Note: On models equipped with the traction control system, the ABS unit is a dual-purpose unit and performs the functions of the anti-lock brake system (ABS) and the traction control system (ASR).
Models without clutch control
1. The ABS system was installed on most models as standard equipment. The system consists of a hydraulic unit that contains three hydraulic electromagnetic valves (one for each front brake and a third for the two rear brakes), electric return pump, four wheel sensors (one per wheel) and an electronic control unit (ECU). The purpose of the system is to prevent the wheels from locking during sudden braking or on a slippery road. This is achieved by automatically releasing the brake on the corresponding wheel, followed by the application of the brake action. In the case of the rear wheels, both brakes are applied simultaneously.
2. The electromagnetic 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 rotation speed of each wheel. By comparing these signals, the ECU can determine the speed at which the car is moving. Given a known speed, the control unit determines when one wheel is slowing down more than the others and is close to locking. Under normal conditions, the system functions like a conventional braking system.
3. If the ECU detects that the wheel is close to locking, it actuates the corresponding electromagnetic valve of the hydraulic unit, which disconnects the corresponding brake caliper of the wheel that is close to locking 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 to the master cylinder, reducing the pressure in the caliper and the brake is released. When the wheel speed returns to normal, the pump is turned off, the solenoid valve restores the pressure in the caliper and the brake action is restored. This cycle can be repeated 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 ABS system is completely dependent on electrical signals. In order to increase reliability, the safety circuit built into the ECU constantly monitors all signals coming to the ECU. If an incorrect signal or low battery voltage is detected, the ABS system is automatically disabled and the warning light on the instrument panel lights up, informing the driver that the system is not working. However, the normal braking system is still in effect.
7. If a malfunction occurs in the ABS system, it is necessary to carry out diagnostics and repair the vehicle at a Mercedes-Benz service station.
Models with All-Terrain Regulator (ASR)
8. On models also equipped with the anti-lock braking system (ASR), the hydraulic unit also functions as an anti-lock braking system.
9. On models with a clutch control system, the hydraulic unit contains four electromagnetic valves (instead of three), a return pump, and a shift lever (which switches the hydraulic unit from the ABS function to the (ASR) function), and a compression pump for the traction 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 required for the operation of the brake portion of the traction control system.
10. The braking part of the system operates as described above, but each rear brake operates independently. An additional wheel sensor is installed on the main gear and the hydraulic unit has an electromagnetic valve for each rear brake.
11. The traction control system protects the rear wheels from losing traction both under heavy braking and when the throttle is closed, depending on the 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 danger of wheel slippage has passed, the hydraulic unit allows the fluid to return to the accumulator and release the brake, allowing the wheel to spin freely again.
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13. On the throttle side, if the wheel is about to lose traction, the second ECU turns on the electromagnetic actuator and closes the throttle valve, reducing engine torque. When the danger of wheel slip has passed, the actuator returns the throttle valve to the normal position and returns throttle control to the driver.
14. Just as with ABS, if a fault is detected in the system (ASR), the vehicle must be diagnosed and repaired at a Mercedes-Benz service station.
Warning: Diagnostics of faults in ABS/ASR systems are performed using special testing equipment. For safety reasons, vehicle owners are strongly advised not to perform tests in ABS/ASR systems using regular workshop equipment.