TM 5-3805-296-23-1
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AIR SYSTEM AND BRAKES
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Introduction
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The Military 621G Scraper uses four wheel drum brakes which are air actuated. There is a brake at each wheel of
the tractor and the scraper. The air system is broken into four different air circuits:
Air compressor and air dryer circuit
Service brake circuit
Park and secondary brake circuit
Accessory air circuit (differential lock)
Air Compressor and Air Dryer Circuit
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The air compressor is located on the left side of the engine. The air compressor governor controls the pressure of
the air compressor by sensing reservoir pressure. If reservoir pressure is low, nothing happens within the governor
and the compressor continues to function. When the reservoir pressure reaches the maximum cutout setting, the
air compressor governor diverts air pressure from the compressor to the compressor unloading valves. When the
reservoir pressure drops to the minimum cut-in pressure of 95 psi (655 kPa), the air compressor unloading valves
close and the compressor delivers air to the reservoirs.
The transmission/chassis ECM monitors pressure sensors in the reservoir and controls a solenoid valve for the air
compressor governor. The transmission/chassis ECM will command the solenoid valve for the air compressor gov-
ernor to close when the air pressure drops below 85 psi (586 kPa) in any of the air reservoirs. When the solenoid
valve closes, the air compressor governor will be disabled and the air compressor will continue to charge the air
system. The solenoid valve will open when the lowest air pressure in any of the air reservoirs is 115 psi (793 kPa)
and the governor will resume normal operation. The air compressor will continue to supply air pressure to the air
reservoirs until the air pressure reaches the maximum cutout setting.
The air compressor supplies air to the air dryer. The air dryer prevents the buildup of moisture in the air system.
The air dryer uses a desiccant cartridge to dry the air. Dry air is then sent to the air reservoirs. When the air com-
pressor governor turns off the air compressor, it will provide air pressure to the air dryer to open a drain valve at the
bottom of the air dryer. Any moisture that is stored inside the air dryer is then drained through a drain hose.
Service Brake Circuit
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When the brake pedal is pushed, pressurized air is sent from the brake air control valve to relay valves on the trac-
tor and the scraper. This air pressure opens the relay valves allowing air from the air reservoirs to flow to the brake
actuators. The service brakes are now applied.
The Military 621G Scraper can be stopped in an emergency by turning the park brake switch ON.
The service brakes can be used in order to disengage the park brakes for the tractor and scraper. When the brake
pedal is pushed, pressurized air is sent from service brake control valve to the double check valve forcing it open.
Air flows to the quick release and holding valve and finally to the tractor and scraper brake actuators. The air pres-
sure will compress the brake actuator springs disengaging the park brakes.
Park and Secondary Brake Circuit
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The park brake and secondary brake circuit can be activated manually by the park brake switch or automatically by
a pressure reduction in the system. When the operator turns the park brake switch to the ENGAGED position, the
park brake switch will send an electrical signal to the transmission/chassis ECM. The transmission/chassis ECM
will send an electrical signal to two park brake solenoid valves closing the valves. When the park brake solenoid
valves are in the CLOSED position, the air pressure in the park brake circuit will escape from the quick release and
holding valves. The park brakes are now engaged.
The quick release and holding valves reduce the time that is required to apply the park and secondary brakes. This
is done by hastening the exhaust of air pressure from the secondary brake chambers.The two double check valves
are located on the tractor and the scraper. The double check valve keeps the secondary brake piston and the ser-
vice brake diaphragm in the brake actuators from being activated at the same time.
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