spring parking brakes, apply the truck or tractor
spring parking brakes.
WARNING
If a trailer or combination vehicle is not equipped
with spring parking brakes, do not park it by pull-
ing out only the trailer air supply valve knob.
This would apply only the trailer service brakes.
If air were to bleed from the trailer brake system,
the trailer brakes would release, possibly causing
an unattended runaway vehicle.
NOTICE
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.
Automatic Slack Adjusters
Automatic slack adjusters should never be manually
adjusted except during routine maintenance of the
foundation brakes (e.g., replacing shoes), during
slack adjuster installation, or in an emergency situa-
tion.
When the brake pushrod stroke exceeds the legal
brake adjustment limit on a vehicle, there is likely a
mechanical problem with the foundation brake com-
ponents or the adjuster is improperly installed.
Visit a repair facility as soon as possible when
brakes equipped with automatic slack adjusters are
determined to be out of adjustment.
NOTICE
Manually adjusting an automatic slack adjuster to
bring the pushrod stroke within legal limits is
likely masking a mechanical problem. Adjustment
is not repairing. In fact, continual adjustment of
automatic slack adjusters may result in prema-
ture wear of the adjuster itself. Further, the im-
proper adjustment of some automatic slack ad-
justers may cause internal damage to the
adjuster, thereby preventing it from properly
functioning.
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 (ATC), wheel spin is controlled during
reduced-traction situations. In normal braking appli-
cations, 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 be-
hind the center dash). The control unit’s main circuit
interprets the speed sensor signals and calculates
wheel speed, wheel retardation, and a vehicle refer-
ence speed. If the calculations indicate wheel lockup,
the main circuit signals the appropriate modulator
valve to reduce braking pressure. During emergency
braking, the modulator valve alternately reduces, in-
creases, or maintains air pressure supply in the
brake chamber to prevent front and rear wheel
lockup.
If equipped with ATC, an additional solenoid valve is
installed. During reduced-traction situations, the ATC
solenoid valve controls air pressure to the modulator
valves, and they in turn increase, hold, or reduce
pressure to the appropriate brake chambers to pro-
vide better 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), modulator 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 indicator
light come on for about three seconds. After three
seconds, the warning lights go out only if all of the
tractor’s ABS components are working.
Brake Systems
12.3
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