The pressure of the oil entering the bellows overcomes the tension of the bellows spring and permits the latch to retain
the air shut-off valve in the open position. If the oil pressure drops below a predetermined value, the spring in the
bellows will release the latch and permit the air shut-off valve to close and thus stop the engine.
The overspeed governor, used on certain applications, consists of a valve actuated by a se of spring-loaded weights.
Engine oil is supplied to the valve through a connection in the oil line between the bellows and the temperature-sensing
valve. An outlet in the governor valve is connected to the engine oil sump. Whenever the engine speed exceeds the
overspeed governor setting, the valve (actuated by the governor weights) is moved from its seat and permits the oil to
flow to the engine sump. This decreases the oil pressure to the bellows, thus actuating the shutdown mechanism and
stopping the engine.
A restricted fitting, which will permit a drop in oil pressure great enough to actuate the shutdown mechanism, is required
in the oil line between the cylinder block oil gallery and the shutdown sensing
To be sure the protective system will function properly if an abnormal engine condition occurs, have the system checked
periodically by your local Detroit Diesel Allison Service Outlet.
Also make sure the air shut-off valves close each time the engine is shut down.
To start an engine equipped with a mechanical shutdown system, first manually open the air shut-off valve and then
press the engine starting switch. As soon as the engine starts, the starting switch may be released, but the air shut-off
valve must be held in the open position until the engine oil pressure increases sufficiently to permit the bellows to retain
the latch in the open position.
During operation, if the engine oil pressure drops below the setting of the pressure sensitive bellows, the spring within the
bellows will release the latch and permit the air shut-off valve to close, thus stopping the engine.
If the engine coolant overheats, the temperature-sensing valve will open and permit the oil in the protective system to
flow to the engine crankcase. The resulting decrease in oil pressure will actuate the shutdown mechanism and stop the
engine. Also if the engine loses its coolant, the copper plug will be heated up by the hot exhaust gases passing over it
and cause the temperature-sensing valve to open and actuate the shutdown mechanism.
Whenever the engine speed exceeds the overspeed governor (if used) setting, the oil in the line flows to the sump,
resulting in a decrease in oil pressure. The oil pressure bellows then releases the latch and permits the air shut-off valve
When an engine is stopped by the action of the shutdown system, the engine cannot be started again until the particular
device which actuated the shutdown mechanism has returned to its normal position. The abnormal condition which
caused the engine to stop must be corrected before attempting to start it again.
AUTOMATIC ELECTRICAL SHUTDOWN SYSTEM
The automatic electrical shutdown system shown in Fig. 3 protects the engine against a loss of coolant, overheating of
the coolant, loss of oil pressure, or overspeeding. In the event one of the foregoing conditions arises, a switch will close
the electrical circuit and energize the solenoid switch, causing the shutdown solenoid to release the air shutdown latch
and stop the engine.
The electrical circuit is de-energized under normal operating conditions. When the engine is started, the oil pressure
switch opens when the oil pressure reaches approximately 10 psi (69 kPa) and the fuel oil pressure switch closes at
approximately 20 psi (138 kPa) fuel pressure. The water temperature switch remains open.
If the oil pressure drops below 10 psi (69 kPa), the oil pressure switch will close the circuit and energize the shutdown
solenoid. This will activate the shutdown mechanism and stop the engine.
A loss of coolant or an increase in coolant temperature to approximately 203 °F (95 ° C) will close the contacts in the
water temperature switch, thus closing the electrical circuit and activating the shutdown mechanism.
The water temperature switch consists of a temperature-sensing valve and a micro-switch. The valve