Fault diagnosis

The control unit (1) is supplied:
- directly from the battery (+VB) on pins 9 and 25, through a 60A fuse (2);
- under ignition key (+15) on pin 23, through a 10A fuse (3).

FAULT DIAGNOSIS

The system has a self-test function which checks if there are any faults on the following components:

• wheel rpm sensors (4 sensors);
• input solenoids (4 solenoids);
• output solenoids (4 solenoids).

If a malfunction is identified, the control unit's safety circuit excludes the A.B.S. system, nevertheless ensuring normal operation of the conventional braking system.

When the A.B.S. system is switched off, this is indicated to the driver by the warning light on the instrument panel coming on. For safety reasons, the electronic control unit, by means of two microprocessors, controls all the logic functions, its software and all the input signals.

If the two microprocessors receive identical input signals, they supply identical output signals.

If there is a logic discrepancy between input signals and output response over a certain period of time, the safety system detects the fault and the A.B.S. is de-activated.

The electronic control unit also has a safety circuit which monitors the efficiency of the system before every start-up and during driving.

The safety circuit carries out the following self-tests:

1. after the ignition has been switched on and for about 2 seconds, it checks the operation of the control unit, the relays which operate the solenoids and the connection of the sensors;
2. after the engine has started, as soon as the speed of 6 km/h is exceeded, it activates the solenoids and recycling pump for an operating check; it also checks that the 4 speed signals are present;
3. whenever the speed of 24 km/h is exceeded, starting from a standstill, it checks that the 4 speed signals are present;
4. during driving it continuously checks the peripheral speed of the wheels against the calculated reference speed, it checks the memory conditions and supervises the operation of the two relays;
5. during driving it constantly checks the battery voltage.