14. Brake system

On cars the working brake system with the hydraulic drive is used.

The system works as follows:

   – the pedal of a brake is connected with the main brake cylinder;
   – when the pedal is pressed, liquid under pressure from the main brake cylinder on metal tubes and flexible hoses is transferred to lobbies and back brake mechanisms;
   – working brake cylinders represent hydraulic modulators which transform the liquid pressure created in the main brake cylinder in movement by an element of the brake mechanism;
   – forward and back pistons of a support of a brake move, and due to friction about brake disks and drums provide the required delay of the car;
   – at a brake pedal otpuskaniye, pressure in system decreases. Brake shoes and pistons come back to a neutral or starting position and braking stops.

At reduction of level of brake fluid, the liquid level sensor in the main brake cylinder includes a control lamp in combinations of devices. If the control lamp burns, first of all check that the parking brake is released as the lamp indicates the lowered level of liquid and the tightened parking brake.

Arrangement of elements of the brake system on the car

1 – brake tube;
2 – brake pedal;
3 – main brake cylinder;
4 – vacuum amplifier of brakes;
5 – forward disk brake mechanism;
6 – support;
7 – back drum brake mechanism (CBS);
8 – parking brake;
9 – cable of the parking brake;
10 – lever of the parking brake;
11 – tube of the highway of high pressure of the brake system;
12 – back disk brake mechanism (ABS)

Anti-blocking system of brakes (ABS)

The anti-blocking system of brakes is intended for an exception of blocking of wheels at intensive braking.

Main ABS functions:

   – reduction of a brake way at the expense of an exception of blocking of wheels at intensive braking;
   – improvements of stability of driving in the course of braking.

The anti-blocking system of brakes works at information basis from four sensors transferred to the ABS block. The system operates each wheel separately and reduces liquid pressure in that wheel brake mechanism which wheel begins to be blocked.

Arrangement of elements of anti-blocking system of brakes (ABS) on the car

1 – block of hydraulic management (HCU);
2 – rotor of the sensor of frequency of rotation of a wheel;
3 – sensor of frequency of rotation of a wheel;
4 – association pipelines

Under normal conditions, the standard brake system carries out braking until blocking of the wheel found by the ABS control unit. During usual braking inlet valves usually open, and final valves are usually closed.

When the ABS control unit finds blocking of a wheel, it operates each valve for increase or reduction of pressure.

The ABS control unit determines the frequency of rotation and norm of delay of each wheel on the basis of information from sensors of frequency of rotation of a wheel. When braking the frequency of rotation of a wheel decreases, and the ABS block defines a difference between the speed of the car and frequency of rotation of a wheel. If delay of some wheels exceeds the expected value, the ABS block defines the beginning of blocking and opens the corresponding electromagnetic valves to reduce pressure of brake fluid.

At the same time the frequency of rotation of a wheel increases, and the electromagnetic valve of dumping of pressure is closed and there is an increase in pressure of brake fluid in the working brake cylinder of the corresponding wheel.

ABS control unit

The ABS control unit performs the following functions:

   – ABS operates;
   – calculates car speed;
   – carries out self-diagnostics of system: codes of malfunctions with self-diagnostic and safety functions are brought in memory by two separated microprocessors.

The ABS control unit continuously checks blocking of each wheel and operates each valve to regulate pressure of brake fluid if it is required. The ABS control unit also checks that components work properly, and switches off ABS, at detection of failures. Then the brake system works as the standard brake system.

Rotation frequency sensor

The sensor of frequency of rotation finds rotation of a wheel and consists of a permanent magnet and the coil. Sensors of forward wheels obtain information from the gear rotors located on power shafts.
When passing teeths of a rotor about the sensor, magnetic field of the sensor therefore tension impulse is developed changes. This tension is proportional to the frequency of rotation of a wheel and can be measured.

Control lamp of ABS

The control lamp of ABS located in a combination of devices lights up for 2 seconds after inclusion of ignition.

If the control lamp of ABS does not die away in 2 seconds after inclusion of ignition or lights up periodically, means in ABS there are violations.

In the presence of violations the brake system works as the standard brake system.

Block of hydraulic management (HCU)

The block of hydraulic management (HCU) consists of the pump with the electric motor and the block of valves to operate ABS pressure.

The sensor is located in the electric motor of the pump and transmits an output signal to the ABS block which defines operability of the pump.

Diameter of a forward opening of a pipe of supply of brake fluid:
Forward left and back right brakes: 12 mm
Forward right and back left brakes: 10 mm

The pump with two contours provides the required pressure, and the block of valves distributes pressure transferred to wheel brake mechanisms.

ABS self-diagnostics

The ABS control unit begins to perform self-diagnostic functions after inclusion of ignition. The ABS control unit finds malfunction for each contour, and a component, comparing a condition of system to a limited set of conditions in the ABS control unit. The ABS control unit stores a code of malfunction and then brings to the diagnostic unit in the form of the codes designated by four figures.

Check of codes of self-diagnostics

1. Turn a key in the ignition lock in position of OFF and connect the diagnostic unit to the diagnostic socket.
2. Turn a key in the ignition lock in position of ON and on the device set model of the car and model of the diagnostic unit.
3. Choose the checked parameter, after initialization of the block.
4. Press check of codes No. 1 and find out whether malfunction is written down.  


The malfunction code – number with four figures which appears continuously so far a code of malfunction is not erased by the diagnostic unit.

5. According to the list of codes of malfunction, reveal a faulty element and restore its working capacity.
6. After deleting of codes of malfunction from memory of the ABS control unit select inspection item No. 4.

Precautions before carrying out repair

1. For prevention of damage of elements of the brake system before carrying out repair or diagnostics satisfy the following conditions:

   – switch off ignition and disconnect each component of the brake system;
   – replace components which are not maintainable as knot;
   – when using electric welding disconnect each component of the brake system.

Brake fluid dissolves paint, plastic and isolation of wires therefore at casual hit of liquid on a paint and varnish covering of the car or its components, wash out their large number waters.