When working on vehicles with ABS systems, the following precautions must be observed:
- when carrying out welding work, the connector should be removed from the electronic control unit of the system;
- if the car is not painted at a Mercedes service station, you should be warned that the electronic unit can only be at a temperature of 95°C for a short time and no more than two hours at a temperature of 85°C;
- when connecting, tighten the battery terminals securely.
When performing any work on the brake system, the ABS system must be checked for proper operation as follows:
- start the engine and check that the yellow lamp in the instrument cluster with the ABS symbol goes out and does not light up again at a speed of 5-7 km/h;
- the system starts to operate from the moment the speed of 5-7 km/h is reached. All pressings on the brake pedal are regulated from the moment the so-called blocking area is reached from the speed of 8 km/h.
The system works as follows. The wheel rotation frequency measured by the sensor determines decelerations and accelerations, the signals of which are transmitted to the electronic control unit. The signals from the individual wheels are combined into a relative signal that determines the approximate speed of travel. By comparing the wheel speed with the relative value, slip signals are generated.
If the brake caliper pressure causes the wheels to lock up due to excessive pressure (a condition that occurs when the corresponding wheel slips), the pressure level in this support is maintained at the same level, i.e. increasing it becomes impossible. If the blockage continues, i.e. the pressure is already too high, the system opens the electromagnetic relief valve to reduce the pressure. Excess fluid is fed back to the brake cylinder by the return circuit pump. If the pressure has dropped so much that the wheel accelerates again, the valve closes and the pressure remains stable again. If the wheel acceleration exceeds the threshold value determined for the given driving conditions, the pressure in the brake system, on the contrary, increases through the inlet solenoid valve. Thus, by receiving the appropriate signals from the control unit, the hydraulic regulator can control the pressure in the system, reducing, increasing or maintaining it at a constant level.
Now a few words about the electronic component of ABS.
Rotation speed sensors are so-called pulse sensors. The car is equipped with four sensors, one for each wheel.
The two front sensors are installed in the steering knuckle, the rear ones in the axle shaft.
The rotation speed is measured by sensors using toothed rims. At the front they are built into the hubs, at the rear – into the axle shafts. The sensor consists of a magnetic core with a winding. The rotation of the toothed ring, located at a certain distance from the sensor, changes the magnetic field, under the influence of which an alternating current is induced in the winding. It changes the frequency in accordance with the rotational speed of the wheel and the number of teeth, i.e. this frequency is proportional to the rotational speed of the wheel.
Removal and installation of sensors is carried out as follows:
- turn off the ignition;
- turn the wheel;
- in an accessible place, "cut" the wire of the sensor being removed and connect the wire of the new one to it. If you have no experience in connecting electrical wires, careful insulation, corrosion protection, etc., it is better to entrust the work to specialists.
ABS system components are directly or indirectly affected when replacing brake calipers, pads, hoses, brake discs, cables and parking brake system components.