Design and function of anti-lock braking system - RA4200124A700X(42-0700)

The ABS anti-lock braking system (or electronic brake slip control) is one of the active safety elements of a vehicle which can decisively reduce the risk of becoming involved in an accident.

For this reason, the ABS is expected to meet the following demands:

1. The driving stability of the vehicle should be assured while braking, both when the braking force is slowly increased up to the locking limit and when suddenly increasing the braking force in the event of an emergency stop.
2. As long as the vehicle speed is adequately below the curve limit speed, braking on a bend should be possible without impairing driving stability while maintaining full steerability (the curve limit speed is the speed at which a vehicle can be driven around a bend without engine power and just fast enough that it will not leave the road under the influence of centrifugal force).
3. When the brake pedal is excessively operated to the extent that an uncontrolled braking system will result in the wheels locking, the ABS will modify the braking pressure in the wheel brakes to the extent that the wheels do not lock and that instead, the adhesion between the wheels and the road is optimally exploited. Consequently, vehicles with ABS as compared with uncontrolled brake systems provide the following advantages when the brakes are fully applied:
   • Higher driving stability
   • Better steerability

The malfunction indicator lamp with the ABS symbol in the instrument cluster lights up when the ignition is switched on and goes out after the engine is started (same as charge-indicator lamp).

Should the lamp fail to go out, this could indicate low voltage in the vehicle electrical system or an open circuit in the current supply to the electronic control unit. Once the vehicle has attained a speed of approx. 5 km/h, the ABS performs a self-test (BITE = B  uilt I  n T  est E  equipment). The ABS malfunction indicator lamp will light up if a fault is recognized.

Fig 1: Identifying ABS Malfunction Indicator Lamp

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

If the problem assumes the form of a permanent fault, e.g., open wire, then the fault remains stored in the ABS control unit until the ignition is switched off. Short periods of undervoltage in the vehicle's electrical system are not recorded. Thus, if the battery voltage remains below 11 volts after the ignition has been switched on and the vehicle has exceeded the test speed, the ABS will remain inactive until such time as the alternator raises the system voltage to above 11 volts. Only then will the ABS malfunction indicator lamp go out.

Any braking in the locking range initiated above 8 km/h (above 12 km/h on the 1st version up to 02/84) can be governed down to a speed of 3 km/h (down to 5 km/h on the 1st version up to 02/84). This means that controlled braking will proceed only after the so-called control speed of 8 km/h (12 km/h on 1st version up to 02/84) has been exceeded.

When the malfunction indicator lamp in the instrument cluster is on, this signals that the ABS is inactive; the antilock system will not operate when the vehicle's brakes are applied. The conventional brake system remains operational. The vehicle should be checked and repaired in a Mercedes-Benz service station as soon as possible.

Design 

The anti-lock braking system comprises the conventional braking system known up to now and the following additional components:

1. Hydraulic unit
2. Wheel speed sensors
3. ABS control unit
4. Harness with overvoltage protection relay
   Fig 2: Identifying ABS Hydraulic Unit Components Location

   Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Model 124 (except 124.034) 

Fig 3: Identifying ABS Hydraulic Unit Components Location - Model 124

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Model 129 

Fig 4: Identifying ABS Hydraulic Unit Components Location - Model 129

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Models 124.034, 140 

Fig 5: Identifying ABS Hydraulic Unit Components Location - Model 124.034,140

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Electrical components at front axle and in engine compartment

Model 124.034, 140 

Electrical components at rear axle and in passenger compartment

Fig 6: Identifying ABS Malfunction Indicator Lamp Components Location - Model 124.034, 140

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Model 201 

Fig 7: Identifying ABS Malfunction Indicator Lamp Components Location - Model 201

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Model 202 

Electrical components at front axle and in engine compartment

Fig 8: Identifying ABS Hydraulic Unit Components Location - Model 202

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Model 202 

Electrical components at rear axle and in passenger compartment

Fig 9: Identifying ABS Malfunction Indicator Lamp Components Location - Model 202

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.