freightliner Cargo Manual page 70

Brakes
CAUTION
Never apply the service and spring parking brakes
simultaneously. To do so transmits excessive input
force to the brake components, which could dam-
age or cause eventual failure of brake actuating
components.
Meritor WABCO ® Antilock Braking
System (ABS), Air Brake Systems
The Meritor WABCO ® Antilock Braking System (ABS) is
an electronic wheel speed monitoring and control sys-
tem that works with the air brake system. ABS 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.
IMPORTANT: For proper ABS system operation,
do not change tire sizes. The sizes of the tires in-
stalled during production are programmed into the
electronic control unit. Installing different sized
tires could result in a reduced braking force, lead-
ing to longer stopping distances.
ABS includes signal-generating tone wheels and sen-
sors located in the wheel hubs of each sensed axle.
The sensors transmit vehicle wheel speed information
7.3
to an electronic control unit (located behind the passen-
ger seat). The control unit's main circuit interprets the
speed sensor signals and calculates wheel speed,
wheel retardation, and a vehicle reference speed. If the
calculations indicate wheel lockup, the main circuit sig-
nals the appropriate solenoid control valve to reduce
braking pressure. During emergency braking, the sole-
noid control valve alternately reduces, increases, or
maintains air pressure supply in the brake chamber to
prevent front and rear wheel lockup.
The electronic control unit also has a safety circuit that
constantly monitors the wheel sensors, solenoid control
valves, and the electrical circuitry. The ABS warning
light comes on after turning on the ignition switch. The
warning light goes out, if all ABS components are work-
ing properly.
If, during vehicle operation, the safety circuit senses a
failure in any part of the ABS system (a sensor, sole-
noid control valve, wiring connection, short circuit, etc.),
the ABS warning light comes on and the control circuit
where the failure occurred is switched to normal braking
action. The remaining control circuit will retain the ABS
effect. Even if the ABS system is partially or completely
inoperative, normal braking ability is maintained. An ex-
ception would be if a solenoid control valve (or combi-
nation solenoid control valve) is damaged and inopera-
tive. As these components are an integral part of the air
brake system, normal braking may be impaired or inop-
erative.