TM 9-2330-294-14
system air from the semitrailer air reservoirs. When
(2) Test.
the air system is depressurized, the fail-safe springs
(a ) Observe low pressure lamp in towing
vehicle as semitrailer air system is pressurized or
apply the semitrailer brakes. The manual release
valve is used to release the fail-safe brakes when the
depressurized. The lamp lights at pressures below 65
psi and goes out at higher pressure.
semitrailer air system is depressurized. The valve
control handle is pushed IN (and will stay IN as long
(b ) As an alternative to (a) above connect an
as the main air system is depressurized). This action
ohmmeter across the two terminals of the switch
connects the fail-safe air chamber to the fail-safe
release air tank, thus pressurizing the fail-safes and
pressurized or depressurized. The resistance should
be near zero at pressures below 65 psi and very high
releasing the semitrailer brakes. (The fail-safe
at pressures above 65 psi.
release tank is isolated from the main air system by
k. Two-Way Check Value Operation and Test.
one-way check valve. This allows the release tank to
remain pressurized even though the rest of the air
(1) Operation (fig. 4-59 and 4-60). Two-way
system is not under pressure).
check valves are used in the suspension air system to
(2) Test.
deliver or to shut off air from the various regulator
(a ) Pressurize fail-safe brake release tank
valves. This valve is tee shaped with two inlet ports
either by pressurizing semitrailer air system or
opposite each other and one outlet port. During
through the fail-safe brake release tank fill valve (fig.
operation air is admitted from the inlet port with the
highest pressure and directed to the outlet port. The
second inlet port is automatically sealed off.
(b ) Disconnect intervehicular emergency air
line.
(2) Test.
(c ) Repressurize main air reservoir by
(a ) Pressurize air system 90 to 120 psi).
opening drain valve in bottom of one tank.
(b ) Verify that air at 85 psi, 50 psi and 15 psi
(d ) Verify 90 to 120 psi at fail-safe release
is available to the suspension system by checking
tank fill valve (fig. 2-3) by checking pressure with a
t h e tank valves on the suspension pressure
tire air gage.
regulators using the towing vehicle tire air gage (fig.
(e ) Position fail-safe manual brake release
(c ) Verify that the suspension shutoff valves
valve handle to the OUT position.
CAUTION
(d ) S e l e c t F U L L L O A D p o s i t i o n o n
Do not use pliers to loosen fitting. Use only
suspension air control valve and allow ap-
proper wrench. Unscrew fitting only a few
proximately 3 minutes for the system to react.
threads,
(e ) Replace the two-way check valve in the
(f ) Loosen an air fitting on air line leading
85 psi line if the air suspension springs do not inflate
from manual release valve to fail-safe brake
or if air vents continuously from the suspension air
chambers (fig. 4-27 and 4-28). No air should escape
control valve.
unless handle is pushed IN.
(f ) S e l e c t H A L F L O A D p o s i t i o n o n
(g ) Push release valve handle IN. Air should
suspension air control valve a n d allow ap-
escape from the loosened fitting.
proximately 3 minutes for the system to react.
(h ) Replace valve if air leaks when handle is
(g ) Replace both two-way check valves if air
OUT, or if it does not flow when the handle is IN.
vents continuously from the air control valve.
(h ) If the air bags are not inflated when the
i. Pressure Regulator Values Operation and Test.
air control valve is in the HALF LOAD position (f )
(1) Operation (fig. 4-27, 4-28, 4-59, and 4-
above either or both two-way check valves may be
60). Four pressure regulator valves are used in the
faulty: continue to (i ) below.
semitrailer suspension system. In each case the
(i ) Select NO LOAD position on suspension
regulators supply air at a constant pressure to some
air control valve and allow system approximately 3
other component of the semitrailer. Suspension
minutes to react.
regulators supply air at 15, 50, 85, and 100 psi.
(j ) Replace both two-way check valves if air
(2) Test. The 15 psi brake air system regulator
leaks continuously from air control valve.
is tested and adjusted in a similar manner to the
(k ) Replace two-way check valve in 50 psi
suspension regulator valves. See c (2) above,
line if air springs are inflated in (i ) or (f ) above.
j. Low Pressure Warming Switch Operation and
(l ) Replace two-way check valve in 85 psi
Test.
line if air bags do not inflate in either (i ) or
(1) Operation (fig. 4-27 and 4-28). This switch
is part of a circuit that warns the M746 driver when
(f ) above.
semitrailer air system pressure drops below 65 psi.
(m ) Replace two-way check valve in 50 psi