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.