TM-9-2320-272-10
TRUCK, 5-TON, 6X6, M939, M939A1, AND M939A2 SERIES TRUCKS (DIESEL)
TECHNICAL MANUAL; OPERATOR'S MANUAL
SEPTEMBER 2004
CHANGE 2
TM-9-2320-272-10 - Page 80 of 481
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TM 9-2320-272-10
Change 2
1-63
1-20. COMPRESSED AIR AND BRAKE SYSTEM OPERATION (Contd)
(2) The secondary constant pressure system is made up of the following
components:
I
K
L
D
E
A
LOW AIR PRESSURE SWITCH
–
Activates warning buzzer and warning
lights when air pressure goes below 60 psi (414 kPa).
PEDAL VALVE
– Allows air pressure from secondary constant pressure
system to flow into secondary signal system when operator depresses brake
pedal.
SECONDARY AIR PRESSURE GAUGE
–
Indicates amount of air
pressure in secondary system.
ONE-WAY CHECK VALVE
– Allows air pressure to flow into secondary
reservoir but prevents it from coming out if constant pressure system fails or
engine stalls.
SECONDARY AIR RESERVOIR
– Stores enough air pressure so the
operator can make five normal brake applications before running out of air if
constant pressure fails or engine stalls.
INTERMEDIATE REAR BRAKE CHAMBERS
–
Converts air pressure to
mechanical force which applies intermediate rear service brake.
DOUBLECHECK VALVE #1
–
Serves two functions:
• Allows system to receive signal pressure from either pedal valve or, when
towed, from brake system of towing vehicle.
• Serves as a tee between front and rear primary signal lines.
• Removed when ABS kit is installed and replaced with a cross tee.
SECONDARY AIR RESERVOIR DRAINVALVE
–
Provides a drain for
moisture and air from secondary air reservoir.
FRONT RELAY VALVE
–
Boosts signal air to rear brake chambers;
regulates air pressure to rear brake chambers so operator has control over
amount of braking; and releases air pressure to rear brake chambers directly
to vent when brake pedal is released (not used if ABS kit is installed).
(ABS) FRONT RELAY VALVE WITH ELECTRONIC CONTROL UNIT
(ECU)
–
Boosts signal air to left rear brake chamber and left intermediate
bake chamber controling brake lock up by venting air to chambers on left side
of vehicle if left wheels lock up when braking.
STOPLIGHT SWITCH
–
As the brake pedal is depressed, switch receives
an air pressure signal at electrical contacts which close to activate circuits to
taillights.
DOUBLECHECK VALVE #2
– Allows either primary or secondary signal
air pressure to activate stoplight switch while keeping the two systems
separate.
REAR-REAR BRAKE CHAMBERS
– Converts air pressure to mechanical
force which applies rear-rear brakes.
C
B
G
M
F
J
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