Fiat Bravo Service Manual page 459

Bravo-Brava Braking system
Anti-lock braking system
33.
If the wheel does not regain its speed within a pre-determined time (t), a new pressure reduction phase
is started.
The wheel normally regains speed until it exceeds the reference speed; at this point a new braking cycle
(point D) commences, characterized by the three regulation phases for reducing, maintaining or re-esta­
blishing on the brake calipers the pressure generated by the driver on the brake pedal.
The logic described is not fixed, but adapts to the dynamic behaviour of the tyres in accordance with the
different grip coefficients and the relevant deceleration/acceleration thresholds at the various speeds.
The number and frequency of the correction interventions is determined by the dynamic behaviour of
the chain consisting of the brake circuit and A.B.S. components, but even more so by the tyre/road sur­
face grip coefficient.
During braking on dry tarmac, six-eight interventions per second can be reached; this frequency is con­
siderably lower on ice or wet roads.
The electronic control unit controls the various phases, supplying pulses of different current intensity to
the solenoids. It also ensures that both rear wheels are supplied with the same braking force applicable
to the rear wheel which is most liable to lock, i.e. that with least grip on the ground (to ensure the best
stability of the trajectory).
NOTE During the intervention cycle, the brake pedal moves slightly depending on the increase or re­
duction of the controlled pressure.
If a wheel is rolling with a flat tyre, the A.B.S. intervenes if necessary to control the braking.
The A.B.S. is also active during braking in reverse gear.
The intervention of the device usually ceases at speeds of under 2.75 km/h to permit complete locking
of the wheels when the vehicle is stopped.
NOTE Since the parameters monitored by the control unit (wheel speed and acceleration) are influ­
enced by the inertia of the wheel/tyre assembly, vehicles with the anti-lock braking system
must only be fitted with the rims, tyres and brake linings recommended and chosen by the
manufacturer.
If snow chains are fitted, the resulting rolling condition leads to signals which, suitably filtered in the
control unit, do not exclude the anti-lock braking device in the event of driving on hard and compact
snow.
In aquaplaning conditions, the electronic control unit detects from the rpm sensor a faulty condition
even during ordinary driving, with no braking, as the driving wheels tend to rotate at a higher speed than
the driven wheels.
This condition would cause the electronic control unit to carry out a regulation cycle which did not meet
requirements; for this reason the anti-lock braking system temporarily switches itself off (for such a short
time that the warning light does not necessarily come on) and switches itself on again as soon as the
aquaplaning ceases.
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