The hydraulic unit is located between the boost piston unit and wheel brake cylinder.
It modulates the brake pressure during ABS controlled braking and controls or regulates the brake pressure during an ASC or DSC control procedure.
Essentially, the design and function of the hydraulic unit correspond to that of ABS/ASC5.
A changeover valve and a prebooster valve must be additionally provided due to possible brake intervention at the front axle during a DSC control procedure.
The following are therefore integrated in the housing:
When terminal 15 is switched on, all valves are applied to 12 V and are switched to ground by the control unit during a control procedure.
Power is not applied to the 4 inlet solenoid valves and the 2 changeover valves are open, the 4 outlet valves and the 2 admission valves are closed.
The return pump is driven by an electric motor which, when active, is powered via the ABS motor relay.
During ABS control, the pump returns the brake fluid drained off during the pressure reduction phase to the master brake cylinder.
It produces the necessary brake pressure during ASC or DSC control with brake intervention.
The return pump has only one direction of rotation, i.e. both during ABS as well as during ASC/DSC control.
The changeover valve must be closed and the prebooster valve open if the pump is to generate brake pressure during an ASC/DSC control phase.
The pump draws in the brake fluid via the open prebooster valve and then delivers it to the wheel brake cylinder, in which the necessary brake pressure is to be built up. The system pressure build-up is secured by the closed changeover valve.
A hydraulically operated pressure relief valve integrated in the two changeover valves prevents the delivery pressure of the pump increasing above 161 (+/-25) bar during an ASC/DSC control phase.