When working on vehicles with an ABS system, 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 branded workshop, it 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, firmly tighten the battery terminals.
During all work on the brake system, the ABS system must be checked as follows:
- start the engine and check that the yellow lamp in the instrument cluster with the ABS designation goes out and no longer lights up at a speed of 5-7 km / h;
- the system starts to operate from the moment when the speed of 5-7 km/h is reached. All brake applications are adjusted from the moment the so-called blocking zone is reached from a speed of 8 km/h.
The system works as follows. According to the wheel speed measured by the sensor, decelerations and accelerations are determined, the signals of which are transmitted to the electronic control unit. Signals from individual wheels are formed into a relative signal that determines the approximate speed of movement. By comparing the wheel speed with a relative value, slip signals are generated.
If the wheel has a tendency to lock due to excessive pressure in the brake caliper (a condition that occurs when the corresponding wheel slips), the pressure level in this caliper is kept at the same level, i.e. it becomes impossible to increase it. If the blocking continues, i.e. the pressure is already too high, the system opens the solenoid bypass valve to reduce the pressure. The excess fluid is fed back to the brake cylinder by a return circuit pump. If the pressure drops enough that the wheel accelerates again, the valve closes and the pressure remains stable again. If the wheel acceleration exceeds a threshold value determined for the given driving conditions, the pressure in the brake system, on the contrary, is increased via the intake solenoid valve. Thus, receiving appropriate signals from the control unit, the hydraulic regulator can control the pressure in the system, reducing, increasing or holding it at a constant level.
Now a few words about the electronic component of ABS.
The speed sensors are the so-called pulse sensors. The vehicle has four sensors, one for each wheel.
Two front sensors are installed in the steering knuckle, the rear sensors are in the axle shaft.
The rotational speed is taken by sensors using gear rims. In front they are built into the hubs, behind - in the axle shaft. The sensor consists of a magnetic core with a winding. The rotation of the ring gear, 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 according to the speed of the wheel and the number of teeth, i.e. this frequency is proportional to the wheel speed.
Removal and installation of sensors is carried out as follows:
- turn off the ignition;
- unscrew the wheel;
- in an accessible place «have a bite to eat» the wire of the removed sensor and connect the wire of the new one to it. If you do not have experience in electrical wiring, careful insulation, corrosion protection, etc., it is better to entrust the work to specialists.
Elements of the ABS system are directly or indirectly affected when replacing brake calipers, pads, hoses, brake discs, cables and parts of the parking brake system.