199002179 - INTRODUCTION - ANTI-LOCK BRAKING SYSTEM (A.B.S.)

CONSTRUCTION SPECIFICATIONS

The vehicle is equipped with an ABS/EBD brake control system.The ABS/EBD system is the Bosch 8.0 type, developed from the ABS system already used on other vehicles.The ABS is fitted parallel to the hydraulic braking system so that should it not work braking would still be guaranteed.The ABS includes the EBD function (Electronic Brake force Distribution), the electronic adjustment and distribution of the braking force between the axles. There is therefore no mechanical device on the braking system for distributing the brake force between the two axles.VIEW OF ASSEMBLY1 - ABS/EBD control unit2 - Left front wheel sensor3 - Right front wheel sensor4 - Right rear wheel sensor5 - Left rear wheel sensor6 - EBD warning light7 - ABS warning light

SPECIFICATIONS

The Bosch 8.0 ABS control unit is connected to the C-CAN and is known as the BSM (Brake System Module) and comes in two versions:- With EBD- With EBD and ESP (Electronic Stability Program) which includes the ASR/MSR/HBA/HHC functions.The ABS control unit is connected to the front wiring, the system cable loom is incorporated with the engine compartment cable loom.The control unit receives and transmits information on the C-CAN

COMPOSITION

Structure

The Bosch 8.0 A.B.S. system consists of:- an electronic control unit integral with the hydraulic control unit- an electrohydraulic control unit that modulates braking pressure by means of eight solenoids, two for each wheel;- four ACTIVE MAGNETORESISTIVE sensors that record wheel angular speed of rotation;- wiring with specific connector

OPERATION

Introduction

The electronic control unit processes signals from the active sensors and brake light control switch. It identifies the wheel or wheels with a tendency to lock (maximum slip between wheel and road surface) by means of the control unit software and modulates the brake fluid pressure selectiely for the front wheels and in tandem for the rear wheels (select-low function).The ABS modulates brake pressure in accordance with three basic stages:- 1st pressure maintenance stage;- 2nd pressure reduction stage;- 3rd pressure increase stage.If operation is requested, the ABS stays active at speeds greater than 2.7 km/h. After that, it cuts out to allow the car to stop.After the key ON and when a speed of 6 km/h is exceeded, the control unit carries out an operation check on the solenoids and pump engine. The control unit also tests the sensors when a speed of 12 km/h is exceeded.The ABS is detected as a component of the C-CAN network by the body computer at the first key ON.

Operating strategies

Pressure increase stage without intervention of ABSWhen the brake pedal is pressed, the electronic control unit (1):- does not supply load solenoid (N.A.) (2)- does not supply discharge solenoid (N.C.) (3)The pressure generated by brake pump (4) therefore reaches brake calipers (5) without undergoing changes.Example of ABS interventionPressure maintenance stageControl unit (1):- supplies load solenoid (N.A.) (2)- does not supply discharge solenoid (N.C.).The hydraulic connection between brake pump (4) and brake caliper (5) is therefore broken.The pressure in brake caliper (5) remains constant even when pressure on the brake pedal is increased.Pressure reduction stageThe control unit:- supplies load solenoid (N.A.) (2)- supplies discharge solenoid (N.C.) (3).The hydraulic connection between brake pump (4) and brake caliper (5) is broken and discharge solenoid (3) opens and brings brake caliper (5) into communication with low pressure accumulator (6) and scavenger pump (7).Control unit (1) also supplies motor (8) driving scavenger pump (7) to return fluid taken from brake caliper (5) to the main circuit.The fluid flows through high pressure accumulator (9) and restriction (10) that performs a damping function.The system is equipped with a check valve (11) fitted in parallel with load solenoid (2) that allows fast reduction of pressure in brake caliper (5) during brake pedal release.Pressure increase stageControl unit (1):- does not supply load solenoid (N.A.) (2)- does not supply discharge solenoid (N.C.) (3)The fluid taken into the main circuit is then free to return to brake caliper (5) and increase the pressure acting on the brake caliper.EBD function (electronic brake force distribution)The EBD function controls brakeforce distribution and thus replaces the conventional mechanical load proportioning valve allowing:- independent action on the rear brake callipers- improved brake force distribution- optimal intervention under all load conditions (static or dynamic), driving conditions (straight line or corners) or car condition (tyres, brakes and suspension)- implementation of a strategy that follows the ideal distribution curve.Fa - Front axle brake forceFp - Rear axle brake forceA - Distribution curve implemented by the brake systemB - Ideal distribution curveC - Distribution curve implemented by a conventional hydraulic load proportioning valveD - Distribution curve implemented by the EBD functionEBD function failure is indicated by simultaneous activation of:- ABS warning light- low brake flud and/or handbrake on warning light.Under these conditions, the car must be driven with extreme caution to the nearest authorised workshop to check the system.RecoveryThe electronic control unit comes with a safety circuit that supervises ABS efficiency.With key on, the safety circuit carries out an initial autotest for 4 seconds to check:- electronic control unit operation- activates the solenoids to check their operation- controls the C-CANEvery time the car starts off from a standstill, the safety circuit checks for signals from active sensors.When the car is in motion, the safety circuit works as follows:- continually compares angular wheel speed with the calculated reference speed- checks memory condition- checks operation of the relays, the solenoids and the pump motor- constantly monitors battery voltage- checks C-CAN efficiencyIf the safety circuit detects one or more faults in system components during the check stage, the safety circuit operates as follows:- deactivates the ABS while still guaranteeing operation of the conventional braking system- indicates the fault status to the driver by turning on a warning light on the control panel.