If the oil pressure drops below 27 psi (186 kPa), the oil
operated automatic shutdown system which incorpo-
pressure switch will close the circuit and energize the
rates a hot wire relay and one oil pressure switch
shutdown solenoid. This will activate the shutdown
mechanism and stop the engine.
Since the fuel pressure builds up rapidly, the fuel oil
A loss of coolant or an increase in coolant temperature
pressure switch could close before the lubricating oil
to approximately 203° F (94° C) will close the contacts
pressure switch opens, thereby effecting a shut down of
in the water temperature switch, thus closing the
the engine. The hot wire relay, however, delays the
electrical circuit and activating the shutdown
closing of the fuel oil pressure switch for 3 to 10
seconds to enable the lubricating oil pressure to build
up and open the oil pressure switch contacts.
The water temperature switch consists of' a tempera-
ture-Tensing valve and a micro switch. The valve
When the lubricating oil pressure falls below 10± 2
contacts a copper plug (heat probe) which extends into
psi (69± 14 kPa), the contacts in the oil pressure
the exhaust manifold outlet. Engine water is directed
switch used in this system will close and current will
over the power element of the valve and should the
flow to the hot wire relay. The few seconds required to
water temperature exceed approximately 203° F
heat the hot wire relay provides sufficient delay to
(94° C), the valve will close the contacts in the micro-
avoid an engine shut dawn when low oil pressure is
switch and energize the shutdown circuit. If a loss of
caused by a temporary condition such as an air bubble
water occurs, the heat of the exhaust gases will be
or a temporary overlap in the operation of the oil
transmitted through the copper plug to the tempera-
pressure switch and the fuel oil pressure switch when
ture sensing valve and cause the shutdown circuit to
starting or stopping the engine.
Depending upon the particular shutdown system used.
If the engine speed exceeds the high speed setting of
the high water temperature switch may be installed in
the overspeed governor, the No. 2 governor switch will
one of the openings in the water manifold, or the
close and activate the shutdown mechanism.
temperature sensing valve and micro-switch unit may
he installed in the exhaust manifold outlet.
When the engine is shut down. the decrease in speed
will open the governor switches and the decrease in oil
In some electrical shutdown applications, the tempera-
and fuel pressures will close the oil pressure switches
ture sensing valve and the copper plug used in the
and open the fuel pressure switch, thus de-energizing
mechanical shutdown system is utilized. The lubricat-
ing oil pressure switch is installed in the oil line to the
valve. When an increase in water temperature causes
The cause of the abnormal conditions must then be
the valve to open and permits the oil to flow back to
determined and corrected before the engine is started
the crankcase. the resulting decrease in oil pressure
again. Also, the air shut off valve must be manually
will close the oil pressure switch and energize the
reset in the open position before the engine can be
electrical shutdown circuit. Low engine oil pressure
will also cause the contacts in the oil pressure switch to
close and energize the shutdown circuit.
Some engines are equipped with an electrically
Likewise, if the oil pressure drops below the setting of
The alarm system shown in Fig. 6 is similar to the
the oil pressure switch, the switch will close and cause
automatic electrical shutdown system, but uses a
the bell to ring. The bell will continue to ring until the
warning bell in place of the air shut off valve solenoid.
engine operator turns the alarm switch off. The alarm
The bell warns the engine operator if the engine
switch must also be turned off before a routine stop
coolant overheats or the oil pressure drops below the
since the decreasing oil pressure will close the oil
safe operating limit.
pressure switch and cause the bell to ring.
When the engine is started and the oil pressure is
If the alarm bell rings during engine operation, stop
sufficient to open the oil pressure switch contacts
the engine immediately and determine the cause of
(opening pressure is stamped on the switch cover). the
the abnormal condition. Make the necessary correc-
alarm switch must be turned on manually to put the
tions before starting the engine again.
system in operation. The water temperature switch is
normally open. Should the engine coolant exceed 200-
An alarm bell may be connected to the electrical
205° F (93-95° C), the water temperature switch will
shutdown system as shown in Fig. 7. In this system. if
close the electrical circuit and sound the alarm bell.