Bendix Commercial Vehicle Systems M-12R ANTILOCK MODULATOR - Manual (Page 3)

3
FIGURE 3 APPLYING: NORMAL SERVICE APPLICATION
EXHAUSTING: NORMAL SERVICE
APPLICATION
When a service application is released, air at the modulator
control port returns to the application valve. Control pressure
above the relay piston exhausts through the supply solenoid
and the check valve in the solenoid housing out to the exhaust
port of the valve that sent the control signal. Relay piston
movement opens the modulator's exhaust, allowing air from
the piston's underside (and from the service brake chambers)
to exhaust through the modulator exhaust port.
ANTILOCK MODE: SOLENOIDS ACTIVATED
If a service application is made and the antilock controller
senses impending wheel lockup, it commands the M-12
™ /
M-12R ™ modulator to modify the service brake application
pressure. When activated by the controller, the M-12 ™ /
M-12R ™ modulator solenoids alter application by exhausting
and applying control air pressure above the relay piston.
The supply solenoid closes, preventing control line pressure
from entering the modulator. Then the exhaust solenoid
opens, allowing control pressure to exhaust from the top
side of the piston through the exhaust port of the solenoid
assembly. This activity occurs in a pulsating manner,
simulating "pumping the brakes."
DELIVERY
PISTON
EXHAUST
SUPPLY
EXHAUST
SOLENOID
CONTROL
DOUBLE
CHECK
TRAILER
CONTROL
BRAKE
VALVE
Figure 2 shows two antilock systems in which the M-12 ™
and M-12R ™ modulator can be used. Figure 2A is a typical
schematic for a full vehicle axle control system, and Figure
2B is a typical trailer installation using the M-12R
™
modulator. With the attachment of different controllers, the
M-12
™ or M-12R ™ modulator can adapt to either system.
OPERATION
APPLYING: NORMAL SERVICE APPLICATION
When a normal service brake application is made, and the
antilock wheel sensors do not sense impending wheel lock
up, service air pressure enters the modulator control port.
Air passes through the normally-open supply solenoid and
into the relay valve portion of the modulator. (The normally-
closed exhaust solenoid remains closed.)
Air acts on the relay piston closing the exhaust and opening
the inlet. Supply air pressure flows to the delivery ports and
out to the service brake chambers.
BALANCED POSITION: NORMAL SERVICE
APPLICATION
The relay valve portion of the modulator reaches a balanced
position when service delivery pressure acting on the
underside of the relay piston approaches that of the control
air acting on the top side of the piston. The piston moves
allowing the inlet valve to close, while the exhaust remains
closed. This prevents the valve from delivering or exhausting
air.
SUPPLY
SOLENOID
SERVICE BRAKE
CHAMBER
RESERVOIR