312000789 - CONSTRUCTION SPECIFICATIONS

CONSTRUCTION SPECIFICATIONS

The vehicle is equipped with a Bosch 8.1 ABS/EBD brake control system.The ABS is fitted parallel to the hydraulic braking system so that should it not work braking would still be guaranteed.The A.B.S. control unit is connected to the CAN through which it exchanges data with the Body Computer Node and the Engine Management Control Unit.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.
1. 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 lightDepending on the electrical system fitted on the vehicle, the Bosch 8.0 A.B.S. control unit is connected to the C-CAN line and is called the Brake Node.

COMPOSITION

Structure

The ABS consists of:

  • an electronic control unit integrated into the hydraulic control unit;
  • an electrohydraulic control unit that modulates braking pressure by means of eight solenoids, two for each wheel;
  • four ACTIVE 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 selectively 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 increase stage.
  • 2nd pressure maintenance stage;
  • 3rd pressure reduction 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 motor. 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 CAN by the Body Computer Node at the first key ON.

Operating strategies

Pressure increase stage without intervention of abs

When the brake pedal is pressed, the electronic control unit (1):

  • does not supply load solenoid (N.O.) (2)
  • does not supply the discharge solenoid (N.C.) (3).
The pressure generated by the brake pump (4) therefore reaches the brake callipers (5) without undergoing changes.

Intervention of the abs

Pressure maintenance stage

The electronic control unit (1):

  • supplies the load solenoid (N.A.) (2)
  • does not supply discharge solenoid (N.C.).
The hydraulic connection between brake pump (4) and brake calliper (5) is therefore broken.The pressure in the brake calliper (5) remains constant even when pressure on the brake pedal is increased.
Pressure reduction stage

The electronic control unit:

  • supplies the load solenoid (N.A.) (2)
  • does not supply the discharge solenoid (N.C.) (3).
The hydraulic connection between the brake pump (4) and the brake caliper (5) is broken and the discharge solenoid (3) opens and brings the brake caliper (5) into communication with the low pressure accumulator (6) and scavenger pump (7).The electronic control unit (1) also supplies the motor (8), driving the scavenger pump (7) to return fluid taken from the brake caliper (5) to the main circuit.The fluid flows through the high pressure accumulator (9) and the restriction (10) that performs a damping function.The system is equipped with a check valve (11) fitted in parallel with the load solenoid (2) that allows fast reduction of pressure in the brake caliper (5) during brake pedal release.
Pressure increase stage

The electronic control unit (1):

  • does not supply load solenoid (N.O.) (2)
  • does not supply the discharge solenoid (N.C.) (3).
The fluid taken into the main circuit is then free to return to the brake calliper (5) and increase the pressure acting on the brake calliper.

Ebd function (electronic brake-force distribution)

The EBD function controls brake force distribution and thus replaces the conventional mechanical load proportioning valve, allowing:

  • acts exclusively on the rear brakes
  • improved brakeforce 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 braking forceFp. Rear axle braking forceA. Distribution curve implemented by the braking systemB. Ideal distribution curveC. Distribution curve implemented by a conventional hydraulic load proportioning valveD. Distribution curve implemented by the EBD function

EBD function failure is indicated by the simultaneous activation of:

  • ABS warning light
  • low brake fluid and/or handbrake-on warning light.
Under these conditions, the car must be driven with extreme caution to the nearest authorised workshop to have the system checked.

Recovery

The electronic control unit comes with a safety circuit which monitors ABS efficiency.

With the ignition key inserted, the safety circuit carries out an initial auto test for 4 seconds to check:

  • operation of electronic control unit
  • operation of the solenoids (checks this by activating them)
  • controls the network
Every 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 the memory condition,
  • checks the operation of the relays, the solenoids and the pump motor,
  • constantly monitors the battery voltage,
  • checks CAN efficiency.

If 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.

Warning lights

There are two warning lights in the instrument panel for signalling the correct operation of the braking system.
1. Brake system warning light2. ABS failure warning light

Warning light operating logic

When the ignition is turned to the ON position, both warning lights come on for around four seconds to allow a check on their correct operation.

Once this period has elapsed, if no faults are detected in the system:

  • the ABS warning light goes out,
  • the braking system warning light remains on if the handbrake has not been applied, otherwise it goes out.
Fault signallingBraking system warning light coming onThe braking system warning light also comes on to indicate the application of the handbrake and when the brake fluid in the reservoir goes below the preset level.ABS warning light coming on

The ABS warning light comes on when:

  • the supply voltage for the ABS control unit is between 7.5 V and 9 V (the correct values for the supply voltage are between 9 V and 18 V).
  • the control unit pump motor is not working properly,
  • one of the wheel speed sensors is not working,
  • a solenoid valve for the front brake circuit is not working properly,
  • the brake lights switch is not working.
In these conditions, the EBD system continues to work.Braking system and ABS warning lights coming on simultaneously

Both warning lights come on when:

  • the supply voltage for the ABS control unit is less than 7.5 V (the correct values for the supply voltage are between 9 V and 18 V).
  • more than one of the wheel speed sensors is not working,
  • a solenoid valve for the rear brake circuit is not working properly,
  • the ABS control unit is not working properly,
  • the ABS electronic control unit has been replaced with one that does not correspond to the system specifications.
In these conditions, the ABS and the EBD are both deactivated.