G343 ENGINE Caterpillar

Systems Operation


Gas, Air Induction, And Exhaust Systems

In addition to components illustrated in the above diagram, some installations have a safety shut-off-valve attachment in the gas supply line. The valve is electrically operated from the ignition system and can also be manually operated to shut off the engine. Manual resetting is required after shutdown to start the engine.


(Engines so Equipped)

The turbocharger is mounted on the exhaust outlet of the engine exhaust manifold. The energy that normally would be lost from the exhaust gases is used to drive the turbocharger.

As the engine starts, the flow of exhaust gases from the exhaust manifold is directed through inlet (5) to the turbine wheel (6). The turbine wheel and compressor impeller (1) are mounted on a common shaft. The exhaust gases pass over the turbine wheel forcing it and the impeller to rotate.

Incoming air, from the air cleaner, flows through inlet (7) to the center of the impeller. The rotating impeller compresses the air and forces it into the carburetor.

The turbocharger bearings are pressure-lubricated by engine oil. The oil enters through port (4) and is directed through passages to lubricate the thrust bearing (3), rings (9 and 11) and the journal bearings (10). Oil leaves the turbocharger through a port in the bottom of the center section and is returned to the engine sump.

1-Compressor impeller. 2-Compressor housing. 3-Thrust bearing. 4-Lubrication inlet port. 5-Exhaust inlet. 6-Turbine Wheel. 7-Air inlet. 8-Exhaust outlet. 9-Ring. 10-Shaft journal bearings. 11-Ring. 12-Turbine housing.

Pressure Differential Regulator

(Engines Equipped With Turbocharger)

The pressure differential regulator mounted on the exhaust manifold extension, serves as an exhaust gas bypass valve. It limits the volume of exhaust gases to the turbine wheel by bypassing a part of the exhaust gases. The exhaust bypass valve is actuated directly by a pressure differential between the atmosphere and turbocharger compressor outlet pressure to the carburetor.

One side of the diaphragm in the regulator senses atmospheric pressure through a breather in the top of the regulator. The other side of the diaphragm senses air pressure from the turbocharger compressor outlet side through a control line. When outlet pressure to the carburetor reaches the proper value, the force of the air pressure upon the diaphragm overcomes the force of the spring and atmospheric pressure. This force unseats the bypass valve, allowing exhaust gases to bypass the turbine wheel.

The bypass passage is in an external pipe between the exhaust manifold extension and exhaust elbow. Under steady load conditions, the valve will assume a fixed position, allowing just enough exhaust gas to the turbine wheel to maintain the proper air pressure to the carburetor.

PRESSURE DIFFERENTIAL REGULATOR (Engines Equipped with Turbocharger)


Air enters the carburetor through air horn (3) and fills outer chamber (4). Air enters inner chamber (7) (mixing chamber) by forcing diaphragm (6) away from ring (14). There are two diaphragms, one on each side of the carburetor. Fuel enters the carburetor through fuel inlet (2), past the power mixture adjustment (5) to the center of the carburetor and into fuel outlet tube (9). Fuel valve (12) is mounted on diaphragm (6). With the diaphragm forced away from ring (14), fuel flows past fuel valve (12) and into inner chamber (7). The fuel and air mix in inner chamber (7). This mixture is drawn down past throttle plate (16) and into the inlet manifold.

With the engine stopped, spring (11) holds diaphragm (6) against ring (14) and holds fuel valve (12) closed. No air or fuel can flow to inner chamber (7). As the engine is started, the vacuum in the cylinders, created by the intake stroke of the pistons, creates a low pressure situation in inner chamber (7). This low pressure is sensed by chamber (8), behind the diaphragm through holes (13). This allows the pressure in chamber (8) to equalize with the low pressure situation in the inner chamber. The higher inlet pressure acting on the outer portion of the diaphragm overcomes the spring force and moves the diaphragm outward. This also moves fuel valve (12) outward and allows air and fuel to flow into the inner chamber. A small volume of air is also metered into the inner chamber through idle adjustment opening (15).

CARBURETOR OPERATION (Operative position shown)
1-Balance line inlet. 2-Fuel inlet. 3-Air horn. 4-Outer chamber. 5-Power mixture adjustment. 6-Diaphragm (two). 7-Inner chamber. 8-Chamber (two). 9-Fuel outlet tube. 10-Carburetor body. 11-Spring (two). 12-Fuel valve (two). 13-Sensing holes. 14-Ring (two). 15-Idle adjustment opening. 16-Throttle plate.

Line Pressure Regulator

1-Spring side chamber. 2-Adjusting screw. 3-Spring. 4-Outlet. 5-Valve disc. 6-Main orifice. 7-Main diaphragm. 8-Lever side chamber. 9-Lever. 10-Pin. 11-Valve stem. 12-Inlet.

A line pressure regulator is required on turbocharged engines. Also, naturally aspirated engines with a gas supply pressure exceeding the desired inlet pressure differential between the gas and air must use a line pressure regulator.

The regulator is adjusted by turning the adjusting screw (2).

Gas flows through the inlet (12), main orifice (6), past the valve disc (5), and through the outlet (4). The chamber (8) on the lever side of diaphragm (7) senses outlet pressure.

As gas pressure in chamber (8) increases to a value higher than the force of the diaphragm spring (3) and air pressure in the spring side chamber (1) (atmosphere on naturally aspirated engines; turbocharger boost on turbocharged engines), the diaphragm is forced against the spring. This pivots the lever (9) at pin (10) and causes the valve stem (11) to move the valve disc to close the inlet orifice. As the diaphragm moves in this manner, air is expelled from the spring side chamber. This movement of air forces a vent valve open and the trapped air is permitted to escape. This is accomplished quickly enough to prevent any lag in the main diaphragm movement due to air compression.

With the inlet orifice closed, gas is pulled from the lever side chamber through the outlet. This reduces the pressure in the chamber and becomes less than pressure in the spring side chamber. Spring and air pressure in the spring side chamber forces the diaphragm toward the lever. This pivots the lever and opens the valve disc, allowing additional gas flow to the carburetor.

When the pressures on either side of the diaphragm are equal, the regulator supplies gas to the carburetor at a constant pressure differential above the incoming air.

Balance Line

The balance line is used only on turbocharged engines. It maintains the correct pressure differential between the line pressure regulator and carburetor inlet.

As the load on the engine changes, turbocharger boost pressure changes in the inlet manifold. The balance line communicates this change in pressure to the spring side of the diaphragm in the line pressure regulator. This pressure change acts to assist the regulator diaphragm to position the line regulator valve to correct the gas pressure to the carburetor. Thus, the correct pressure differential between the line pressure regulator and carburetor inlet is maintained.

Cooling System

Radiator Or Heat Exchanger

1-Aftercooler. 2-Water temperature regulator housing. 3-Radiator. 4-Cylinder head. 5-Auxiliary water pump. 6-Diesel engine cylinder block. 7-Diesel engine water pump. 8-Oil cooler.

Coolant is circulated by a gear driven centrifugal-type water pump. Temperature regulators located at the front of the cylinder head, restrict coolant flow through the radiator, until the coolant reaches operating temperature.

Coolant from the engine water pump is directed through a passage in the flywheel housing to the engine oil cooler. From the engine oil cooler, coolant is directed through the external piping into the cylinder block, around the cylinder liners, and into the cylinder head to cool the area around the cylinder head valves. Coolant is then directed to the water temperature regulator housing.

Until the coolant reaches the temperature required to open the temperature regulators, coolant bypasses the radiator and flows directly back to the water pump.

A pressure relief cap assembly is used to control the pressure in the cooling system. The cap allows pressure (and some water if the cooling system is too full) to escape when system pressure exceeds the relief pressure of the cap.

Raw water is circulated by a gear-driven, centrifugal-type water pump. Coolant is directed to the aftercooler and then to waste.

Lubricating System

1-Camshaft journal oil supply line. 2-Turbocharger oil supply line. 3-Oil filter. 4-Oil manifold in cylinder block. 5-Turbocharger oil drain line. 6-Oil pump (located within oil pan). 7-Oil pan (sump). 8-Oil cooler.

The lubrication system consists of a sump (oil pan), oil pump, oil cooler and oil filter. The engine contains an oil manifold and oil passages to direct lubricant to the various components.

The oil pump draws lubricant from the sump and forces it through the oil cooler, oil filter, and then into the oil manifold. Oil flows through connecting passages to lubricate the engine components. A regulating valve in the pump body controls the maximum pressure of the oil from the pump.

When the engine is started, the lubricating oil in the oil pan is cold (cool). This cool oil forces bypass valves in the oil filter base to open, and allows an immediate oil flow through the engine. When pressure through the oil cooler and oil filter has been equalized, the turbocharger lubrication valve closes. Filtered oil is then delivered to the turbocharger.

As oil viscosity and pressure decrease the oil filter bypass valve closes. Oil temperature continues to increase and the oil cooler bypass valve closes. Oil now flows through the oil cooler and oil filter before reaching the engine components.

A contaminated or restricted oil filter element will not prevent lubricating oil from being delivered to the engine components. The oil filter bypass valve will open, allowing oil to bypass the element.

An oil manifold, cast into the cylinder block, directs lubricant to the main bearing supply passages. Oil is also directed up through the cylinder head to lubricate the camshaft journals and the camshaft idler (drive) gears.

Oil spray orifices in the cylinder block spray oil on the underside of the pistons. This cools the pistons and provides lubrication for the piston pins, cylinder walls and piston rings.

The connecting rod bearings receive oil through drilled passages in the crankshaft between the main bearing journals and connecting rod journals.

When the engine is warm and running at rated speed, the oil pressure gauge should register in the "operating range". A lower pressure reading is normal at idling speeds.

Oil Pressure Gauge

The instrument panel contains an oil pressure gauge which is connected to the ignition system.

In the event of low oil pressure, a magnetic switch is energized and interrupts the magneto output, thus stopping the engine.

Before cranking an engine, the magnetic switch and gauge reset button should be pushed in. This overrides grounding the magneto as is usual in the case of low oil pressure. Normal oil pressure releases the lockout and the gauge switch is ready to signal a shutdown in case of low oil pressure.

1-Contact adjustment screw. 2-Reset button.


Governor Operation

When the engine is operating, the balance between the centrifugal force of revolving weights (15) and the force of spring (7) controls the movement of valve (14). The valve directs pressure oil to either side of positioning piston (13). Depending on the position of the valve (14), piston (13) will move the shaft (1) to increase or decrease fuel to the engine to compensate for load variation.

Pressurized lubrication oil enters passage (10) in the governor cylinder (3). The oil encircles sleeve (2) within the cylinder. Oil is then directed through a passage in piston (13) where it contacts valve (14).

When engine load increases, engine RPM decreases and revolving weights (15) slow down. The weights move toward each other and allow governor spring (7) to move valve (14) forward. As the valve moves, an oil passage around it opens to pressure oil. Oil then flows through passage (12) and fills the chamber on spring side of piston (13). The pressure forces the piston and shaft forward increasing the amount of fuel to the engine. Engine RPM increases until the revolving weights rotate fast enough to balance the force of the governor spring.

When engine load decreases, engine RPM increases, revolving weights (15) speed-up, and the toes on the weights move valve (14) rearward, to allow the oil behind piston (13) to flow through a drain passage opened at the rear of the piston. At the same time, the pressure oil between sleeve (2) and piston (13) forces the piston and shaft to rear to decrease the amount of fuel to the engine. Engine RPM decreases until the revolving weights balance the force of the governor spring.

When the engine is started, speed limiter plunger (8) restricts the movement of the governor control linkage. When operating oil pressure is reached, the plunger in the speed limiter retracts and the governor control can be moved to the HIGH IDLE position.

When engine RPM is at LOW IDLE, an adjusting screw bears against a stop at the carburetor. To stop the engine, the magneto is grounded.

Oil from the engine lubricating system lubricates the governor weight bearing. The various other parts are splash lubricated. Oil from the governor drains into the governor drive and timing gear housings.

1-Shaft. 2-Sleeve. 3-Cylinder. 4-Drive gear (weight assembly). 5-Lock ring. 6-Seat. 7-Governor spring. 8-Speed limiter plunger. 9-Collar. 10-Oil passage. 11-Oil passage. 12-Oil passage. 13-Piston. 14-Valve. 15-Weights. 16-Lever assembly. 17-High idle adjusting screw.

Solid State Ignition

1-Transformer. 2-Wiring harness. 3-Spark plug cover. 4-Solid state magneto. 5-Timing bolt.

The solid state ignition system consists of five basic components - a solid state magneto, ignition transformers for each cylinder, wiring harness, spark plugs and engine instrument panel.

Instrument Panel

The instrument panel contains a magnetic switch, manual stop switch (1), oil pressure gauge (2), and a water temperature gauge (5) which are connected to the magneto. The safety-shut-off gas supply line valve attachment is also operated by the instrument panel upon shut down.

In the event of normal stopping, high water temperature or low oil pressure, the magnetic switch is energized and interrupts the magneto output, thus stopping the engine. On solid state systems a different magnetic switch is required to carry the higher output of the magneto.

1-Stop switch. 2-Oil pressure gauge. 3-Magnetic switch reset button. 4-Oil pressure gauge reset button. 5-Water temperature gauge.

Before cranking a cold engine, the magnetic switch and oil pressure gauge reset buttons (3) and (4) should be pushed in. This overrides grounding the magneto as is usual in the case of low oil pressure. Normal oil pressure releases the lock out and the gauge switch is ready to signal a shut down in case of low oil pressure.

When the water temperature gauge switch is operating properly, a hot engine cannot be started until the engine has cooled. Holding in the magnetic switch reset button will override the oil pressure and water temperature gauge switches.


The solid state magneto is a self-contained electric generating unit. Current is produced in the alternator section of the magneto. Current is stored in the capacitor and released, then distributed through the circuit board in the pulser distributor section. This system uses capacitive storage and low tension distribution.

Two moving parts, the magnet rotor and pulser rotor shaft both rotating on ball bearings will give long service life. Engine timing will remain as set, because no mechanical rubbing or wearing parts exist that cause gradual timing changes.


The solid state magneto utilizes non-wearing electronic switching to handle high surge currents and insures ignition at the highest of combustion chamber pressures. With solid state ignition, energy storage and voltage step up are accomplished separately by the use of electronic switching. This system eliminates breaker points, contactors, and brushes. There is no arcing and only minimum wear. An electromagnetic spark advance is used. An ignition spark of high intensity is produced to fire the air-fuel charge under all operating conditions.

The alternator creates a voltage as the magnet rotor is driven by the engine through a drive coupling. The alternating current is rectified and stored in a capacitor (4). A zener diode, on the power board, regulates the capacitor voltage level for proper firing. As the pulser rotor (8) passes each pulser coil (trigger circuit) (7), a triggering voltage is produced and sent to the electronic switch (silicon-controlled rectifier) (9) for the firing cylinder. The switch is then 'turned on' and allows the capacitor (4) to discharge, through the distribution board (5), the low voltage high current impluse to the ignition transformer. In turn, the transformer produces a high voltage low current impulse which is sent across the spark plug electrodes. This same process recurs as the pulser rotor passes each pulser coil.

Test the solid state magneto output by the spark delivered at the spark plug terminal.

Ignition Transformers

The ignition transformers step up the magneto voltage to the high voltage required to fire the spark plugs. For good operation, the terminals must be clean and tight. The transformer terminal marked + (positive) is connected by a wire to the magneto plug connector marked G.


Do not pierce the insulation on the high voltage wire to the spark plug. A hole in the insulation will cause misfiring.

Spark Plugs And Adapters

Natural gas engine spark plugs use a dual ground electrode. This allows the spark plug to run longer before it needs to be regapped or replaced.

The boot covers the high tension lead and the terminal portion of the spark plug. This prevents water, dirt and other foreign material from getting into the spark plug adapter.

Starting And Charging Systems

The electrical system is a combination of three separate electric circuits: the charging circuit, the starting circuit and the lighting or load circuit. Each circuit is dependent on some of the same components. The battery (batteries), ammeter, cables and wires from the battery are common in each of the three circuits.

The charging circuit is in operation when the engine is operating. The electricity producing (charging) unit is a generator or alternator. A regulator in the circuit senses the state of charge in the battery and regulates the charging unit output to keep the battery fully charged.

The starting circuit operates only when the start switch is actuated.

The low amperage load and charging circuits are both connected on the same side of the ammeter while the starting circuit connects to the other side of the ammeter.

System Components


The generator keeps the battery charged, and supplies current to operate the electrical components.


Generator Regulator

The generator regulator controls the output of the generator. The regulator incorporates three controls: the cutout relay, the voltage regulator and the current regulator. Each control has contact points which are operated by electromagnets.

Springs hold the cutout relay points open and the voltage regulator and current regulator contact points closed. The spring tension for each unit is a force opposing the force of the electromagnets.

The cutout relay prevents the battery from motorizing a generator that is not producing enough voltage. Generator voltage approximately equal to battery voltage will close the cutout relay points. This closes the circuit between the generator and the battery. The generator can now supply the battery and the components of the electrical system with power.


The voltage regulator prevents the generator from producing damaging high voltage. Generator voltage slightly higher than battery voltage opens the regulator points causing the generator output voltage to lower. Low generator voltage allows the spring to close the regulator points and generator voltage is again high. The action of the voltage regulator points, opening and closing, controls the output voltage of the generator. The points can open and close as often as 200 times per second.

The current regulator limits the current produced by the generator to allow the generator to continue producing voltage equal to battery voltage. When the generator produces current equal to the current regulator setting, the regulator contact points open. Open points lower the generator current. Low current allows the spring to close the points and generator current is again high. The opening and closing of the current regulator points, limits the current produced by the generator. The points can open and close as often as 200 times per second.

This is a schematic wiring diagram of a generator regulator in a battery charging system.

When generator electric loads are low and the battery requires very little charging, the VOLTAGE REGULATOR contact points are operating. When electric loads are high, the CURRENT REGULATOR contact points are operating. The contact points of the two units, will never open at the same time.

Starting Motor

The starting motor used with direct electric start incorporates a solenoid. The action of the solenoid engages the pinion with the ring gear on the engine flywheel, when the solenoid is energized. The pinion always engages before the electric contacts in the solenoid close the circuit between the battery and the starting motor. An overrunning clutch prevents overspeeding of the starting motor. Releasing the start-swich disengages the pinion from the ring gear on the flywheel.



A solenoid is a magnetic switch that utilizes low current to close a high current circuit. The solenoid has an electromagnet with a movable core. There are contacts on the end of the core. The contacts are held open by a spring that pushes the core away from the magnetic center of the coil. Low current will energize the coil and form a magnetic field. The magnetic field draws the core to the center of the coil and the contacts close.

Wiring Diagrams

A variety of electrical systems can be used with these engines. Some systems are available with one 24 volt starting motor. Other systems without electric starting motors are provided for use with air starting and hydraulic starting.

Negative Ground Systems

These systems are most often used in applications where no special precautions are necessary to prevent local radio interference and/or electrolysis of grounded components.



Insulated Systems

These systems are most often used in applications where radio interference is undesirable or where conditions are such that grounded components would corrode from electrolysis.




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