Bendix M-21-M-22 ANTILOCK MOD ASSY User Manual
®
Bendix® M-21™ and M-22™ AntiLock Modulator Assembly
SUPPLY
SOLENOID
CONNECTOR
SUPPLY
DELIVERY
(CAST-IN ID #2)
EXHAUST
(CAST-IN ID #3)
SD-13-4793
DELIVERY
(CAST-IN ID #2)
DELIVERY
MOUNTING
HOLES
SUPPLY
(CAST-IN ID #1)
EXHAUST
(CAST-IN ID #3)
M-21™ MODULATOR M-22™ MODULATOR
FIGURE 1 - M-21™ AND M-22™ MODULATOR
DESCRIPTION
There are two Bendix antilock modulators available from
Bendix Commercial Vehicle Systems LLC, the older M-21
modulator and the newer M-22™ modulator. Both are
functionally the same.
The M-21™ and M-22™ antilock system modulators (Figure
1) are essentially high capacity, on/off air valves that
incorporate a pair of electrical solenoids for control. The
solenoids provide the electro-pneumatic interface or link
between the antilock controller electronics and the air brake
system. The modulator can be used to control the braking
function on an individual wheel or two service actuators. When
used to control both service chambers on an axle or two
chambers on the same side of a tandem axle, the modulator
is sometimes mounted ahead of a quick release valve, which
SUPPLY
(CAST-IN ID #1)
SOLENOID
CONNECTOR
MOUNTING
HOLES
provides quick exhaust of service applications during normal
braking. (Figure 2) In the case of individual wheel control
applications, the modulator is always the last control valve
™
through which air passes on its way to the service brake
actuator and a solenoid assembly which contains one
normally open solenoid (NOS), one normally closed solenoid
(NCS), and two inlet and exhaust diaphragm valves. A
Packard, three pin, 280 series weather resistant electrical
connector is an integral part of the modulator solenoid
assembly and serves to carry control commands from the
antilock controller to the modulator. Two, .35 inch diameter,
mounting holes are provided for frame or cross member
mounting. The single supply, delivery and exhaust port on
both modulators is identified with a cast, embossed numeral
for positive identification.
DELIVERY
1
AXLE CONTROLWHEEL CONTROL WHEEL CONTROL
TO
ANTILOCK
CONTROLLER
™
M-21
OR M-22™ MODULATOR
QUICK RELEASE VALVE
M-21™ OR M-22™ MODULATOR
TO ANTILOCK
CONTROLLER
ANTILOCK
CONTROLLER
bw
TO
™
M-21
OR M-22™ MODULATOR
QUICK RELEASE VALVE
TO
ANTILOCK
CONTROLLER
TO
ANTILOCK
CONTROLLER
REAR AXLE SYSTEMFRONT AXLE SYSTEMS
M-21™ OR M-22
MODULATOR
CONTROLLER/
RELAY ASSEMBLY
™
M-21
OR M-22
MODULATOR
™
™
SERVICE BRAKE
CHAMBER
FIGURE 2 - WHEEL AND AXLE CONTROL SYSTEMS
SERVICE BRAKE
CHAMBER
They are as follows;
Identification Air Line Connection
1 Supply
(incoming air from foot valve or relay)
2 Delivery
(air delivery to service actuators)
3 Exhaust
(air exhaust during antilock control)
OPERATION
NON-ANTILOCK APPLICATION (FIGURE 3)
During normal non-antilock braking, both solenoids are deenergized (no electrical power). Brake application air enters
the supply port of the modulator and flows to, and through,
the open exhaust solenoid then to the exhaust diaphragm.
Air pressure, along with spring force, seats the exhaust
diaphragm on the exhaust passage thus preventing the
escape of service air. Simultaneously, application air flows
to the supply diaphragm and forces it away from its seat. Air
flows past the open supply and out the modulator delivery
port to the service brake chambers.
SERVICE &
SPRING BRAKE
CHAMBER
NON-ANTILOCK HOLD (FIGURE 4)
When the desired air pressure is attained in the service
brake chambers, the brake system is in the holding position.
In the holding position both solenoids in the modulator remain
de-energized and the balance of the internal components
remain in the same position as they assumed during
application.
2
EXHAUST PORT
EXHAUST
DIAPHRAGM
EXHAUST
SOLENOID
BRAKE VALVE
SUPPLY
SOLENOID
BRAKE
CHAMBER
FIGURE 3 - M-21™, M-22™ MODULATOR NON-ANTILOCK APPLICATION OF SERVICE BRAKES
EXHAUST PORT
SUPPLY
DIAPHRAGM
EXHAUST
DIAPHRAGM
SPRING
EXHAUST
SOLENOID
SPRING
BRAKE
CHAMBER
FIGURE 4 - M-21™, M-22™ MODULATOR NON-ANTILOCK APPLICATION HELD POSITION
SUPPLY
DIAPHRAGM
SPRING
BRAKE VALVE
SUPPLY
SOLENOID
SPRING
3
EXHAUST PORT
EXHAUST
DIAPHRAGM
EXHAUST
SOLENOID
BRAKE VALVE
SUPPLY
SOLENOID
BRAKE
CHAMBER
FIGURE 5 - M-21™, M-22™ MODULATOR NON-ANTILOCK EXHAUST OF SERVICE BRAKES (NORMAL)
SUPPLY
DIAPHRAGM
SPRING
SPRING
NON-ANTILOCK EXHAUST
The manner in which air exhausts through the modulator
differs, depending upon how rapidly the brake application is
released by the driver.
Normal Exhaust (Figure 5) - During a normal, relatively
"slow", brake release, air moves back through the modulator
in the reverse direction as it flowed during application. The
internal components of the modulator will remain in the same
position as they assumed during application until air pressure
decreases to approximately one half psi, at which time the
supply diaphragm will seat on the supply passage. Air will
generally not be expelled from the modulator exhaust port
during normal brake release.
Rapid Exhaust (Figure 6) - The rapid exhaust operation
described in the following text only occurs when the
modulator is controlling two type 30 (or larger) service
chambers. During a very rapid brake release the modulator
will exhaust air in a different manner. An example of this
would be the case if the driver made a severe brake
application then lifted his foot from the foot valve. During a
rapid brake release, air from the exhaust diaphragm flows
back through the open exhaust solenoid and to the
application device's exhaust. With pressure removed from
the control side of the exhaust diaphragm, air, returning to
the modulator from the service chambers, lifts the exhaust
diaphragm from its seat. Returning air flows past the open
exhaust and out the modulator exhaust port. Air will also
flow past the open supply diaphragm and back to the
application device's exhaust.
ANTILOCK OPERATION
GENERAL
If a service brake application is made and the antilock system
detects an impending wheel lockup, the antilock controller
will immediately begin modification of the brake application
using the modulator.
In order to modify the brake application, the coils of the two
solenoid valves contained in the modulator are energized or
de-energized in a pre programmed sequence by the antilock
controller. When a solenoid coil is energized, a shuttle within
the solenoid moves, and depending upon the function of the
specific solenoid, it either opens or closes thereby causing
the exhaust or re-application of air pressure to the brake
actuator. The solenoids in the modulator are controlled
independently by the antilock controller. By opening and
closing the solenoid valves in the modulator, the antilock
controller is able to simulate what the driver does when he
"pumps the brakes". It must be remembered, however, that
unlike the driver the antilock controller is able to "pump"
each modulator, along with the brakes connected to it,
independently and with far greater speed and accuracy.
4
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