E7020 Abs

Description

The electronic anti-lock brake system (BOSCH 5.7 ABS) governs braking pressure sent to the wheels and prevents loss of grip under all tyre and road surface conditions.

The system is designed to complement the normal mechanical braking system and not to replace it. In this way it ensures effective safety in the case of anomalies: the ABS uses the same fluid as the conventional mechanical braking system.

Four sensors fitted to the four wheels inform the electronic control unit of wheel speed at every moment. In this way, they detect wheel lock, wheel slip and loss of grip.

Under these conditions, the control unit sends specific instructions to solenoids that modulate hydraulic circuit pressure to eliminate lock and restore the car to within grip limits. This ensures the shortest possible stopping distance, without losing control of the steering.

The sensors are active, i.e. they consist of a Hall-effect receiver that faces a magnetic coding device built into the hub bearing. Under these conditions, the signal is less influenced by electromagnetic interference and temperature.

The control unit also controls brake force distribution over the rear axle and thus replaces the mechanical load proportioning valve used previously (EBD function: Electronic Brake Distribution).

The control unit is equipped with a self-diagnostic function: when an error occurs in the ABS system, a warning light is turned on and the system is simultaneously deactivated. Under these conditions, the car brakes using only the conventional system.

When the control unit detects an error in the EBD function, it simultaneously turns on the 'insufficient brake fluid level' and 'handbrake on' warning light as well as the ABS warning light.

Under these conditions, the rear brake distribution control is also deactivated and the car must be driven with great care.

For more details Characteristic of working principle 3340 Anti-lock brake system (abs) .

The ABS control unit computes vehicle speed on the basis of values supplied by drive wheel sensors (the ABS control unit computes an average reading) and true wheel circumference supplied by the body computer. This speed signal is then sent to nodes that require the information via the CAN network. The ABS control unit also generates a discrete signal that copies the vehicle speed information computed and transmitted via the CAN network.

The control unit supply lines are protected by a special maxifuse located in the engine bay control unit. ignition-operated power supply from a special fuse in the underfacia control unit.

Description of operation

The ABS electronic control unit M50 is supplied (at pins 2 and 6) directly by the battery via a line protected by maxifuse F4, housed in the engine compartment junction unit B1.

The ignition-operated supply (INT/A) reaches pin 23 from the line protected by the fuse F42 of under facia control unit B2.

The control unit is earthed via pins 1 and 5.

The four sensors K70, K71, K75, K76 send wheel speed signals to pins 28-12, 15-16, 13-14 and 30-31 of M50 respectively.

Brake pedal switch I30 sends an enablement signal to pin 32 of control unit M50: the ABS cannot intervene unless the brake pedal is pressed. The switch I30 is supplied when the ignition is switched on (INT) from fuse F37 of junction unit B2.

Pin 37 di M50 emits a discrete speed signal that is sent tol Body Computer M1; the same signal is also copied via the CAN network.

Pins 24 and 40 of ABS control unit M50 connect to Body Computer M1 and instrument panel E50 via the CAN line to manage the ABS failure warning light and - if the problems affect the EBD function - the low brake fluid and handbrake warning lights.

Self-diagnostic data can be read by connecting to connector G of Body Computer M1 - pin 1: it receives signals from pin 11 of control unit M50 via a special diagnostic line.

Wiring diagram

Components

Component location