignition lock position 2. The valves are activated by the ABS control unit switching the corresponding connections to ground potential.
Depending on the driving situation determined with the aid of the speed sensors, each control circuit can assume the following operating statuses by corresponding 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 open (power applied) |
During controlled braking, the solenoid valves are opened and closed again 5 to 12 times per second by the ABS control unit. The return pump is permanently activated.