TIAGO

BRAKES

Brakes Held ON

When the brake pedal is held on, the diaphragm

will momentarily continue to move forward and

thus com- presses the outer edges of the reaction

disc. This movement causes the centre of the disc

to extrude, pressing back the input rod and thus

closing the atmospheric port. Thus the output

force is balanced against that of the input force.

Further movement of the brake pedal either opens

the vacuum port or the atmospheric port

depending on whether the brake pedal is released

or depressed.

. Brakes Released

Immediately after the brake pedal is released, the

vacuum port is opened and the atmospheric

pressure in the rear chamber is extracted into the

front chamber and from there to the inlet manifold

via the non- return valve. The atmospheric port

remains closed whilst the input rod assembly

returns to its original position as shown in figure.

The diaphragm is then again ‘suspended’ in

vacuum until the brake pedal is depressed.

6.8 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 jeopardise

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.