General description

GENERAL DESCRIPTION

The electronic wheel anti-lock system (BOSCH ABS 5.3) adjusts the braking pressure transmitted to the wheels preventing loss of road-holding under all tyre and road conditions.

The system has been designed to integrate, and not replace, the normal mechanical braking system, guaranteeing a high degree of safety in the event of a failure: in fact it operates on the same brake fluid as the conventional mechanical circuit.

Four sensors, located on the four wheels, inform the electronic control unit of the speed of each wheel continuously, thereby recording locking situations affecting the wheels, skidding and loss of grip.

In these situations, the control unit suitably operates the solenoid valves that modulate the pressure in the hydraulic circuit, eliminating wheel locking and bringing the car back to the limit of roadholding, which means that the braking distance is reduced to a minimum, without losing control of steering.

The adjustment solenoid valves are, in this version of the system, two (one for pressure charging and one for relief) for each wheel.

In this version, the control unit controls distribution of the braking load in the rear axle, replacing the braking load proportioning valve used previously ("EBD" function: Electronic Brake Distribution").

For further details on the hydraulic operation of the system see GROUP 33-BRAKES.

Components

The system comprises:

• four magnetic induction sensors which read the speed of the wheels: L28; L29; L30; L31.
• the integrated electronic and hydraulic control unit N51, which houses the following:
  • the electronic control module (CPU)
  • the eight solenoid valves
  • two brake fluid pumps with corresponding motor
  • two damping accumulators and two reservoirs
  • a safety valve
• the connector for self-diagnosis T8
• the brake switch H3 (the same that turns on the stop lights) which signals the system the braking condition.

The ABS includes a self-diagnosis system which continually monitors all the system parameters and components: in the event of a failure or fault, the system cuts itself off automatically leaving the conventional servo-assisted mechanical braking system operational: the driver is alerted of this situation by a special warning light "ABS" [symbol] on the instrument cluster (C10).

A fault on the "EBD" function turns on the "handbrake on" warning light [symbol].

Suitably connecting to the diagnosis connector (T8) located next to the control unit, it is possible to use the signals of the flashing code to quickly locate the faulty component (see "Fault-finding").

The connector T8 can also be used to connect to the ALFA ROMEO Tester system.

FUNCTIONAL DESCRIPTION

System supply:

With a line protected by wander fuse G125a (10A) the key- operated voltage supplies pin 15 of the ABS control unit N51, the battery voltage reaches pin 17 and 18 of the same N51 from the line protected by fuse G125b (60A).

The electronic control unit is connected to earth via pin 19 of N51, while the pump is via pin 16.

Sensors and solenoid valves:

Directly inside the control unit N51, the module is connected with the adjustment solenoid valves, which modulate the pressure on the brakes of the four wheels; outside, it is connected with the four sensors L28 - L29 - L30 - L31 (pin 3-4, 6-7, 1-2 and 8-9) which signal the speed of the single wheels, and with the brake switch H3 (pin 14), which sends a consent signal: in fact the ABS system cannot come into operation if the brake pedal is not pressed.

Self-diagnosis:

When the control unit detects problems concerning the "ABS" through the self-diagnosis system, it sends a signal to the instrument cluster C10 which turns on the "ABS failure" warning light: this signal is sent from pin 20. If the problems concern the "EBD" function, the handbrake warning light is turned on, from pin 21.

The diagnosis connector T8 connected to pin 11 and 12 allows connection of the control unit with the ALFA ROMEO Tester or "reading" of the flashing code (see "Fault-finding").