Note: On models equipped with road traction control, the ABS unit is a dual-use unit and functions as an anti-lock wheel system (ABS) and clutch control (ASR).
Models without clutch control
1. The ABS system was installed on most models as standard equipment. The system consists of a hydraulic assembly which contains three hydraulic solenoid valves (one for each front brake and a third for the two rear brakes), electrically driven return pump, four wheel sensors (one per wheel) and electronic control unit (ECU). The purpose of the system is to prevent the wheels from locking up during hard braking or on slippery roads. This is achieved by automatically releasing the brake on the respective wheel, followed by applying the brake action. In the case of the rear wheels, both brakes are applied simultaneously.
2. The solenoid valves are controlled by the ECU, which receives signals from four wheel sensors (the front sensors are mounted on the hubs and the rear sensors are mounted on the final drive), which determine the speed of rotation of each wheel. By comparing these signals, the ECU can determine the speed at which the vehicle is moving. At a known vehicle speed, the control unit determines the moment when one of the wheels slows down more than the others, and is close to blocking. Under normal conditions, the system functions like a normal braking system.
3. If the ECU determines that a wheel is close to blocking, it will actuate the corresponding hydraulic block solenoid valve, which will disconnect the corresponding brake caliper of the wheel that is close to blocking from the master cylinder.
4. If the wheel speed continues to decrease at an abnormal rate, the ECU turns on the electric return pump and pumps hydraulic fluid back into the master cylinder, reducing caliper pressure and the brake is released. When the wheel speed returns to normal, the pump turns off, the solenoid valve repressurizes the caliper and the brake action is restored. This cycle can repeat up to 10 times per second.
5. The operation of the valves and the return pump creates pulsations in the hydraulic circuit. When the ABS system is operating, these pulsations can be felt on the brake pedal.
6. The operation of the ABS system is completely dependent on electrical signals. To improve reliability, a safety circuit built into the ECU constantly monitors all signals to the ECU. If an incorrect signal is detected or a low battery voltage is detected, the ABS system is automatically disabled and a warning lamp lights up on the instrument panel, informing the driver that the system is not working. However, the conventional braking system is still active.
7. If a malfunction occurs in the ABS system, it is necessary to diagnose and repair the car at a Mercedes-Benz service station.
Models with Traction Control (ASR)
8. On models equipped with road grip control (ASR) the hydraulic unit also performs the functions of an anti-lock wheel system.
9. On models with clutch control, the hydraulic block contains four solenoid valves (instead of three), return pump, and shift lever (which switches the hydraulic unit from the ABS function to the (ASR)), and the compression pump of the clutch control system. Two Electronic Control Units are installed, one controls the hydraulic modulator and the other controls the position of the throttle actuator. In addition to these differences, a compression pump and an accumulator are installed to create and maintain the hydraulic pressure necessary for the operation of the brake part of the road grip control system.
10. The brake part of the system operates as described above, however, each rear brake operates independently. An additional wheel sensor is installed on the main gear and the hydraulic unit has a solenoid valve for each rear brake.
11. The Traction Control System protects the rear wheels from losing traction on both heavy braking and throttle applications, depending on vehicle speed. In extreme cases, a combination of both methods may be used.
12. On the brake side, if a wheel begins to lose traction, the hydraulic unit uses the pressure stored in the accumulator to smoothly apply the brake to the corresponding wheel. When the risk of wheel slip has passed, the hydraulic assembly allows the fluid to return to the accumulator and release the brake action, allowing the wheel to spin freely again.
13. On the throttle side, if the wheel is about to lose traction, the second ECU activates the electromagnetic actuator and closes the throttle, reducing engine torque. When the danger of wheel slip has passed, the actuator returns the throttle to the normal position and returns control of the throttle to the driver.
14. As in the case of the ABS, if a malfunction is detected in the system (ASR) the vehicle must be diagnosed and repaired at a Mercedes-Benz service station.
Warning: Diagnosis of malfunctions in ABS / ASR systems is carried out using special test equipment. For safety reasons, vehicle owners are strongly advised not to test the ABS/ASR systems using normal workshop equipment.