The Bosch designed Anti Lock Brake System for the mercedes operates from the interaction of hydraulic unit with three fast switching solenoid valves, an elrctronic control unit with overvoltage protection three speed sensors and a wire harness with relays. The yellow warning light on the insrument panel with the ABS symbol is turned on with the ignition,this light will go out once the engine is running the antilock brake system is programed to go out after 7.5MPH.

When any fault is found in the ABS system the warning light will go on, the Built In Test Equipment will begin to check the ABS afer 3 MPH and if there is default then the conventional brake system still remains operational. When battery runs below 10.5 volts the ABS system will remain off untill the alternator runs up over 10.5 volts then the ABS system light will go out, following a repair to the abs system the check engine light goes out after 7 MPH.

After any check or replacement of any ABS component a check is needed with the following test adapter # 126 859 09 21 00. Independent from the master cylinder pressure the hydraulic unit will compensate brake fluid pressure to brake calipers during regulation, pressure increase above master cylinder pressure is not possible.

Brake fluid pressure is increased, held or decreased with three hydraulic unit solenoid valves that control left front and right front and rear brakes. In the build up pressure stage, the pressure increases by the intake valve opening to pressure from the master cylinder the pressure holding stage which is after the reduction stage pressure from the hydraulic unit is constant, input and output valves in solinoid valve are closed.

During pressure reduction stage, the brake fluid flows from reservoir to return pump, to maintain constant fluid volume, the pump returnes the fluid to Mercedes Benz master cylinder against prevailing brake master cylinder pressure. Each circuit has a noise silenser to dampen brake fluid delivery noise, solenoid valves and return pump relays are on a 12 pole socket of the hydraulic unit, a diode id sodered in the socket.