PREVOST X3-45 Multiplex, Maintenance Manual

Page: 626 / 1422

4
Rapid Exhaust (Figure 7) -
The Rapid Exhaust operation
described in the following text occurs when the modulator is
controlling service chamber(s). During a rapid brake release
the quick release modulator will exhaust air differently to a
“slow” brake release.
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, the air previously delivered
to the brake chamber is vented through the M-32
™
modulators as follows:
For the M-32QR ™ Modulator:
The bias valve moves to its
closed position, closing the air return route to the brake
valve’s exhaust. Air pressure against the exhaust valve within
the M-32
™
modulator overcomes the spring force and allows
air to exhaust through the M-32QR
™
modulator exhaust port.
Residual air pressure between the bias valve and the brake
pedal flows back to the brake valve exhaust.
For the M-32 ™ Modulator:
As in the “slow” brake release,
air pressure travels back to the brake valve’s exhaust, but
also the air pressure against the exhaust valve within the
M-32
™
modulator overcomes the spring force and allows air
to exhaust through the M-32
™
modulator exhaust port.
ANTILOCK OPERATION
GENERAL
If a service brake application is made and the antilock system
detects an impending wheel lockup, the antilock controller
will make a controlled brake application using the modulator.
In order to control the brake application, the coils of the two
solenoid valves contained in the modulator are energized or
de-energized in a preprogrammed sequence by the antilock
controller. When a solenoid coil is energized, and depending
whether the exhaust or hold solenoid is energized, it either
opens or closes, thereby causing the exhaust or
reapplication of air pressure to the brake actuator. The
solenoids in the modulator are controlled independently by
the antilock controller (ECU).
An experienced driver (of a vehicle without ABS) who
encounters wheel lock-up may sometimes “pump the brakes”
in order to attempt to prevent wheel lock-up and maintain
vehicle control. In the case of an ABS braking system, the
driver does not need to “pump the brakes” since the antilock
controller is able to apply and release the brakes using the
modulators, with far greater speed and accuracy. Depending
on the number of modulators used, some systems are able
to apply braking power to wheels independently (see page
2).
ANTILOCK EXHAUST
(Figure 8)
When wheel lock is detected or imminent, the antilock
controller energizes the supply and exhaust solenoids in
the modulator.
Energizing the supply solenoid allows application air to flow
to the control side of the supply diaphragm. Air pressure
acting on the supply diaphragm, along with the spring force,
enables the diaphragm to prevent further delivery of air to
the brake chamber.
Energizing the exhaust solenoid shuts off the air normally
applied to the control side of the exhaust diaphragm to keep
it closed. Air pressure acting on the exhaust diaphragm,
overcomes the spring force, and allows air to exhaust through
the exhaust port.
ANTILOCK HOLD MODE
(Figure 9)
The antilock controller will place the modulator in the Hold
position when it senses that the correct wheel speed (braking
force) has been attained. The antilock controller will also
FIGURE 7: M-32
™
AND M-32QR
™
MODULATORS RAPID NON-ANTILOCK EXHAUST OF SERVICE BRAKES
Supply or Hold
Diaphragm
Open
Spring
Brake
Chamber
Brake
Valve
Exhaust
Diaphragm
Exhaust
Valve
Open
Exhaust
Port
M-32
™
Modulator
M-32QR ™
Modulator
Bias
Valve
Delivery
Port
Supply
Port
Supply or Hold
Diaphragm
Open
Spring
Brake
Chamber
Brake
Valve
Exhaust
Diaphragm
Exhaust
Valve
Open
Exhaust
Port
Delivery
Port
Supply
Port