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 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.

The sensors are the "active" type, i.e. they consist of a Hall effect receiver which faces a magnetic codifier in the hub bearing: under these circumstances the signal is less affected by electro-magnetic interference and temperature.

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 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).

The warning light is "intelligent". It lights up when a fault is detected in the system and also when open circuit or short-circuit to earth is detected in the circuit connecting the warning light to the respective ECU, thanks to an internal pilot circuit which checks correct connection.

An "EBD" function fault turns the "handbrake engaged" [symbol] warning light on as well as the ABS warning light. The self-test signals can be used to rapidly identify faulty components by suitably connecting to the unified diagnostic socket (T20). The ABS ECU also sends the vehicle speed value to the tachometer and to other electronic systems.

On versions with EURO 3 standard engines, the ABS control unit converses with the engine management control unit via the CAN line at high speed and signals any irregularities in the road surface, distinguished by the speed sensors fitted on the individual wheels.

In this way the engine management control unit is capable of distinguishing between engine torque variations due to the unevenness of the road surface and those due to failed ignition in the combustion chamber (misfire).

The speedometer signal is also sent in the same way (average value of the four wheel speeds).

FUNCTIONAL DESCRIPTION

System supply:

With a line protected by wander fuse G125a (10A) (from October 2000 from fuse F3 of the additional fuse box G2) 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.