244000427 - ANTI-LOCK BRAKES (ABS BOSCH 5.3)

COMPOSITION

The Bosch 5.3 ABS is extremely compact (easy to fit), light and reliable.The use of new microhybrid type electronic components, the improvement of the magnetic flows achieved through the new design compact shape of the valve bodies and the reduction in the number of components as a result of pressing the jets directly in the valve seat has allowed the improvements in the solenoid valve modulating specifications.In addition to the anti-lock function, the system controls the distribution of the brake force between the front and rear axles via the EBD function (Electronic Brake-force Distribution) allowing the traditional load proportioning valve on hydraulic braking systems to be dispensed with. In addition to the ABS and EBD systems, some versions may be equipped with an ASR function (ANTI-SLIP REGULATOR).The main system components are:

electronic control unit;
an electro-hydraulic control unit that modulates the braking pressure at the brake calipers by means of eight solenoid valves, two for each wheel;
four sensors (5) and (8), one for each wheel, that detect the rotation speed of the actual wheels.

Diagram showing Bosch 5.3 ABS anti-lock brakes

The following complete the system:

the hydraulic system pipes;
specific electrical wiring;
a switch on the brake pedal for detecting the braking conditions;
a warning light (7) in the control panel.

COMPONENTS

Electro-hydraulic unit

The electro-hydraulic unit consists of two sections fastened to one another: an electronic type control unit and an electro-hydraulic type one.The electronic control unit is connected to the ABS wiring by means of a multiple connector with 31 terminals.According to the signals coming from the sensors and, with the assistance of programmes stored in its memories, the electronic control unit controls the electro-hydraulic control unit.The electro-hydraulic control unit is connected to the brake pump and to the ABS components by means of the braking system pipes.

Electronic control unit

The electronic control unit consists of hybrid circuits with resistances, diodes, transistors and integrated logic circuits. The core of the system consists of two CMOS microprocessors with 16 K of ROM which run the same programme independently with feed back. Both receive the same input signals that they each process individually and only when the results obtained are identical does the electronic control unit give the operational command to the electro-hydraulic control unit.If this is not the case, for example, if there is a problem in the anti-lock braking system, the device switches itself off and braking takes place conventionally; the failure warning light in the control panel comes on at the same time.The information relating to the fault is stored in a non volatile memory; in effect, one of the two microprocessors has a CMOS EEPROM whose contents are preserved even if the battery voltage fails. The task of the this memory is to preserve the fault codes so that they can be read in a service situation using the diagnostic equipment.

The (alternating or analogue type) signals sent by the rpm sensors to the electronic control unit are transformed into square wave (or digital type) signals by the inlet amplifier.The frequency of these signals provides the control unit with the corresponding speed, acceleration or deceleration values for the individual wheels.A reference speed is processed from the combination of the individual wheel peripheral speeds and it is continuously updated thereby indicating the speed of the vehicle at any given moment.When the driver press the brake pedal, the wheels can decelerate at different rates: comparing the peripheral speeds of the individual wheels with the reference speed allows a constant check on the slipping of each individual wheel.If the braking force causes one wheel to slip in relation to the others, the electronic control unit gives the command to the electro-hydraulic control unit solenoid valves to reduce the braking force at the wheel where there is a loss of grip. The wheel concerned therefore picks up speed.

The peripheral speed of a braking wheel decreases to a greater extent than that of the vehicle, in extreme conditions with the wheels completely locked through the braking action (peripheral wheel speed = 0) and the vehicle still moving, the difference between these two speeds is at its greatest.This difference is known as creeping or the slipping coefficient when it is expressed as a percentage.0% slip = free wheel.100% slip = wheel locked and vehicle moving.During braking the friction coefficient increases when braking takes place with limited slipping (rolling) and then decreases when the slipping of the tyre is accentuated until it locks.By means of a considerable number of pratical tests and experiments it is generally possible to produce the maximum braking force with slipping values of between 5% and 15% with a maximum figure of 20%.This defines an optimum sector which the anti-lock brakes tend to return to on any type of vehicle.

The electronic control unit also stores the acceleration and deceleration levels that should never be exceeded for each individual wheel.The rolling of the braking tyre is therefore kept in check by means of a systematic, continuous and extremely rapid comparison of the slipping, deceleration and acceleration figures for the wheel.As soon as the combined slip and acceleration/deceleration levels are exceeded, the electronic control unit intervenes operating the electro-hydraulic control unit solenoid valves in the three regulation stages decreasing, maintaining or increasing the pressure produced by the driver on the brake pedal at the brake calipers, restoring the braking condition to the optimum system values.These stages determine an intermittent, but extremely rapid regulation cycle that is repeated until the vehicle stops. The electronic control unit operates the different stages, providing the solenoid valves with impulses of different current intensity. It also ensures that both rear wheels are provided with the same braking force applied to the rear wheel most likely to lock, i.e. the one with less grip (to ensure the best trajectory stability).In the case of a wheel where the tyre is limp, the ABS intervenes, if necessary, controlling the braking conditions.The ABS is also activated when braking whilst reversing.Usually the intervention of the device stops at speeds below 2.75 km/h to allow the wheels to lock fully with the vehicle stationary.

Since the parameters that the control unit checks (wheel speed and acceleration) are affected by the inertia of the wheel/tyre assembly, vehicles equipped with anti-lock brakes should only be fitted with the wheel rims, tyres and brake pads recommended by the Manufacturer.

If chains are fitted, the rolling condition gives rise to signals that, suitably filtered in the control unit, ensure that the anti-lock brakes are not switched off when driving over hard, compact snow.In limited grip conditions and/or incorrect distribution of the drive torque (acquaplaning), the electronic control unit is informed, by the rpm sensors fitted on each wheel, of the irregular conditions whilst driving because the drive wheels tend to rotate at a different speed from the driven ones.This condition leads to an electronic control unit adjustment cycle that does not meet the requirements; the anti-lock brakes are temporarily switched off (without the warning light necessarily coming on as it is so brief) and are then switched back on when the reduced grip conditions are over.The electronic control unit has a safety circuit which has the task of supervising the efficiency of the braking system before setting off and whilst driving.The safety circuit carries out the following self- checks:

1 after inserting the ignition key and for around 4 seconds, it checks the operation of the control unit, the relays that operate the solenoid valves and the connection of the sensors;
2 after the engine has been started, as soon as 6 km/h is exceeded, it operates the solenoid valves and the recovery pump for an operational check; it also checks for the presence of the 4 speed signals;
3 each time 24 km/h is exceeded, starting with the vehicle stationary, it checks for the presence of the 4 speed signals;
4 whilst driving it constantly compares the peripheral speed of the wheels with the reference speed calculated, checks the memory conditions and oversees the operation of the two relays;
5 whilst driving it constantly checks the battery voltage.

If one of these problems is detected, the anti-locking braking system guarantees the normal operation of the conventional braking system and switches itself off notifying the driver by lighting up the failure warning light in the instrument panel.The electronic control unit is informed that the driver is braking by the signal arriving from the switch on the brake pedal. This information is useful both for controlling the braking and, especially in certain conditions, such as for example, during sharp acceleration that causes the wheels to slip or in the case of uneven road surfaces (cobblestones) that can lead to variations in the speed of the wheels as a result of matters unrelated to the braking in progress.Under these circumstances the microprocessors process a strategy linked to the variations in the speed of the wheels at these times ensuring that the braking in progress is within the correct parameters. Since there are specific braking control conditions, a failed connection of the brake pedal switch to the control unit does not adversely affect the efficiency of the system. For this reason it is not signalled by the warning light coming on and the ABS is not disabled.

Electro-hydraulic control unit

The electro-hydraulic control unit is connected to the brake pump and to the brake caliper cylinders by means of the braking system pipes and together with the electronic control unitmakes up the electro-hydraulic unit.Its task is to vary the pressure of the brake fluid in the brake caliper cylinders corresponding to the control signals cming from the electronic control unit.It consists of eight two-way solenoid valves (two for each hydraulic circuit) and a dual circuit recovery pump (2).The eight solenoid valves and the recovery pump are operated by the electronic control unit according to the signals for the four rpm sensors. In particular, the pump allows the recovery of the brake fluid during the pressure reduction stage making it available again upstream of the solenoid valves for the subsequent pressure increase stage.The accumulators make it possible to absorb brake fluid during the pressure reduction stage.The unit is connected to the braking system by means of connectors that can be identified by the codes illustrated.The electro-hydraulic control unit cannot be overhauled and should not be tampered with.It is supplied as spares filled with (DOT 3) brake fluid and with the solenoid valves not supplied.The operation of bleeding and refilling the braking system is the same as that for a traditional system.

To avoid errors when connecting the various braking system circuits during repair operations, the electro-hydraulic unit connections are different sizes (M10x1 and M12x1) and the connectors can also be identified by the codes illustrated.

Wheel rpm sensors

The rpm sensors provide the electronic control unit with all the information needed to operate the electro-hydraulic unit.They measure the driving speed, acceleration, deceleration and wheel slipping.The sensors are the inductive type and are fitted in special housings located in the front and rear wheel dampers.The magnetic flow lines close through the teeth of a flywheel facing the sensor rotated by the wheel. The passage from full to empty depending on whether or not a tooth is present causes a variation in the magnetic flow that is sufficient to produce an electro-motive force at the sensor terminals and consequently an alternating electrical signal at the electronic control unit.The internal sensor elements (coil and permanent magnet) are completely submerged in a protective resin and surrouneded by a plastic casing. A brass fitted on the sensor casing is designed to fasten the latter without distortions.

The recommended distance (gap) between the end of the sensor and the flywheel for obtaining correct signals should be:

0.64 - 1.30 mm for the front wheels;
0.25 - 1.15 mm for the rear wheels;

This distance is not adjustable. If the gap is found to be outside the tolerance limits, check the condition of the sensor and phonic wheel.The sensor resistance is equal to 1600 ± 100 Ohm

Front wheel rpm sensor position.

Each time an rpm sensor is fitted it has to be lubricated with

... DATA ERROR - CROPPED TEXT \| Ошибка данных - Текст обрезан ...