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 TM 9-2320-312-24-1
THEORY OF OPERATION - CONTINUED
0003 00
AIR SYSTEM
1.
The air system supplies clean, dry compressed air to operate the tractor airbrake system and to control miscellaneous air-
operated components (fifth wheel unlock and driver's seat). The system consists of an engine-mounted air compressor
with governor, air dryer, three air reservoirs with pull-cord drain valves, and air lines and fittings.
2.
A dual air pressure gauge on the instrument panel monitors air pressure in both primary and secondary air reservoirs.
BRAKE SYSTEM
1.
The M878A2 is equipped with a standard dual airbrake system. Compressed air, generated by the engine-mounted air
compressor, is stored in three air reservoirs and distributed, upon demand from the floor-mounted brake pedal, through
air lines and valves to airbrake chambers at each wheel.
2.
The airbrake system features the following:
a.
floor-mounted brake pedal (two joined pedals) to operate the tractor (and trailer) brakes;
b.
manually-operated parking brake control valve to apply and release the tractor parking brakes;
c.
manually-operated trailer brake control valve to apply and release the trailer service brakes independently of the
tractor brakes; and
d.
manually-operated trailer air supply control valve to apply and release the trailer parking brakes.
3.
The mechanical components of the brake system are automatically-adjusting slack adjusters on front axle, manual slack
adjusters on rear axle, s-cam camshafts, and 16.5 in brakeshoes.
STEERING SYSTEM
1.
The power steering system consists of a engine-mounted hydraulic pump, hydraulic reservoir, power steering gear, and
hydraulic hoses. The hydraulic pump provides the power-assist for the steering system.
2.
Steering linkage from the steering gear connects to the left side of the Eaton E-1460I front steer axle.
FIFTH WHEEL SYSTEM
1.
The M878A2's fifth wheel can be hydraulically raised 31 in (79 cm), using a control lever on the instrument panel. This
feature allows for the following:
a.
adjustment of fifth wheel height to differing trailer coupling heights;
b.
moving ("spotting") trailers without having to raise their landing gear; and
c.
negotiating steep ramps during RO/RO procedures.
FIFTH WHEEL
FIFTH WHEEL
FRAME SUB-
ASSEMBLY
HYDRAULIC
CYLINDERS
HYDRAULIC
RESERVOIR
0003 00-3
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