D346 MARINE ENGINE Caterpillar

Shutoff Devices



Emergency shutoff devices stop the engine when operating conditions are such that engine damage may result if operation is continued. If a shutoff device has stopped the engine, see the topic ALARM SWITCHES for an aid in troubleshooting. Additional measures may be required depending upon the circumstances and conditions present. Authorized dealer personnel should analyze major engine problems.

Familiarize yourself with the types and locations of the shutoff devices on your engine, the conditions which can cause each control to function, and the resetting procedures required to start your engine.

Have all shutoff controls on the engine checked twice a year by your Caterpillar dealer. To test the devices, abnormal operating conditions must be simulated which could cause engine damage if the tests are incorrectly performed.


If severe or prolonged overheating of the engine has occurred, contact your Caterpillar dealer to have your engine checked for damage.


Always determine the cause of the shutdown, and have the necessary repairs made before restarting your engine.

Electric Shutoff Controls

The operation of all electric shutoff controls is similar: A critical operating condition actuates a switch in the sensing unit of the electric shutoff control. The switch closes the circuit to the shutoff solenoid which moves linkage to bypass the fuel to the cylinders; thus stopping the engine. The shutoff control may require resetting before the engine can start.

High Water Temperature Shutoff

The shutoff switch is located in the water temperature regulator housing. Excessive water temperature closes the switch. No resetting procedure is required: As the coolant cools, the switch opens.


The sensing element must be submerged in the coolant to operate. A low coolant level cannot actuate the shutoff.

Low Oil Pressure Shutoff Switch

This device is usually mounted on the side of the engine with oil lines connected to the switch. Low oil pressure closes the switch.

Manually operated systems require resetting by pushing the button on the switch until it latches. After the engine is started and oil pressure builds, the knob will return to the extended position.


This switch cannot provide protection if the reset knob does not move to the extended position after the engine starts.

Automatic start-stop systems use a switch which resets itself.

Overspeed Shutoff Switch (Electro-Mechanical)

This switch is mounted either on the tachometer drive or on the governor. Excessive engine speed closes a switch by centrifugal force.

To reset the switch, push the button marked RESET. The knob will remain down until the engine overspeeds.

Overspeed Shutoff Switch (Electronic)

This switch uses a flywheel housing mounted magnetic pickup to sense engine speed. The control box is mounted on the engine block. Excessive engine speed closes a circuit breaker. To reset switch, push arm of breaker to the run position.

Fuel/Oil Pressure Switch

This switch is mounted in the fuel manifold between the clean side of the fuel filter and the fuel injection pump housing. Low fuel pressure (below 3 PSI, 2 kg/cm2) opens the switch after the engine has been stopped by one of the above shutoff controls. This switch prevents the battery from discharging after the engine is stopped.

The switch can be used to de-energize the shutoff solenoid after stopping the engine, or to open the circuit between the alternator regulator and the charging alternator after the engine is stopped.

This switch is also used on automatic start-stop applications equipped with an electric governor. In this application, when adequate oil pressure is reached, the switch closes to allow the electric governor to control engine speed.

This switch does not require resetting.

Mechanical Shutoff Control

The mechanical shutoff control consists of a low oil pressure and an engine overspeed shutoff device. On 6 cylinder engines, the mechanical shutoff is mounted on the side of the engine. On V-engines, the shutoff is mounted in the vee between the two banks of cylinders.

A high water temperature shutoff may be installed in the temperature regulator housing.

If either an overspeed condition, or a low oil pressure condition occurs, a spring-loaded rod is tripped inside the control which moves and holds the rack in the shutoff position. If a high water temperature condition occurs, the excessive heat causes a valve to move in the temperature sensor, which dumps oil flowing to the low oil pressure shutoff. The low oil pressure actuates the low oil pressure shutoff to stop the engine.

Long periods of cranking can trip the low oil pressure shutoff and prevent the engine from starting. The shutoff control must then be reset before the engine can be started.


Normal stopping should not trip the shutoff control. The basic cause for the shutoff control to stop the engine can be determined by the manner in which the shutoff control must be reset:

1. Push the reset lever (or handle).

If the rod latches, an overspeed condition stopped the engine; and a cause for overspeeding should be investigated.

If the rod will not latch, the engine was stopped because of a low oil supply to the shutoff control. This could be caused either by low oil pressure in the engine; or by high water temperature in the cooling system. Therefore, investigate both the lubrication system and cooling system for the cause of the shutdown.

Reset the control by the following method:

1. Push the reset button firmly.

2. Push the reset lever (or handle) until the rod latches.

If the reset lever does not move freely, check the reset cable for kinks, binds, or dry lubrication. Make needed repairs or adjustments.

Air Inlet Shutoff Control

The air shutoff control is an attachment to the mechanical safety shutoff described above. If an overspeed, low oil pressure, or high water temperature condition occurs, the spring loaded rod in the shutoff control is released which pushes the rack into the shutoff position and also pulls a plate shut in each inlet manifold of the engine. Both the air inlet shutoff and the mechanical shutoff must be reset.

If an overspeed condition shut the engine down:

1. Pull the reset lever on the air shutoff control to the RESET position.

Release the lever. The lever should latch in the RUN position.

2. Push the reset lever on the mechanical shutoff control until the rod latches.

If either a low oil pressure, or a high water temperature condition shut the engine down:

1. Pull the reset lever on the air shutoff control to the RESET position.

Release the lever. The lever should latch in the RUN position.

2. Firmly push the reset button on the mechanical shutoff control.

3. Push the reset lever on the mechanical shutoff control until the rod latches.

Hydraulic Shutoff Controls

The hydraulic shutoff control is mounted on (or near) the governor. When oil pressure drops below 8 PSI, the fuel injection pumps will bypass the fuel from the cylinders.

To start the engine, move and hold the reset lever against the stop while cranking the engine. After the engine starts, the lever will stay in the running position.

A high water temperature shutoff can be used with this shutoff control Excessive coolant temperature causes the temperature sensor to open a dump valve which empties the pressure oil from the low oil pressure shutoff device. The low oil pressure shutoff control shuts down the engine.


The water temperature sensor must be submerged in the coolant to operate. A low coolant level, or a failed water pump cannot actuate the shutoff.

If excessive water temperature stopped the engine, make necessary repairs, allow the water sensor to cool (no resetting of the water temperature shutoff switch is required) and start the engine as instructed above.

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