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BRAKES
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7. WORKING OF ANTI-LOCK BRAKING
SYSTEM (ABS):
1. ABS is an aid to retain steering control and
stability while braking.
2. ABS will not prevent accidents resulting from
excessive cornering speeds, following another
vehicle too closely or aquaplaning, i.e. where a
layer of water prevents adequate contact between
tyre and road surface.
3. ABS will not improve braking performance or
stopping distance beyond that which is physically
possible in relation to road conditions and the
vehicles tyres or brakes.
4. The additional control provided by ABS must
never be exploited in a dangerous or reckless
manner which could jeopardize the safety of driver
or other road users.
The ABS ECU takes wheel speed information from
sensors, located within the hub carrier, and
monitors the relative deceleration/acceleration of
each wheel at all times. These signals are used to
calculate the rotational deceleration (negative
acceleration) rates for each wheel during braking
operation. In the event of a wheel slip condition
(i.e. rotational deceleration being outside of
allowable limits) the hydraulic system will control
the brake line pressure by operating the
appropriate solenoid valves within the hydraulic
modulator and thus releasing the brake pressure
from that particular wheel.
Once the wheel deceleration has recovered to
within allowable limits the modulator then allows
pressure to be re-applied to the particular brake
caliper or wheel cylinder from which the brake
pressure was released.
When ABS intervention is necessary the warning
lamp will not illuminate but the driver will feel the
brake pedal pulsating and may experience audible
feedback from the modulator indicating that ABS
control is taking place.
The ABS and EBD warning lamps will illuminate
for a system check when the ignition is switched
on. Both lamps will then extinguish to indicate the
system is healthy. (As the EBD lamp is shared
with the Parking brake lamp this will not extinguish
until the parking brake is released.) If a fault
occurs with the system or any of its constituent
components the relevant warning lamp in the
instrument cluster will illuminate to inform the
driver for the respective fault.
A. BRAKING WITHOUT ABS IN OPERATION:
In case of normal braking operation when no wheel
speed differences are detected and neither ABS
nor EBD are active, when the driver presses the
brake pedal, brake pressure developed in tandem
master cylinder (TMC) reaches to each wheel
through hydraulic control unit (HCU). During this
phase, both inlet & outlet solenoid valves are
switched OFF (or not excited) by the ECU. Hence
the inlet valve is open and outlet valve remains
closed allowing the brake fluid to flow freely
through the inlet valve and to the relevant wheel.
When the driver releases brake pedal, the brake
fluid flows back from the wheel, through the
respective one-way valve towards TMC.
B. BRAKING WITH EBD INTO OPERATION:
In case of a braking where the ABS ECU detects a
slip difference between the front and rear wheels,
EBD comes into operation. If the difference in
wheel slip between the front and rear wheels
exceeds a predetermined threshold the ABS
activates the pressure inlet valves for the rear
wheels, closing the valves and preventing further
pressure build up at that rear axle.
If during the braking event, this slip ratio between
the front and rear axle reduces below the threshold
level the inlet valves will be de-activated and
switch back to their normally open state allowing
further pressure increase at the rear axle. In this
way the rear axle can follow more closely to the
Ideal braking force distribution curve for the rear
axle.
If the driver releases the brake, the inlet valves will
be deactivated and fluid can return to the TMC via
the inlet valve and the one-way valve.
EBD control only acts upon the rear axle and can
only limit pressure being applied to a rear wheel; it
cannot perform pressure reductions.