Meritor WABCO Pneumatic
Antilock Braking System (ABS)
Pneumatic ABS is an electronic wheel speed moni-
toring and control system that works with the air
brake system. It passively monitors vehicle wheel
speed at all times, but controls wheel speed during
an emergency or reduced-traction stop. In normal
braking applications, the standard air brake system is
in effect.
Pneumatic ABS Operation
The Meritor WABCO ABS is a four-sensor system. It
combines one front-axle control channel with one
rear-axle control channel to form one control circuit.
Example: The sensor and solenoid control valve at
the left front wheel form a control circuit with the sen-
sor and solenoid valve at the right rear axle.
ABS includes signal-generating tone wheels and sen-
sors located in the wheel hubs of each sensed axle.
The sensors transmit vehicle wheel speed informa-
tion to an electronic control unit.
IMPORTANT: For proper ABS system operation,
do not change tire sizes. The sizes of the tires
installed during production are programmed into
the electronic control unit. Installing different
sized tires could result in a reduced braking
force, leading to longer stopping distances.
During emergency or reduced traction stops, fully
depress the brake pedal until the vehicle comes to a
safe stop. Do not pump the brake pedal. With the
brake pedal fully depressed, the ABS system will
control all wheels to provide steering control and a
reduced braking distance.
Although the ABS system improves vehicle control
during emergency braking situations, the driver still
has the responsibility to change driving styles de-
pending on the existing traffic and road conditions.
For example, the ABS system cannot prevent an ac-
cident if the driver is speeding or following too
closely on slippery road surfaces.
Even if the ABS system is partially or completely in-
operative, normal braking ability is usually main-
tained.
IMPORTANT: If a solenoid control valve (or
combination solenoid control valve) is damaged
and inoperative, normal braking may be im-
paired.
Automatic Traction Control
Vehicles with electronic engines and ABS may have
Automatic Traction Control (ATC). On these vehicles,
f420477a
05/24/2012
AA
A. Fill level for brake fluid.
Fig. 13.2, Master Cylinder Reservoir
Hydraulic and Air Brake Systems
13.4
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