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9.1.2. Brake fluid

Brake fluid — light yellow color, is poisonous, aggressive to a paint and varnish covering, is neutral to metal and rubber details. Keeps liquid state even at –40 °C and has very high temperature of boiling — about +260 °C.
Lack of brake fluid — its hygroscopicity. Water can get into brake fluid, for example with damp air, – through a tank or microscopic openings in brake hoses and rubber cuffs. Content in brake fluid of water not only leads to corrosion of metal details of system, but also significantly influences decrease in a boiling point. Already at the 2,5% content of water temperature of boiling decreases to +150 °C. At strong load of brakes it can be dangerous because brake mechanisms very strongly heat up. Near the heated brakes in liquid vials of steam which reduce pressure in system can be formed and can lead to the fact that the brake pedal when pressing will "fail". In such cases it is necessary to press several times quickly a brake pedal for increase in pressure in a hydraulic system.
The effect of steam formation at the parking of the car after sharp braking is especially dangerous. In the absence of cooling from the running air stream working knots of the brake system heat up even stronger, the maximum temperature remains within 15 min. after a stop, and only half an hour later brake fluid cools down up to the normal temperature.

Fig. 8.1. Scheme of separate elements of the ABS system: 1 – control unit; 2 – the brake mechanism of a forward wheel with the sensor of frequency of turns; 3 – hydraulic block; 4 – the main brake cylinder with the amplifier; 5 – mechanism of the parking brake; 6 – the brake mechanism of a back wheel with the sensor of frequency of turns

In respect of scheduled works replacement of brake fluid is provided every two years (the minimum requirement according to the DOT 4 specification). At the same time it is necessary to use only brake fluid recommended for your model by the producer of the car.

Fig. 8.2. Installation of sensors of frequency of rotation on forward wheels: 1 – brake disk; 2 – nave of a forward wheel; 3 – guard; 4 – a bolt with an internal hexagon, 25 N · m; 5 – rotation frequency sensor; 6 – rotary fist

Fig. 8.3. Measurement of frequency of rotation of a wheel: 1 – rotation frequency sensor; 2 – coil; 3 – magnetic core; 4 – gear rotor; 5 – coaxial wire

 Anti-blocking system (ABS)
ABS control unit (fig. 8.1).
Constantly processes information arriving from wheel sensors of frequency of rotation and compares it to the programmed values. If the difference of values indicates danger of blocking when braking one or several wheels, the control unit activates the hydraulic block which reduces working pressure in the brake system of the corresponding wheel until it does not receive a free wheeling and will not be applied brakes partially again. Depending on a condition of a roadbed this cycle during one braking passes for only a few milliseconds.
Hydraulic block.
It is the main knot of the ABS system, it is established in a forward left part of a motor compartment. Turns on the electric pump and the valvate block with magnetic valves. When pressing a brake pedal the main brake cylinder under pressure gives brake fluid via the valvate block to wheels. At the same time the valvate block regulates pressure in each of the hydraulic actuators of system uniting a forward wheel and the back wheel located from it on diagonal. At inclusion the ABS control unit sends team in the form of an electric signal: "To reduce pressure of working liquid". Brake fluid in this case on a small circle via the valvate block comes back to a tank. Operation of the pump can be noticed on an easy pulsation of a pedal of a brake.
Wheel sensors of frequency of rotation.
Each sensor 5 (fig. 8.2) is installed on the wheel, at small distance from the gear disk (a rotor of impulses) fixed directly on a nave.
Gear ledges of a disk 4 (fig. 8.3) depending on the frequency of rotation of a wheel (speed) quicker or more slowly pass by the magnetic core 3 of the sensor 1. Each tooth of a disk when passing induces short increase in tension in the coil 2. Its frequency depends on the frequency of rotation of a rotor and, respectively, a wheel. On a coaxial wire the 5th electric signal comes to the control unit.
Thus, information on the frequency of rotation of a wheel is transferred in the form of an electric signal to the control unit.

 Anti-blocking system
The corresponding relays are responsible for operation of magnetic valves and the electric pump. They are established on the holder under the ABS control unit.
Violations in work of the ABS system.
The control lamp of the ABS system lights up at inclusion of ignition and dies away if the engine works not later than through 2 pages. If the lamp continues to burn, so in the ABS system there is a malfunction. Nevertheless the movement can be continued — brakes will function, as on the car without ABS. For elimination of malfunction it is necessary to address to the service center Mercedes where will diagnose the system control unit by means of the special device.