Audi A4

since 1994 release

Manual of the car

Audi A4
+ Running gear
+ Regular servicing
+ Engines
+ Turbo-supercharging
+ System of an exhaust
+ Cooling system
+ Fuel tank and fuel pump
+ Air filter and channels of absorption
+ System of injection
+ Coupling
+ Transmission and main transfer
+ Suspension bracket of wheels and steering
- Brakes
   Independent work on the brake system
   So the brake system works
   Check of brakes
   Brake fluid
   Check of level of brake fluid
   Check of the brake system on tightness and existence of damages
   Replacement of brake fluid
   Disk brakes
   Measurement of thickness of overlays of forward disk brakes
   Check of a condition of brake disks
   Replacement of overlays of disk brakes
   Back disk brakes
   Measurement of overlays of back disk brakes
   Emergency brake
   Check of idling of the lever of the emergency brake
   Main brake cylinder
   Brake amplifier
   Check of the brake amplifier
   Works on hydraulics of the brake system
   Pumping of the brake system
   The help at malfunctions
   ABS and EDS
   What does ABS?
   Function of separate knots
   Electronic system of distribution of brake effort (EBV)
   Violations in work of the ABS system
   Electronic blocking of differential (EDS)
+ Wheels and tires
+ Electrotechnical equipment
+ System of ignition
+ Lighting
+ Alarm equipment
+ Tools and devices
+ Heating and ventilation
+ Body details
+ Salon
Search of malfunctions
Technical characteristics


Function of separate knots

Hydraulic module

The hydraulic module (1) of system of anti-blocking is placed ahead at the left in a motive compartment. It is connected by the central socket (2) with the control unit under a back seat.

It is ahead at the left in a motive compartment: the module is built in between the brake pipelines of the main brake cylinder and brake tubes conducting to brakes of wheels. According to orders of an electronic control unit pressure in contours of the brake system remains or to constants, or goes down, or again increases. But pressure cannot be higher than that which you create, pressing a brake pedal. Pressure is regulated by four quickly switching electromagnetic valves – on one on each wheel.

If electromagnetic valves are cut off power, then pressure increases. If the maximum current arrives, then there is a pressure decline, at average current pressure remains at one level. The phase of removal of pressure is of special interest: as for pressure decline it is impossible just to produce the brake fluid (for this purpose a brake pedal would be necessary to wring out completely) arriving here from the main brake cylinder somewhere, brake fluid is pumped out the powerful return pump back to the main brake cylinder. You notice it on behavior of a pedal of a brake which begins to pulse slightly when the return pump works, i.e. ABS begins to work. If attentively to listen, then noise of the working pump is heard. This noise is suppressed with special dampers in the hydraulic module a little.

Speed sensors

Such sensors of speed (2) are installed on all four wheels in openings (1) of a suspension bracket of an axis. Behind them according to the frequency of rotation of wheels gear elastic washers rotate.

Only four sensors fix the speed of rotation of each wheel and transfer this information to an electronic control unit. On its basis the electronic control unit regulates operation of the hydraulic module.

Sensors of speed consist of the magnetic core and the coil and are mounted at small distance from a gear disk – a rotor. The rotor – its name speaks for itself – rotates together with a wheel and forces to rotate zubtsevidny cambers on its perimeter by the sensor quicker or more slowly depending on speed. Each tooth passing under the sensor induces short rise in tension in the sensor. In such a way in the sensor the alternating voltage which changes the frequency according to the speed of rotation of a wheel is created. The signal arising thus is processed by an electronic control unit as information on wheel speed.

In Audi A4 rotors are placed on two or on four (quattro) driving wheels on the external hinge of a power shaft and on naves of not driving back wheels.

Electronic control unit

The combined control unit (shooter) of system of anti-blocking and electronic blocking of differential is approximately in the middle of the car under a back seat.

The electronic control unit is under a back seat and processes information arriving from speed sensors.

At the same time it operates the hydraulic module so that wheels were not blocked. Along with processing of signals and the subsequent logical part, the control unit still has the scheme of blocking. By means of it the device can check, distinguish itself failures and control working tension. If malfunctions are fixed, it switches off ABS and the control bulb in the combined device lights up.