The ABS hydraulic unit is located in the engine compartment between the pipelines of the master brake cylinder and the wheel brakes (see illustration 12.0).
In accordance with the commands given by the ABS electronic control unit, the pressure in the wheel brakes is maintained constant, reduced or increased again. However, this pressure cannot be higher than that which appears in the main brake cylinder as a result of the impact on the brake pedal. Solenoid valves are responsible for adjusting the pressure - one for each wheel brake mechanism. When an electric current is supplied to them with a maximum voltage, they reduce the pressure, and at a minimum, they increase. Due to the fact that the pressure in the system that appears when the brake pedal is pressed cannot be removed, a special evacuation pump is installed to reduce it, which diverts part of the brake fluid back into the master brake cylinder. By pulsing the brake pedal, you can determine the functioning of the ABS. Above the ABS hydraulic unit are two relays. One, larger, supplies power to the evacuation pump, and the smaller one «answers» for solenoid valves.
An ABS sensor is installed on each wheel, which takes into account the number of wheel revolutions and sends this information to the control unit (see illustration 12.0a).
Based on it, the control unit controls the ABS hydraulics. ABS sensors are a core with a coil. Sensors receive information about the number of revolutions of the wheel by induction from the gear on the wheel hub.
ABS control unit on vehicles with a body «sedan» located under the left rear seat (see illustration 12.0b), for Avant cars - on the left side of the trunk behind the trim (see illustration 12.0v).
The ABS electronic unit, along with hydraulic control, has a self-diagnostic function. If a malfunction is detected, the electronic unit disables the ABS and a warning light on the instrument panel lights up.
Cars manufactured before 7/92 have an ABS switch on the instrument panel. With it, when driving on icy or gravel roads, the ABS can be disabled to provide a shorter braking distance.
ABS malfunction
When the ABS is turned off, the corresponding warning light on the instrument panel lights up. If the car's engine is turned off, then when it is started, the ABS, despite the shutdown, turns on automatically.
The warning light on the instrument panel comes on when the ignition is switched on. The lamp should go out immediately after the engine starts or with the start of movement, the speed of which is more than 6 km / h. If the on-board network voltage drops below 10 V, the warning light also comes on. The warning light may also come on if one of the wheels rotates faster than the others within 20 seconds. In this case, you must turn off the ignition and turn it on again.
If the warning light stays on, the ABS is not working. The cause of the ABS failure must be determined by a specialist workshop. ABS-related work may not be performed by all workshops.
The motorist can independently check the plug connections of the ABS control unit with wheel speed sensors or with the ABS hydraulic unit.
Attention! If the ABS warning light on the instrument panel is constantly on, regardless of whether the ABS is intentionally disabled or if it is out of order, then you can safely continue driving. In this case, the brakes function like vehicles without ABS.
Electronic differential lock
As an option, the AUDI 80 can be equipped with an electronic differential lock in combination with ABS. Using ABS, EBD improves traction under adverse driving conditions, for example, when one of the front wheels is spinning when starting off. This situation is registered by the wheel speed sensor and transmitted to the ABS and EBD control unit, which actuates the disc brake mechanism of this wheel. The wheel disc brake mechanism is driven through a pair of additional solenoid valves on the ABS hydraulic unit. The ABS opens the accumulator, applying pressure to the wheel cylinder and braking the wheel until both wheels have equal grip. As soon as this moment is reached, the control unit closes the solenoid valves and stops braking the wheel.
The electronic differential lock also has a self-diagnostic function, preventing erroneous operation of the front brakes. When the vehicle speed exceeds 40 km/h, the EBD is switched off. The same happens if the control unit detects a malfunction of the EBD. The ABS continues to work. If one of the wheel speed sensors fails or there are malfunctions in the electronic unit, then the EBD and ABS are disabled. The functioning of the brake system of the AUDI 80 is retained.
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