automatically causes the trailer air supply valve to
pop out.
NOTICE
Do not use the spring parking brakes if the ser-
vice brakes are hot, such as after descending a
steep grade. To do so could damage the brakes.
Allow hot brakes to cool before using the spring
parking brakes.
Do not use the spring parking brakes during
freezing temperatures if the service brakes are
wet. To do so could cause them to freeze. If the
brakes are wet, drive the vehicle in low gear and
lightly apply the brakes to heat and dry them.
If the trailer is not equipped with spring parking
brakes, pulling out the parking brake valve applies
the tractor spring parking brakes and the trailer ser-
vice brakes. When the tractor and trailer parking
brakes (or trailer service brakes) are both applied,
the trailer brakes are released by pushing in the
trailer air supply valve, leaving the tractor parking
brakes applied. Air pressure in the primary or sec-
ondary reservoir must be at least 65 psi (448 kPa)
before the tractor spring parking brakes, or the trailer
service or spring parking brakes, can be released.
Automatic Slack Adjusters
WARNING
Manually adjusting an automatic slack adjuster to
bring the pushrod stroke within legal limits is
likely masking a mechanical problem. Adjustment
is not a repair. Before adjusting an automatic
slack adjuster, troubleshoot the foundation brake
system and inspect it for worn or damaged com-
ponents. Improperly maintaining the vehicle brak-
ing system may lead to brake failure, resulting in
property damage, personal injury, or death.
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.
Meritor WABCO® Antilock
Braking System
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.
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.
The electronic control unit (ECU) also has a safety
circuit that constantly monitors the wheel sensors,
traction control valve (if equipped), modulator valves,
and the electrical circuitry.
The Meritor WABCO ABS combines one front-axle
control channel with one rear-axle control channel to
form one control circuit. For example, the sensor and
modulator valve on the left-front axle form a control
circuit with the sensor and modulator valve on the
right-rear axle. If, during vehicle operation, the safety
circuit senses a failure in any part of the ABS, the
vehicle ABS warning indicator illuminates and the
control circuit where the failure occurred is switched
to normal braking action. See
Fig. 12.3. The remain-
ing control circuit will retain the ABS effect. Even if
the ABS is completely inoperative, normal braking
Brake System
12.3
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