Fiat 1998 Marea Weekend, Service Manual

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Marea • Marea Weekend
Braking system
Anti-lock brakes
33.
The wheel speed signals are sent by the active sensors to the electronic control unit.
The frequency of these signals provides the controlunit with the corresponding speed, acceleration or
deceleration values for the individual wheels.
A reference speed is processed from the combination of the individual peripheral wheel speeds which,
being constantly updated, indicates the speed of the vehicle at any moment.
When the driver presses the brake pedal the wheels may decelerate in differing degrees: by comparing
the peripheral speed of the individual wheels with the reference speed it is possible to constantly control
the creeping of each individual wheel.
If the braking force causes one wheel to creep in relation to the others (known in English as split), the elec-
tronic control unit gives the command to the electro-hydraulic control unit solenoid valves to reduce the
braking force at the wheel where there is a loss of grip. The wheel concerned then regains speed.
NOTE The peripheral speed of a braking wheel decreases to a greater extent than that of the vehicle, with
the wheels completely locked through the braking action (peripheral speed of the wheel = 0) and
the vehicle still moving the difference between these two speeds is at the maximum value.
This difference is known as creeping or the slipping coefficient when expressed as a percentage.
Creeping 0% = free wheel
Creeping 100%
= wheel locked and vehicle moving
During braking the friction coefficient tends to increase when the braking takes place with limited
creeping (rolling) and decrease when the sliding of the tyre is accentuated until it locks.
By means of a considerable number of practical tests and experiments it has been proven that, ge-
nerally, it is possible to reach the maximum braking force with creeping values of between 10%
and 20%. This defines an optimum area within which the anti-lock brakes operate for all types of
vehicles.
The electronic control unit also contains deceleration and acceleration threshold figures in its memory
which each individual wheel should never exceed.
Therefore, by means of a systematic, continuous and extremely rapid comparison of the wheel slipping,
deceleration and acceleration values the rolling of the tyre during braking is kept in check.
As soon as the acceleration/deceleration and combined split threshold values are exceeded, the elec-
tronic control unit intervenes, operating the electro-hydraulic control unit solenoid valves in the three
regulation stages to decrease, maintain or restore the pressure created by the driver on the brake pedal to
the brake calipers, restoring the braking conditions to the optimum values for the system.
These stages determine an intermittent, but extremely fast adjustment cycle which is repeated until the
vehicle stops. The electronic control unit controls the various stages, supplying the solenoid valves with
impulses of variable lengths. It is also ensures that both the rear wheels receive the same braking force
applied to the rear wheel most likely to lock, i.e. the one with less adhesion (to ensure optimum trajecto-
ry stability).
The A.B.S. system is also activated when braking in reverse gear.
Usually, the intervention of the device ceases at speeds below 5 kph to allow the wheels to lock com-
pletely with the vehicle stationary.
Since the parameters which the control unit controls (wheel speed and acceleration) are affected by the
inertia of the wheel/tyre assembly, vehicles equipped with anti-lock brakes
should only be fitted
with the wheel rims, tyres and brake pads recommended by the Manufacturer.
Copyright by Fiat Auto
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