E7020 Abs

Description

The electronic anti-lock brake system (BOSCH 5.3 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.

Two adjustment solenoids (one for pressure charging, the other for pressure discharging) are fitted for each wheel.

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 brakeforce 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 see DESCRIPTION AND OPERATION: 3340 Anti-lock brake system (abs)

The supply lines to the control unit and the various system components (sensors and actuators) are protected by a specific maxifuse and other jumper fuses, located next to the junction unit.

Description of operation

ABS electronic control unit M50 is supplied (at pins 17 and 18) directly by the battery via a line protected by the ABS maxifuse of B6 up to 2/99 or by ABS maxifuse located directly on the battery A1 - connector C from 2/99.

The supply controlled by the ignition (15/54) reaches pin 15 from the line protected by shunt fuse B30 up to 3/98) or by B31 from 3/98 to 2/99 or from fuse F6 from supplementary fuse box B98A from 2/99.

The control unit is earthed via pins 19 and 16.

The four sensors K70, K71, K75, K76 send wheel speed signals to pins 6-7, 4-5, 8-9 and 1-2 of M50 respectively.

Brake pedal switch I30 sends an enablement signal to pin 14 of control unit M50: the ABS cannot intervene unless the brake pedal is pressed.

If the control unit M50 detects problems relating to the ABS function, it sends the instrument panel E50 a signal, from pin 4, which lights up the "ABS failure" warning light; if the problems concern the EBD function, the handbrake warning light is lit up from pin 21.

Pin 22 of M50 produces a speedometer signal (average speed of all four wheels) which is sent to the instrument panel E10. ( Wiring diagram E4021 SPEEDOMETER ).

The ABS includes an autodiagnostic system: when the control unit detects problems relating to the ABS function it sends a signal from pin 20 to the check instrument E30 which contains the "ABS failure" warning light. If the problems involve the EBD function, the control unit also sends a signal from pin 21 to instrument E30 to the "insufficient brake fluid level" and "handbrake applied" warning light.

These are earth signals and cause the warning lights to come on because the supply for instrument E30 is controlled by the ignition (INT) from the line protected by the controls light fuse B11 up to 2/99 or from fuse F4 of supplementary fuse box B98B from 2/99.

The autodiagnostic data can be read by connecting to connector R10: it receives signals from pin 11 of the control unit M50 via the special diagnostic line.

Wiring diagram

Components

Component location