TM 9-2330-294-14
Figure 4-26. Fail-safe chamber assembly schematic diagmm (semitrailer No. 201 and on ).
the emergency relay valves. The emergency relay
d. Fail-Safe Unit Manual Compression and
valves, mounted between the rear and forward axles,
Release (fig. 4-25 or
4-26).
CAUTION
speed brake action by releasing air from the main air
Do not force release bolt beyond its normal
reservoir to the brake chambers. Air pressure causes
stop when turning in either direction.
the wedge in each brake actuating assembly to move
NOTE
forward and press the brake shoes against the drum.
Compressing and releasing fail-safe unit
The emergency relay valves route service air to the
power spring is made easier if air pressure is
brakes if the parking brake is applied (M746 tractor)
used to take the spring load off the release
and also automatically apply the brakes if the
nut. In addition, the normal stops are more
semitrailer breaks away from the towing vehicle.
perceptible.
Three main air tanks and a fail-safe release tank are
To compress fail-safe power spring, turn release bolt
located between frame cross-members of semitrailer
clockwise approximately 25 full turns. To release
No. 1 through 200. Two main air tanks and a fail.
fail-safe power spring, turn release bolt coun-
safe release tank are located between frame cross-
terclockwise approximately 25 full turns.
menbers of semitrailer No. 201 and on. On
e. Brake Compressed Air System (fig. 4-27 or 4-
semitrailers No. 1 through 200 the low-pressure
warning switch relays a signal to the towing vehicle
28), Compressed air is transmitted from the towing
vehicle through the service and emergency air lines
cab low-pressure warning light (M 746) when air
pressure falls below 65 psi. Refer to paragraphs 1.
that are coupled to the towing vehicle at the front of
the gooseneck. Air pressure in the range of 90 psi to
6e and 2-2 through 2-4 for a detailed description of
120 psi is kept in the system through the emergency
4-39 for service instructions for air lines, fittings and
air line when the towing vechicle engine is running.
valves.
When the brake pedal in the towing vehicle is
applied, air is delivered through the service air line to