Hydraulic block
It is located in the engine compartment on the left rear: the hydraulic unit is located between the brake lines from the master brake cylinder and the brake lines to the wheel brakes. In accordance with the commands of the electronic control unit, the pressure in the brake system is maintained constant, reduced or created again. However, this pressure cannot be higher than what is created in the brake master cylinder after the driver presses the brake pedal. Pressure control is handled by four quick acting solenoid operated valves, one per wheel.
If the valves are not energized, the pressure increases. At maximum power supply, the pressure decreases, at medium power supply, the pressure is maintained constant. The pressure reduction phase is especially interesting: since you cannot let the brake fluid coming from the brake master cylinder just go into the reservoir (otherwise, as a result of this, the brake pedal would simply fail), a powerful return pump pumps the brake fluid back to the brake master cylinder. You notice this process in the brake pedal, which begins to pulsate slightly if the return pump is working, that is, the ABS comes into action. If you listen closely, you can hear the noise of the pump. Noise isolation in the hydraulic block slightly muffles the sounds of the pump. Along with the plug connection, two more relays are located at the top of the hydraulic unit. The larger one supplies power to the return pump; a smaller relay controls the solenoid valves.
hydraulic block (1) The anti-lock braking system is located in the engine compartment at the front left. If you remove the protective cover, you can see the relays of the solenoid valves (2) and return pump relay (3).
Left: anti-lock braking system control unit (arrow) located on the left under the rear seat.
Right: similar speed sensors (arrow) are located in a vehicle equipped with an anti-lock braking system on all four wheels.
Speed sensors
The speed of each individual wheel is determined by the speed sensor (there are four of them in total) and transmits this information to the control unit. For its part, the control unit gives a command to the hydraulic unit. The speed sensors themselves consist of a magnetic core and a coil and are installed at a short distance from the toothed washer - the rotor. The rotor rotates along with the wheel, which causes the jagged bulges around its perimeter to pass near the sensor faster or slower depending on the speed. Each tooth running under the sensor causes a short-term increase in voltage in it. Thus, an electrical impulse is created in the sensor, which changes its frequency in accordance with the speed of the wheel. This signal is processed in the control unit as speed information.
Electronic control unit
The control unit is located on the left under the rear seat bench and processes the incoming signals from the speed sensors. At the same time, it controls the hydraulic unit so that the wheels do not block. In addition to complex signal processing, the control unit that follows this solution element also contains a blocking circuit. With its help, the device can check itself, the control light on the dashboard lights up.
Relay
Three relays are involved in the operation of the ABS:
The solenoid valve relay is located under a cover on the hydraulic unit.
The return pump relay is also located under a cover on the hydraulic unit.
The ABS step relay and the surge protection relay are assembled into a combination relay.
Location: on plug-in field 4 additional relay carrier II (chapter «Body electrical system»). Since 7/92, the function of this relay is performed by the ABS control unit.
Main switch
On models prior to 6/92, the ABS master switch located on the switch panel can be used to disable the ABS in the instrument panel so that some reduction in braking distance can be achieved in extreme road conditions. So, for example, if gravel is scattered on the road or ice is covered with snow, when braking, a wedge of gravel or snow is created in front of the tires, which additionally helps braking.
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