Vehicle Operation Systems (Cont)
1-15. AIR SYSTEMS.
The air system (fig. 1--17) consists of an engine driven
air compressor (1) and four air reservoirs (2, 3, 4, and 5). Reservoir (4) is used on
all vehicles except the M983 tractor which has reservoir (6) instead.
The air system includes the necessary valves and air lines to control the vehicles
air operated parts. Pressurized air from the air compressor is passed through the
air dryer (7) to the quick buildup reservoir (2). The air dryer removes dirt and
moisture from the pressurized air.
For Non-FHTV model vehicles, air from reservoir (2) goes to the throttle treadle
(8). Depending on how far the throttle treadle is depressed, 0 to 60 psi (0 to 414
kPa) is supplied to the engine throttle air cylinder (9) and to the transmission
modulator (10). This air pressure controls the vehicle speed. For FHTV model
vehicles, vehicle speed is controlled electronically.
Once air pressure in reservoir (2) rises above 75 psi (517 kPa), a valve opens and
allows reservoirs (3, 4 or 6, and 5) to be pressurized up to 120 psi (827 kPa). Air
from reservoir (4 or 6) goes to the brake treadle valve (11). This air controls the
rear axle service parking brakes (12). Air pressure in this system is shown by the
red needle on the AIR PRESS gage (13). Air from reservoir (3) goes to the brake
treadle valve (11). This air controls the front axle service brakes (14). Air
pressure in this system is shown by the green needle on the AIR PRESS gage
The PARKING BRAKE valve (15) controls air from reservoirs (3 and 5) and
applies or releases the rear axle service (parking) brakes. Reservoirs (3, 4 or 6,
and 5) are interconnected so that if one reservoir fails, air is supplied to release
the rear axle service (parking) brakes from whichever reservoir is functioning. If
air pressure falls below 60 to 75 psi (414 kPa to 517 kPa), a buzzer will sound and
the AIR indicator (16) will light. Later versions of the parking brake valve
incorporate an automatic feature that applies the parking brakes when system
air pressure drops to 30 psi (206 kPa) or less.
On the M984A1 the front brake actuator valve (17) is used to apply the front axle
service brakes when using heavy-duty winch.