Construction and description of the brake system

Hydraulic elements of brake and auxiliary systems BAS and ESP

Diagram of the hydraulic circuit of the anti-skid system ESP

Electrical controls for brake and auxiliary systems BAS and ESP

The design of the capacitive sensor of rotation and acceleration of the body of the ESP system

Location of the sensor of rotation and acceleration of the body of the anti-skid system ESP

The functioning of the capacitive sensor of the steepness of the turn and acceleration of the body of the ESP system



The hydraulic brake system consists of a master cylinder, a brake booster, an ABS unit and front and rear disc brakes. The hydraulic braking system is divided into two circuits. One circuit acts on the front brakes, the second circuit - on the rear wheels. If one of the circuits fails, for example, due to fluid leakage, the vehicle is braked by another circuit. Fluid pressure in both circuits is created by a dual master brake cylinder acting from the brake pedal.


The brake fluid reservoir is located in the engine compartment, on the driver's side under a cover above the brake master cylinder. It supplies brake fluid to the entire system. The volume of fluid in the tank must be constantly monitored.

Description of the operation of the ABS anti-lock brakes, BAS brake booster and electronic stability program (anti-skid system) given in the section «Controls and techniques for safe operation».

The front brakes have a floating caliper. This design requires only one piston to drive both brake pads. The rear brakes have a fixed caliper.

The brake booster stores part of the vacuum created in the engine intake pipe. Since a diesel engine does not have the necessary suction vacuum, diesel vehicles have a special vacuum pump mounted in front of the cylinder head and driven by the camshaft.

With the help of an appropriate valve, the force from the brake pedal is increased by the vacuum.

The foot parking brake acts through cables on the brakes of the rear wheels. The rear wheels are additionally equipped with drum brakes integrated into the disc brakes. The drum brakes are actuated by the parking brake pedal only. The brake pads on the rear wheels are set automatically, requiring only rare adjustments to the parking brake, such as after a repair.

Features of auxiliary systems ABS, ESP and BAS

Function of the hydraulic circuit of the auxiliary brake systems

As part of the hydraulic modulator of auxiliary brake systems (A7/3) includes components of closed-loop dynamic control systems of ABS, ASR and ESP systems.

Pressure and return pump (A7/3m1)

Self-priming pressure and return pumps (p1, p2) built into the hydraulic modulator assembly (A7/3) and are switched by pulse signals during the pressurization and depressurization phases of the active control system ASR and ESP, as well as during the regulation of the return flow when ABS is activated.

Solenoid valves for maintaining and relieving pressure (A7/3y6-y13)

One 2/2-way valve is used to control the pressure in the circuits of each of the wheels in the injection / hold and hold / reset phases of the ABS, ASR and ESP control modes.

Low pressure tank (11)

Low pressure tank (11) is filled with brake fluid in the ABS, ASR or ESP depressurization phase and ensures its transfer to the delivery and return pump (p1/p2).

Circuit control solenoid valves (A7/3y18 and y19)

Solenoid valves-switches (y18 and y19) provide a cut-off of the active pressure circuits of the front and rear axles from the GTZ during the operation of the ASR and ESP. Also, the valves provide pressure relief when it rises above 150 atm. The brake fluid passed through the switch valves is sent back to the GTZ.

Suction solenoid valves (A7/3y22 and y23)

intake solenoid valves (y22, y23) open in the pressure build-up phases ASR/ESP.

Brake pressure sensors (B34, B34/1, B34/2)

ESP Mk20 (models before 7/01) equipped with two brake pressure sensors. Sensor 1 (B34/1) monitors the pressure in the front brake circuit, sensor 2 (B34/2) - in the back.

In ESP Mk25 (models from 8/01) only one sensor is used (B34), which monitors the pressure in the forward circuit.

The information produced by the sensors is transmitted to the control unit and used in calculating the parameters of the closed control loop.

Noise reduction

Self-starting pressure and return pumps (p1, p2) run as needed to minimize noise levels.

Various damping components (13, 14) provide further noise cancellation. Each brake circuit is equipped with a separate damper (12), to reduce the noise generated by the pump.

Slope of turn and body acceleration sensor

Sensors that monitor angular velocity and lateral g-forces are combined into an assembly of steepness of turn and lateral g-forces (B24/5), which saves the space they take up. The micromechanical sensor assembly converts the lateral and vertical projections of angular accelerations into electrical signals. Elements of various masses under the influence arising during the non-inertial movement of the vehicle (turning and accelerating) overloads provide varying degrees of deformation. A special electronic converter converts the received signals and transmits them via the CAN bus to the control unit of the anti-slip and anti-skid systems (N47).

The principle of operation of the sensor

The sensitive element of the sensor is formed from a micromechanical ring (A), equipped with eight spring jumpers (b), providing its movement and electromagnetic action. During rotation in the assembly, additional Coriolis forces arise, proportional to the rotation speed, fixed electromagnetically and after conversion in the ACIS module into the form of analog signals, issued to the dashboard control unit.

The principle of measuring transverse overloads is based on the use of a spring-mass element with a capacitive detector. The operating voltage is supplied from the ESP control unit (N47-5).

The transverse loads arising during the making of turns provide a displacement of the spring-mass element from the equilibrium position by an amount proportional to the value of the resulting overload. Any change in the position of the element results in a change in the capacitance of the detector. Further, the recorded deviation is converted into a signal voltage, which allows the ESP control unit to (N47-5) to quantify the values of fixed transverse overloads.

The electronic control unit ensures that the system, in the presence of damage (e.g. cable break), or when the voltage drops, it turns off on its own. The situation is indicated on the instrument panel by the orange ABS warning lamp. At the same time, the ESP and BAS systems are turned off at the same time, which is indicated by the ignition of the ESP indicator. The main brake system retains its performance. During braking, the car behaves as if there is no ABS system.

If, for example, the ESP warning lamp lights up while driving, this indicates a malfunction of the brake booster or anti-skid system. BAS and ESP are disabled. The normal system, however, remains operational.

If the red warning light comes on while driving (symbol: call sign) braking system, you must immediately stop and find out the cause. Causes may be insufficient brake fluid or a cocked parking brake.

If the ABS warning light comes on while driving:

1. Stop the car, turn off the engine and start it again.

2. Check battery voltage. If the voltage is less than 10.5V, charge the battery.

Attention! If the ABS warning lamp comes on at the beginning of the movement and then goes out after a while, this indicates a low battery voltage, which rises after the generator starts.

3. Check if the battery terminals are securely fastened.

4. Put the car on stands, remove the wheels and check the electrical wires going to the wheel speed sensors for external damage. Other checks must be carried out in a workshop. Electronics has self-diagnostics, existing faults are automatically registered by the system. Checking records and troubleshooting are carried out at the service station.

Attention! Disconnect the ABS connector before performing electric welding. The connection is located in the engine compartment, at the top, on the driver's side, behind the removable cover. The connection is undocked only when the ignition is switched off. When performing paintwork, it is not allowed to heat the control unit to a temperature above +90°C.

When cleaning the brake system, dust is released that can be harmful to human health, so brake dust must not be inhaled.

Working with the brake system requires special cleanliness and strict adherence to instructions. In the absence of the necessary experience, it is advisable to contact the service station.

Note. When driving on wet roads, periodically press the brake pedal to remove moisture from the brake discs.

During the rotation of the wheel, moisture is discharged from the brake discs under the action of centrifugal force, but a silicone film, rubber abrasion products, grease and other contaminants remain, reducing the effectiveness of the brakes.

After installing new brake pads, the latter should run in. Therefore, the first 200 km of the run should not be unnecessarily heavily braked.

Corroded disc brakes create a shaking effect when braking that does not disappear with time. In this case, the brake discs must be replaced.

Dirt sticking to the surface of the brake pads and rain grooves lead to the formation of grooves on the surface of the brake discs, which leads to a decrease in braking efficiency.