1983 88 & 1991 BRAKES Mercedes Benz Anti Lock Brake System

DESCRIPTION

The Mercedes-Benz Anti-Lock Brake System (ABS) is a Bosch design. It operates from the interaction of a hydraulic unit with 3 fast-switching solenoid valves, 3 speed sensors, an electronic control unit with overvoltage protection and a wire harness with relays.

A Yellow warning light with the "ABS" symbol, on instrument panel, lights as the ignition is turned on. The ABS warning light will go out once the engine is running. The ABS system is programmed to activate after 7.5 MPH.

When vehicle speed is above 3 MPH, Built-In Test Equipment (BITE) will begin check of ABS. If any fault is found, the warning light will again go on. When warning light goes on, ABS is switched off and vehicle will brake without ABS control. The conventional brake system remains operational.

If battery is below 10.5 volts, when ignition is turned on and test speed exceeded, ABS will remain off until alternator increases voltage to above 10.5 volts. Warning light will then go out.

Following repair, during which no direct components of ABS were involved, a simple light test will check system. Check that light goes out after 7 MPH.

NOTE: The entire ABS system must be checked if repair includes any ABS component or if units are replaced following an accident. Use ABS Test Adapter 126-859-09-21-00 together with brake test bench. OPERATION HYDRAULIC UNIT

Independent of master cylinder pressure, hydraulic unit will compensate brake fluid pressure to wheel cylinders during regulation. Pressure increase above master cylinder pressure is not possible.

The 3 hydraulic unit solenoid valves control left front, right front and rear brakes. By activating valves with current of varying amperage, brake fluid pressure in individual calipers may be increased, held or decreased.

In pressure "build-up" stage, pressure increases by opening intake valve to pressure supplied by master cylinder. In pressure "holding" stage, which precedes "reduction" stage, pressure from hydraulic unit to wheels is constant. Output and input valves in solenoid valve are closed.

During pressure "reduction" stage, brake fluid flows from reservoir to return pump. To maintain fluid volume, pump returns fluid to main cylinder against prevailing pressure.

To dampen delivery noise, each circuit has a silencer. Relays for solenoid valves and return pump are on 12-pole socket of hydraulic unit. A diode is soldered in socket. Hydraulic unit is connected to ground by cable.