The electronic control unit processes signals from the active sensors and brake light control switch. It identifies the wheel or wheels with a tendency to lock (maximum slip between wheel and road surface) by means of the control unit software and modulates the brake fluid pressure selectiely for the front wheels and in tandem for the rear wheels (select-low function).The ABS modulates brake pressure in accordance with three basic stages:
pressure maintenance stage
pressure reduction stage
pressure increase stage
If operation is requested, the ABS stays active at speeds greater than 2.7 km/h. After that, it cuts out to allow the car to stop. After each key ON and when a speed of 6 km/h is exceeded, the control unit carries out an operation check on the solenoids and pump engine. The control unit also tests the sensors when a speed of 12 km/h is exceeded.The ABS is detected as a component of the C-CAN network by the body computer at the first key ON.
Operating strategies
Pressure increase stage without intervention of ABS
When the brake pedal is pressed, the electronic control unit (1):
does not supply load solenoid (N.O.) (2)
does not supply discharge solenoid (N.C.) (3)
The pressure generated by brake pump (4) therefore reaches brake calipers (5) without undergoing changes.
Example of ABS intervention
Pressure maintenance stage
Control unit (1):
supplies load solenoid (N.O.) (2)
does not supply discharge solenoid (N.C.).
The hydraulic connection between brake pump (4) and brake caliper (5) is therefore broken.The pressure in brake caliper (5) remains constant even when pressure on the brake pedal is increased.
Pressure reduction stage
The control unit:
supplies load solenoid (N.O.) (2)
supplies discharge solenoid (N.C.) (3)
The hydraulic connection between brake pump (4) and brake caliper (5) is broken and discharge solenoid (3) opens and brings brake caliper (5) into communication with low pressure accumulator (6) and scavenger pump (7).Control unit (1) also supplies motor (8) driving scavenger pump (7) to return fluid taken from brake caliper (5) to the main circuit.The fluid flows through high pressure accumulator (9) and restriction (10) that performs a damping function.The system is equipped with a check valve (11) fitted in parallel with load solenoid (2) that allows fast reduction of pressure in brake caliper (5) during brake pedal release.
Pressure increase stage
Control unit (1):
does not supply load solenoid (N.O.) (2)
does not supply discharge solenoid (N.C.) (3)
The fluid taken into the main circuit is then free to return to brake caliper (5) and increase the pressure acting on the brake caliper.
EBD function (Electronic Brake-Force distribution)
The EBD function controls brakeforce distribution and thus replaces the conventional mechanical load proportioning valve allowing:
exclusive action on the rear brake calipers
improved brakeforce distribution
optimal intervention under all load conditions (static or dynamic), driving conditions (straight line or corners) or car condition (tyres, brakes and suspension worn)
implementation of a strategy that follows the ideal distribution curve.
EBD function failure is indicated by simultaneous activation of:
ABS warning light
low brake flud and/or handbrake on warning light.
Under these conditions, the car must be driven with extreme caution to the nearest authorised workshop to check the system.
Recovery
The electronic control unit comes with a safety circuit that supervises ABS efficiency.With key on, the safety circuit carries out an initial autotest for 4 seconds to check:
electronic control unit operation
solenoid activation to check operation
the C-CAN network
Every time the car starts off from a standstill, the safety circuit checks for signals from active sensors.When the car is in motion, the safety circuit works as follows:
continually compares angular wheel speed with the calculated reference speed
checks memory condition
checks operation of both relays, the solenoids and the pump motor
continually checks battery voltage
checks brake pedal switch efficiency
checks C-CAN network efficiency
If the safety circuit detects one or more faults in system components during the check stage, the safety circuit operates as follows:
deactivates the ABS while still guaranteeing operation of the conventional braking system
indicates the fault status to the driver by turning on a warning light on the control panel.
