Fig. 3. - Automatic Electrical Shut-Down System Diagram.
contacts a copper plug (heat probe) which extends into the exhaust manifold outlet. Engine water is directed over the
power element of the valve and should the water temperature exceed approximately 203° F (95° C), the valve will close
the contacts in the micro- switch and energize the shutdown circuit. If a loss of water occurs, the heat of the exhaust
gases will be transmitted through the copper plug to the temperature-sensing valve and cause the shutdown circuit to be
If the engine speed exceeds the high speed setting of the overspeed governor, the governor switch will close and
activate the shutdown mechanism.
When the engine is shut down, the decrease in speed will open the governor switch, and the decrease in oil and fuel
pressures will close the oil pressure switch and open the fuel pressure switch, thus de-energizing the circuit.
The cause of the abnormal conditions must then be determined and corrected before the engine is started again. Also,
the air shut-off valve must be manually reset in the open position before the engine can be started.
Fig. 4. - Automatic Electrical Shut-Down System Incorporating Hot Wire Relay.
Some engines are equipped with an electrically operated automatic shutdown system which incorporates a hot wire relay
(Fig. 4). Since the fuel pressure builds up rapidly, the fuel oil pressure switch could close before the lubricating oil
pressure switch opens and stop the engine. The hot wire relay, however, delays the closing of the fuel oil pressure switch
for several seconds to enable the lubricating oil pressure to build up and open the oil pressure switch contacts.
When the lubricating oil pressure falls below 10 ± 2 psi (69 ± 14 kPa), the contacts in the oil pressure switch used in this
system will close and current will flow through the hot wire relay to the solenoid. The few seconds required to heat the
hot wire relay provides sufficient delay to avoid stopping the engine when low oil pressure is caused by a temporary
condition such as an air bubble or a temporary overlap in the operation of the oil pressure switch and the fuel oil pressure
switch when starting or stopping the engine.
The water temperature switch, which remains open during normal engine operation, is installed in the side of the
thermostat housing. The switch contacts close when the water temperature reaches approximately 205 ° F (96 ° C) and