BENDIX SD-13-4793 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
™
SUPPLY
(CAST-IN ID #1)
SOLENOID
CONNECTOR
DELIVERY
MOUNTING
HOLES
valve, which 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 identifi ed
with a cast, embossed numeral for positive identifi cation.
1
bw
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
TO
ANTILOCK
CONTROLLER
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;
Identifi cation 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
de-energized (no electrical power). Brake application air
enters the supply port of the modulator and fl ows 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 fl ows to the supply diaphragm and forces it
away from its seat. Air fl ows 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.
fl ow past the open supply diaphragm and back to the
application device's exhaust.
ANTILOCK OPERATION
Normal Exhaust (Figure 5) - During a normal, relatively
"slow", brake release, air moves back through the
modulator in the reverse direction as it fl owed 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
fl ows 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 fl ows past the open
exhaust and out the modulator exhaust port. Air will also
GENERAL
If a service brake application is made and the antilock
system detects an impending wheel lockup, the antilock
controller will immediately begin modifi cation 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 specifi c 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.
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