Hydraulic module
Hydraulic module (1) anti-lock system is located in the front left in the engine compartment. It is connected by a central connector (2) with control unit under the rear seat.
It is located at the front left in the engine compartment: the module is integrated between the brake lines of the master brake cylinder and the brake pipes leading to the wheel brakes. In accordance with the orders of the electronic control unit, the pressure in the circuits of the brake system remains either constant, or decreases, or increases again. But the pressure cannot be higher than what you create by pressing the brake pedal. Pressure is controlled by four quick-switching solenoid valves, one for each wheel.
If the solenoid valves are de-energized, the pressure rises. If the maximum current is supplied, then a decrease in pressure occurs, with an average current strength, the pressure remains at the same level. Of particular interest is the depressurization phase: as to reduce the pressure, you cannot simply release the brake fluid somewhere (To do this, the brake pedal would have to be fully depressed), coming here from the main brake cylinder, then the brake fluid is pumped out by a powerful reverse pump back to the main brake cylinder. You notice this by the behavior of the brake pedal, which begins to pulsate slightly when the return pump is working, i.e. the ABS starts to work. If you listen carefully, you can hear the sound of the pump running. Special dampers in the hydraulic module dampen this noise a bit.
Speed sensors
These speed sensors (2) mounted on all four wheels in holes (1) axle suspensions. Behind them, in accordance with the frequency of rotation of the wheels, toothed elastic washers rotate.
Only four sensors record the speed of rotation of each wheel and transmit this information to the electronic control unit. On its basis, the electronic control unit regulates the operation of the hydraulic module.
The speed sensors themselves consist of a magnetic core and a coil and are mounted at a short distance from the toothed disk - the rotor. The rotor - its name speaks for itself - rotates with the wheel and causes the tooth-like bulges around its perimeter to rotate past the sensor faster or slower depending on the speed. Each tooth passing under the sensor induces a short voltage rise in the sensor. In this way, an alternating voltage is created in the sensor, which changes its frequency in accordance with the speed of rotation of the wheel. The resulting signal is processed by the electronic control unit as information about the number of wheel revolutions.
In the Audi A4, the rotors are placed on two or four (quattro) drive wheels on the outer joint of the drive shaft and on the hubs of the non-driven rear wheels.
Electronic control unit
Combined control unit (arrow) anti-lock and electronic differential lock is located approximately in the middle of the car under the rear seat.
The electronic control unit is located under the rear seat and processes information from the speed sensors.
At the same time, it controls the hydraulic module so that the wheels do not block. In addition to signal processing and the subsequent logic part, the control unit also has a blocking circuit. With this, the device can check itself, recognize faults and monitor the operating voltage. If faults are detected, it switches off the ABS and the warning light in the combined instrument panel comes on.
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