MCi Crusader II 1989 Maintenance Manual page 173

Date
1-1-89
Page·-. __
4-_43 _ _
MC-9 MAINTENANCE MANUAL
~SUPPLY FIESEFIVOIR
Figure 4-58. Purge Cycle Diag~am.
Purge Cycle -
Refer to figure 4-58. When air brake
system pressure reaches the cutout setting of the governor,
the compressor unloads (air compression stops) and the
purge cycle of the air dryer begins. The line connecting the
governor unloader port to the AD-4 end cover control port is
pressurized when the governor unloads the compressor. Air
pressure at the AD-4 end cover control port is also present.
on the purge valve piston. The purge valve piston moves in
response to air pressure causing the purge valve to open to
atmosphere.
Contaminants in the end cover sump are expelled imme-
diately when the purge valve opens. Also. air which was
flowing through the desiccant cartridge changes direction
and begins to flow toward the open purge valve. Oil and solid
contaminants collected by the oil separator are removed by
air flowing from the desiccant drying bed to the open purge
valve.
The initial purge and desiccant cartridge decompression
lasts only a few seconds and is evidenced by an audible
burst of air at the AD-4 exhaust.
The actual reactivation of the desiccant drying bed beg-
ins as dry air flows from the purge volume through the
desiccant cartridge purge orifice and into the desiccant
drying bed. Pressurized air from the purge volume expands
after passing through the purge orifice; its pressure is
lowered and its volume increased. The flow of dry air
through the drying bed reactivates the desiccant material by
removing the water vapor adhering to it.Generally 15 to 30
seconds are required for the entire purge volume of a stand-
ard AD-4 to flow through the desiccant drying bed.
The end cover single check valve prevents air pressure
in
the brake system from returning to the air dryer during the
purge cycle. After the 30-second purge cycle is complete.
the air dryer is ready for the next charge cycle to begin. The
purge valve will remain open after the purge cycle is com-
plete and will not close unt!I air brake system pressure is
reduced and the governor signals the compressor to charge.
MAINTENANCE
The desiccant cartridge should be replaced or rebuilt
when the desiccant is contaminated and does not have
adequate water adsorbing capacity. However, the follow-
ing checks should be made before replacing the desiccant
cartridge to ascertain that the water accumulation is not
related to the below listed conditions.
A. An outside air source has been used to charge the
· coach air system. This air did not pass through an air dryer.
B. Air usage is exceptionally high and not normal for the
coach. This may be due to accessory air demands or some
unusual air requirement that does not allow the compressor
to load and unload (compressing and ·non-compressing
cycles) in a normal fashion. Check for high air system
leakage.
C. The air dryer has been installed on a coach that has
been previously used with a defective air dryer. This sytem
will be saturated with moisture and several weeks of opera-
tion may be required to dry it out.
Every 900 operating hours or 25.000 miles. or every three
months. perform the following maintenance procedure:
1. Check for moisture in the air brake sytem by opening
reservoirs. drain cocks or valves. and checking for the pres-
ence of water. If mositure is present. the desiccant may
require replacement however. the conditions above should
be considered before the d~siccant cartridge is replaced.
NOTE: In areas where more than an ap-
proximate· 30°F (17°C) range of temper-
ature is common, small amounts of water
can accumulate in the air brake system
due to condenutlon. The presence of
small amounts of. water is normal.
In addition, a small amount of oil in
the system may be normal and should
not be considered a reason to replace
the desiccant. OIi-stained desiccant can
function adequately.
2. Check mounting bolts for tightness. Ret1ghten to 300
lb-,n (34 Nm) torque.
3. Check the operation of the check valve in the end
cover of the AD-4. Build the air system to governor cut-out
(115-118 psi/793-814 kPa) and observe the test air gauge
installed in the wet air tank reservoir. A rapid loss of pressure
could indicate a failed check valve. This can be confirmed by
checking at the purge valve exhaust.
NOTE: Purge valve will be open when
governor cut-out pressure is reached.
Allow two minutes for purge cycle before
tesUng the check valve.
4. Check for excessive leakage at the purge valve by
coating the exhaust with a soap solution while the compres•
sor ,s loaded (compressing air).
5. Check the operation of the safety valve (figure 4-59)
by pulling the exposed stem while the compressor is
loaded (compressing air). There must be an exhaust of air
while the stem is held and the valve should reseat when
the stem is released.