3 Examples of Troubleshooting for ABS/ASR Braking System of Mercedes-Benz Cars

ABS/ASR is a standard configuration for Mercedes-Benz cars equipped with W140 chassis. The failure of ABS/ASR will directly affect the power output and driving safety of the car. ABS/ASR faults can usually be divided into two categories. One is caused by the malfunction of ABS/ASR components (such as wheel speed sensors, brake light switches or hydraulic control components, etc.). For this type of fault, you only need to query the fault information first and then do targeted maintenance work to eliminate the fault. The other type is caused by the malfunction of components other than ABS/ASR. The most typical one is the poor performance of the electronic throttle, which causes the ABS/ASR control unit to be unable to obtain the actual throttle opening signal through the CAN bus, ASR failure, and the ABS/ASR control unit stores fault information such as CAN bus communication failure and lights up the ASR warning light. For this type of fault, corresponding maintenance (such as maintenance of the electronic throttle) must be carried out to eliminate the fault. Although the ABS/ASR control unit may also fail, it is very rare and should be treated with caution. This article uses 3 faults as examples to illustrate the troubleshooting methods of ABS/ASR faults.

Fault 1: On a 5500 Mercedes-Benz sedan, the ASR warning light sometimes lights up intermittently while driving, and once the light is on permanently, the steering wheel will feel heavy to operate.

According to the working principle of ABS/ASR, when ABS/ASR fails, the transmission of wheel speed signal will be interrupted, and other electronic control systems that require wheel speed signal to work normally will also be affected. Therefore, as long as the ASR fault is eliminated, the problem of heavy steering wheel operation will disappear.

Connect the original STAR fault detector, click "Traction" in "Control units" to enter the ASR electronic control unit diagnostic menu, and then click "Fault codes" to query the fault information. The STAR fault detector displays "003 Right front axle VSS sensor L6/2 implausible", which means that the signal of the right front wheel speed sensor is not real and the fault nature is currently present. After clearing the fault information, a road test was carried out. After about 10 minutes, the ASR light came on. Query the fault information again, and its content remains the same. After lifting the car, click the "Actual Values" function of the STAR fault detector to observe the output signal of each wheel speed sensor. First, turn the right front wheel by hand, and the STAR fault detector always displays 0km/h; then turn the other 3 wheels, and the corresponding wheel speeds are displayed. This shows that the ABS/ASR control unit does not receive the signal from the right front wheel speed sensor. Measure the resistance of the right front wheel speed sensor, which is about 1.0kΩ, which meets the standard; at the same time, check the connection between the wheel speed sensor and the ABS/ASR control unit, which is normal. Then I checked the right front wheel speed sensor signal ring gear at the right front wheel hub and found no other signs of badness except some dirt. After cleaning and reinstalling the right front wheel speed sensor signal ring gear, I tested the car and the fault still existed.

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In order to determine whether the right front wheel speed sensor is faulty, it was decided to use the signal substitution method for inspection, that is, after disconnecting the right front wheel speed sensor connector, use two wires to connect the wire on the right front wheel speed sensor wire side connector in parallel to the wire of the left front wheel speed sensor, and then test drive. The test drive result did not change, so it was suspected that the ABS/ASR control unit was faulty. The ABS/ASR control unit housing was opened for inspection, and there was no sign of open circuit and ablation on the circuit board. Considering that the price of the ABS/ASR control unit is not low, the ABS/ASR control unit of another car of the same model was used for replacement test, and the fault symptoms disappeared; the suspected faulty ABS/ASR control unit was installed on other cars, and the ASR warning light also lit intermittently. This proves that the ABS/ASR control unit is indeed faulty, and the fault is completely eliminated after replacing the ABS/ASR control unit.

Fault 2: A 1995 5320 Mercedes-Benz sedan, when the ASR warning light was on, the speedometer failed at the same time, and the steering wheel was heavy to operate.

The fault information found by the STAR fault detector was "002 Left front speed sensor (L6/1)", which means that the left front wheel speed sensor was faulty. The road test after clearing the fault information showed that the fault did exist. After returning to the factory, the protective rubber sleeve of the left front wheel speed sensor harness was checked and there was no sign of damage. The left front wheel speed sensor was removed and a small amount of iron filings were found on the sensor head. After cleaning, the signal gear ring was checked from the sensor seat hole and there was no missing teeth or dirt. After the left front wheel speed sensor was reinstalled, the output signals of the four wheel speed sensors were observed using the method described in Fault 1. The result was that, except for the left front wheel speed sensor, the other three wheel speed sensors all had wheel speed signal outputs of 0km/h to 8km/h. The resistance of the left front wheel speed sensor was measured to be 1.1kΩ, which met the standard. In this case, the signal substitution method was used for inspection. During the road test, the ASR warning light did not light up, and the speedometer and steering wheel operation returned to normal, which determined that the left front wheel speed sensor was not performing well. After replacing the left front wheel speed sensor with a new one, the fault was completely eliminated.

The wheel speed sensor used in Mercedes-Benz cars is a magnetic induction type. A common fault of this type of wheel speed sensor is insensitivity at low speeds. If you only measure its resistance, you cannot determine its performance, because the source of the fault may be a problem with the sensor's permanent magnet rather than a damaged coil.

Fault 3: A Mercedes-Benz car owner reported that when he stepped on the brake pedal during driving, as the car speed decreased, there was obvious vibration noise from the front wheels, the ABS warning light was always on, and the steering wheel was heavy to operate. If the ignition switch was turned off at this time and the engine was running again, the ABS light would go out normally while the car was driving, and the steering wheel would be easy to operate.

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First, a road test was conducted. The fault symptoms were completely consistent with the situation reported by the car owner. The fault information obtained by the STAR fault detector was that the right front wheel speed sensor signal was not true and the fault nature was memory storage. Disconnect the ignition switch, unplug the right front wheel speed sensor wire side connector, and measure the resistance of the right front wheel speed sensor, which was 1.2kΩ, which met the standard, indicating that the right front wheel speed sensor coil had no short circuit or open circuit fault. After lifting the car, check the installation status of the right front wheel speed sensor and find that there is an additional paper gasket on its mounting seat. The owner said that this was installed during the inspection at other repair shops. It is estimated that it is used to adjust the installation gap of the right front wheel speed sensor, but the fault still exists. Check the right front wheel speed sensor signal ring in the sensor mounting seat hole, and the signal ring is covered with dirt. It is believed that the dirt of the right front wheel speed sensor head and its signal ring is the direct cause of the distortion of the right front wheel speed sensor signal, but after removing the dirt and paper gasket, the fault symptoms did not change at all during the road test.

Checking the signal output of each wheel speed sensor, it was found that the signal of the right front wheel speed sensor would jump directly from 3km/h to 7km/h only if the right front wheel was turned faster. This indicated that there was a problem with the sensitivity of the sensor, and it was determined that the right front wheel speed sensor was faulty. To further confirm this, the signal replacement method was used for inspection. During the road test, the ABS warning light did not light up, and other fault symptoms also disappeared. This indicated that the right front wheel speed sensor was indeed faulty. After replacing the new right front wheel speed sensor, the fault was eliminated.

Due to the decreased sensitivity of the right front wheel speed sensor, the wheel speed sensor will quickly provide a wheel speed signal of 0km/h when the vehicle speed decreases, causing the ABS/ASR control unit to mistakenly believe that the right front wheel is locked and reduce the braking force on the wheel, while in fact the wheel is still rotating, resulting in excessive difference in the braking conditions of the two front wheels and strong chassis vibration.