The hydraulic unit consists of the solenoid valves, an accumulator chamber per brake circuit and a two-circuit return pump.
The solenoid valves are switched by the ABS/DSC 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 switchover valve are necessary for DSC 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 positive supply for the solenoid valves is routed via the valve relay. This relay is activated as of ignition lock position 2 by the ABS/DSC control unit.
The return pump pumps the brake fluid flowing from the wheel brake cylinders via the corresponding accumulator back into the relevant circuit of the master brake cylinder. The positive supply for the return pump is routed via the motor relay. This relay is activated as of ignition lock position 2 by the ABS/DSC control unit.
The accumulators in the hydraulic unit serve the purpose of temporarily taking up the brake fluid which accumulates suddenly during pressure reduction.