CAUTION
Never apply the service and spring parking
brakes simultaneously. To do so transmits exces-
sive input force to the brake components, which
could damage or cause eventual failure of brake
actuating components.
Meritor WABCO
®
Antilock Braking
System (ABS)
The Meritor WABCO Antilock Braking System (ABS)
is an electronic wheel speed monitoring and control
system that works with the standard air brake sys-
tem. ABS passively monitors vehicle wheel speed at
all times, and
controls
wheel speed during emer-
gency stops. If equipped with Automatic Traction
Control, wheel spin is controlled during reduced-
traction situations. 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
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.
ABS includes signal-generating tone wheels and sen-
sors located in the wheel hubs of each sensed
wheel. The sensors transmit vehicle wheel speed
information to an electronic control unit (located on
the frame rail). The control unit’s main circuit inter-
prets the speed sensor signals and calculates wheel
speed, wheel retardation, and a vehicle reference
speed. If the calculations indicate wheel lockup, the
main circuit signals the appropriate solenoid control
valve to reduce braking pressure. During emergency
braking, the solenoid control valve alternately re-
duces, increases, or maintains air pressure supply in
the brake chamber to prevent front and rear wheel
lockup.
If equipped with Automatic Traction Control, an addi-
tional solenoid valve is installed. During reduced-
traction situations, the valve increases or reduces air
pressure supply in the brake chamber to provide bet-
ter traction whenever wheel spin occurs.
The electronic control unit also has a safety circuit
that constantly monitors the wheel sensors, traction
control valve (if equipped), solenoid control valves,
and the electrical circuitry.
After the ignition switch is turned on, the tractor
warning light (TRACTOR ABS) and, if equipped with
Automatic Traction Control, the wheel spin (WHEEL
SPIN) indicator light come on for about three sec-
onds. After three seconds, the warning lights go out
only if all of the tractor’s ABS components are work-
ing.
Vehicles with electronic engines and ABS may have
automatic traction control (ATC). On these vehicles,
the ATC system automatically limits wheel spin dur-
ing reduced-traction situations.
If the vehicle has ATC, there will be a momentary
contact rocker switch on the dash labeled NORM/
SPIN and ATC.
When the ATC system is in the NORMAL mode, it
will apply gentle braking to the spinning wheel, to
feed power to the wheel(s) with better traction. If
both wheels are spinning, the system will signal the
electronic engine to reduce power.
Pressing NORM/SPIN will temporarily allow more
drive wheel spin to help burn through a thin layer of
ice, or to help throw off accumulated mud or snow.
SPIN mode is indicated by a flashing WHEEL SPIN
light. Pressing NORM/SPIN again will cycle the sys-
tem back to normal operation.
The Meritor WABCO ABS system combines one
front-axle control channel with one rear axle to form
one control circuit. For example, the sensor and sole-
noid control valve on the left-front axle form a control
circuit with the sensor and solenoid control valve on
the right rear axle. If, during vehicle operation, the
safety circuit senses a failure in any part of the ABS
system (a sensor, solenoid control valve, wiring con-
nection, short circuit, etc.), the tractor warning light
(TRACTOR ABS) comes on and the control circuit
where the failure occurred is switched to normal
braking action. The remaining control circuit will re-
tain the ABS effect. Even if the ABS system is com-
pletely inoperative, normal braking ability is main-
tained. An exception would be if a solenoid control
valve (or combination solenoid control valve) is dam-
aged and inoperative. As these components are an
integral part of the air brake system, normal braking
may be impaired or inoperative.
IMPORTANT: If any of the ABS warning lights
do not work as described above, or come on
while driving, repair the ABS system immedi-
ately to ensure full antilock braking capability.
Steering and Brake Systems
6.4
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