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.