1674 DIESEL TRUCK ENGINE Caterpillar

Systems Operation


General Information

NOTE: For Specifications with illustrations, make reference to ENGINE SPECIFICATIONS FOR 1674 DIESEL TRUCK ENGINE, Form No. REG01429. If the Specifications in Form REG01429 are not the same as in the Systems Operation and the Testing and Adjusting, look at the printing date on the back cover of each book. Use the Specifications given in the book with the latest date.

The 1674 Diesel Truck Engine is a 638 cu. in. (10.5 liters) displacement, 4-stroke cycle, six cylinder, turbocharged, diesel engine. The cylinder bore is 4.75 in. (120.6 mm) and the piston stroke is 6.00 in. (152.4 mm). The firing order is 1-5-3-6-2-4. The engine weighs approximately 2280 lbs. (1.034 kg) without coolant or oil.

Inlet air, filtered by a dry-type air cleaner, is compressed by a turbocharger before entering the engine cylinders. The turbocharger is driven by the engine exhaust.

A plunger and barrel-type fuel injection pump meters and pumps filtered fuel to a precombustion chamber for each cylinder. The fuel is delivered to the precombustion chamber under high pressure.

A hydraulic governor controls the fuel injection pump out-put to maintain a constant engine RPM under varying work loads. A speed limiting device, in the governor, limits engine speed until engine oil pressure builds up.

There are four in-head valves (two inlet and two exhaust) for each cylinder. Two overhead camshafts, and forked rocker arm assemblies, are located in a housing on top of the cylinder head. The forked rocker arm assemblies act as a direct mechanical link between the lobes on the camshafts and the valve stems.

The timing gears are located at the rear of the engine.

The starting system is direct electric and uses either a 12 or 24-volt starting motor.

Coolant for the engine is used to cool the engine lubricating oil. A full-flow temperature regulator, in the cylinder head at the front of the engine, provides for quick engine warm-up, and allows free circulation of coolant after operating temperature has been reached.

Lubrication for the engine is supplied by a gear-type pump. The pump provides full pressure lubrication to the engine internal and external parts.

The lubricating oil is both cooled and filtered. Bypass valves in the oil cooler assembly provide unrestricted flow of lubricating oil to the engine parts when oil viscosity is high or, if either the oil cooler or the oil filter element should become clogged.

Fuel System

1. Fuel injection valves (six, located in cylinder head, under valve cover). 2. Fuel injection pump. 3. Fuel priming pump. 4. Vent valve. 5. Fuel filter base and final fuel filter. 6. Fuel injection pump housing. 7. Fuel transfer pump.

The fuel system is a pressure type with a separate injection pump and injection valve for each cylinder. Fuel is injected into a precombustion chamber, not directly into the cylinder.

A transfer pump supplies fuel to a manifold in the injection pump housing. Before fuel is delivered to the manifold, it is filtered by a primary filter that removes dirt particles, and later by a final filter that removes more minute particles.

The transfer pump can supply more fuel than is required for injection, so a bypass valve is built into the pump. The valve limits the maximum pressure within the supply system.

The injection pumps receive fuel from the manifold and force it under high pressure to the injection valves. The injection valves spray atomized fuel into the precombustion chambers.

An air vent valve in the system permits removal of air. Air is removed by opening the valve and pressurizing the fuel system. The system can be pressurized by using the priming pump. The vent valve must be open until a stream of fuel, without air bubbles, flows from the vent line.

Fuel Injection Pump Operation

The injection pump plungers and the lifters are lifted by lobes on the camshaft and always make a full stroke. The lifters are held against the cam lobes by springs.

1. Check valve. 2. Inlet port. 3. Fuel manifold. 4. Gear segment. 5. Pump plunger. 6. Spring. 7. Fuel rack. 8. Lifter. 9. Camshaft.

The amount of fuel pumped each stroke is varied by turning the plunger in the barrel. Action of the governor moves the fuel rack which turns the pump gear segment on the bottom of the pump plunger.

Governor Operation

The hydraulic governor maintains speed at the rpm selected.

When the engine is operating, the balance between the centrifugal force of revolving weights and the force of a compressed spring controls the movement of a valve. The valve directs pressure oil to either side of a rack-positioning piston. Depending on the position of the valve, the rack is moved to increase or decrease the fuel to the engine to compensate for load variation.

Pressurized lubrication oil, directed through passages in the fuel injection pump housing, enters a passage in the governor cylinder. The oil encircles a sleeve within the cylinder. The oil is then directed through a passage in the piston where it contacts the valve.

When the engine load increases, the revolving weights slow down. The weights move toward each other and allow the governor spring to move the valve forward. As the valve moves, an oil passage around the piston opens to pressure oil. The oil flows through this passage and fills the chamber behind the piston. The pressure forces the piston and rack 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.

1. Collar. 2. Speed limiter plunger. 3. Lever. 4. Seat. 5. Governor spring. 6. Thrust bearing. 7. Oil passage. 8. Drive gear (weight assembly). 9. Cylinder. 10. Bolt. 11. Spring seat. 12. Weight. 13. Valve. 14. Piston. 15. Sleeve. 16. Oil passage. 17. Fuel rack.

When the engine load decreases, the revolving weights speed-up and the toes on the weights move the valve rearward, allowing the oil behind the piston to flow through a drain passage opened at the rear of the piston. At the same time, the pressure oil between the sleeve and the piston forces the piston and rack rearward. This decreases the fuel to the engine and the engine slows down. When the force of the revolving weights balances the governor spring force, the rpm of the engine will be the same as before.

When the engine is started, the speed limiter plunger restricts the movement of the accelerator. When operating oil pressure is reached, the plunger in the speed limiter retracts and the accelerator can be depressed to the high idle position.

When the engine rpm is at low idle, a springloaded plunger within the lever assembly in the governor bears against the shoulder of the low idle adjusting screw. To stop the engine, the plunger must be forced past the shoulder on the adjusting screw.

Oil from the engine lubricating system lubricates the governor weight bearing. The various other parts are splash lubricated. The oil from the governor drains into the fuel injection pump housing.

Fuel Injection Valve

Fuel, under high pressure from the injection pumps, is transferred through the injection lines to the injection valves. As high pressure fuel enters the nozzle assembly, the check valve within the nozzle opens and permits the fuel to enter the precombustion chamber. The injection valve provides the proper spray pattern.

1. Body. 2. Nut. 3. Glow plug. 4. Nozzle assembly. 5. Precombustion chamber.

Fuel Ratio Control

An air line connects the air inlet manifold to the fuel ratio control. This air line gives the fuel ratio control an indication of the amount of air flowing through the inlet manifold. This lets the fuel ratio control keep the movement of the fuel rack in relation to the air available in the inlet manifold. With the correct air to fuel ratio in the cylinders exhaust smoke is kept to a minimum.

Collar (3) mechanically connects to the fuel rack with governor bolt (4). Bolt assembly (7) goes through a groove in collar (3) and the head of bolt assembly (7) fits behind collar (3).

When the operator moves the governor control to make the engine rpm go faster, the governor spring will move collar (3) into contact with the head of bolt assembly (7). Bolt assembly (7) causes a restriction in the movement of collar (3) and bolt (4) until spring (1) and the turbocharger boost of air pressure inside housing (5) causes diaphragm (2), spring (6) and bolt assembly (7) to remove the restriction to collar (3) and bolt (4). This permits the fuel rack to move giving more fuel to the cylinders as the turbocharger boost of air pressure goes higher along with faster engine rpm.

1. Spring. 2. Diaphragm. 3. Collar. 4. Bolt. 5. Housing. 6. Spring. 7. Bolt assembly.

Variable Timing Drive

The variable timing drive couples the fuel injection pump camshaft to the engine rear timing gears. As engine rpm increases, the variable timing drive advances the timing beginning at engine speed of 1325 rpm. At 2200 rpm the timing is fully advanced.

1. Power piston. 2. Power piston cavity. 3. Control valve spring. 4. Power piston return spring. 5. Oil inlet passage. 6. Drain port. 7. Flyweights. 8. Control valve. 9. Shaft assembly.

During engine low rpm operation, the flyweight force is not sufficient to overcome the force of control valve spring (3). In this case control valve (8) moves to the closed position. Oil merely flows through the power piston cavity (2).

As the engine rpm increases, flyweights (7) overcome the force of control valve spring (3). The control valve moves to the closed position, blocking the oil drain port (6). Pressurized oil, trapped in power piston cavity (2), overcomes the force of spring (4) and moves power piston (1) outward. This causes the fuel injection pump camshaft to index slightly ahead of the shaft (9) portion of the variable timing drive. Any outward movement of the power piston increases the force on the control valve spring. This tends to reopen the control valve, letting oil escape from the power piston cavity. As oil begins flowing from the cavity again, return spring (4) moves the power piston inward.

1. Power piston. 2. Power piston cavity. 3. Control valve spring. 4. Power piston return spring. 5. Oil inlet passage. 6. Drain port. 7. Flyweights. 8. Control valve. 9. Shaft assembly.

At any given rpm, a balance is reached between the flyweight force and the control valve spring force. The resultant position of control valve (8) will tend to maintain proper pressure behind the power piston. The greater the rpm, the smaller the drain port opening, and the further outward the power piston is forced.

As the power piston is moved outward, the angular relationship of the ends of the drive change. As the power piston moves forward in the internal helical spline, the fuel injection pump timing advances.

The gear teeth on shaft assembly (9) drive the governor drive pinion.

Air Inlet And Exhaust System

1. Exhaust manifold. 2. Inlet manifold and aftercooler. 3. Engine cylinder. 4. Air inlet. 5. Turbocharger compressor impeller. 6. Turbocharger turbine wheel. 7. Exhaust outlet.

This engine has an exhaust driven turbocharger to give a boost of air to the cylinders.

The exhaust gases enter the turbine housing and are directed through the blades of a turbine wheel, causing the turbine wheel and a compressor wheel to rotate.

Filtered inlet air from the air cleaners is drawn through the air inlet of the compressor housing by the rotating compressor wheel. The air is forced to the aftercooler and inlet manifold of the engine. It is compressed by action of the compressor wheel.


The aftercooler is installed on the top of the cylinder head. Water flow through the aftercooler, lowers the temperature of the inlet air from the turbocharger. With cooler air, an increase in weight of air will permit more fuel to burn. This gives, an increase in power.

When the turbocharger gives a pressure boost to the inlet air, the temperature of the air goes up. A water-cooled aftercooler, is installed between the turbochargers and the air inlet manifold. The aftercooler causes a reduction of air temperature from the turbocharger.


The turbocharger is mounted to the engine exhaust manifold. All the exhaust gases from the diesel engine pass through the turbocharger.

When engine load increases, more fuel is injected into the engine cylinders. The increased volume of exhaust gas causes the turbocharger turbine wheel and compressor impeller to rotate faster. The higher rpm of the impeller increases the quantity of inlet air. As the turbocharger provides additional inlet air, more fuel can be burned; hence more horsepower derived from the engine.

4. Air inlet. 5. Compressor wheel. 6. Turbine wheel. 7. Exhaust outlet. 8. Compressor housing. 9. Thrust bearing. 10. Sleeve. 11. Lubrication inlet port. 12. Turbine housing. 13. Sleeve. 14. Sleeve. 15. Oil deflector. 16. Compressor journal bearing. 17. Oil outlet port. 18. Turbine journal bearing. 19. Exhaust inlet. 20. Air outlet.

The turbocharger bearings are pressure-lubricated by engine oil. The oil enters the top of the center section and is directed through passages to lubricate the thrust bearing, sleeves and the journal bearings of the turbocharger. Oil leaves the turbocharger through a port in the bottom of the center section and is returned to the engine sump.

Maximum rpm of the turbocharger is controlled by the rack setting, the high idle speed setting and the height above sea level at which the engine is operated.

------ WARNING! ------

If the high idle rpm or the rack setting is higher than given in the book RACK SETTING INFORMATION (for the height above sea level at which the engine is operated), there can be damage to engine or turbocharger parts.


The fuel rack adjustment is done at the factory for a specific engine application. The governor housing and turbocharger are sealed to prevent changes in the adjustment of the rack and the high idle speed setting.

The engine can be operated at a lower altitude than specified without danger of engine damage. In this situation the engine will perform at slightly less than maximum efficiency. When operated at a higher altitude, the rack setting and high idle speed setting must be changed.

Valve And Valve Mechanism

This is an overhead valve (OHV) four stroke cycle engine; i.e., four separate piston strokes are required to complete the firing of one cylinder.

There are four in-head valves (two inlet and two exhaust) for each cylinder. The valves are actuated by two overhead camshafts (one inlet and one exhaust) and forked rocker arm assemblies, located in a housing on top of the cylinder head. The exhaust camshaft is driven by the rear mounted timing gears. The inlet camshaft is driven by the exhaust camshaft drive gear. Valve rotator assemblies cause the valves to rotate during engine operation.

Correctly adjusted valves operate for many hours before they need to be reconditioned. Eventually, however, the valve faces and seats become pitted, causing loss of compression. The cylinder head contains valve seat inserts that can be replaced when the seats have been ground to the extreme limits.

Cored passages in the cyliner head direct coolant around the precombustion chambers and the area around the valve seats. The exhaust manifold is bolted to the cylinder head. The inlet manifold is formed by passages cast in the cylinder head.

1. Exhaust camshaft. 2. Rocker arm assemblies. 3. Inlet camshaft. 4. Roller. 5. Rocker arm shaft. 6. Valve spring. 7. Valve rotator. 8. Valve retainer and lock. 9. Valve guide. 10. Exhaust valve. 11. Inlet valve. 12. Valve seat insert.

Lubrication System

1. Oil passage through rocker arm shafts. 2. Turbocharger oil reservoir in center section. 3. Oil cooler. 4. Oil filter. 5. Oil filter base (includes an oil filter bypass valve and an oil cooler bypass valve). 6. Oil pump (in rear part of oil pan). 7. Oil pan (sump). 8. Oil manifold (in cylinder block).

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 viscous oil does not flow immediately through the oil cooler and oil filter. This cool oil forces bypass valves, in the oil cooler and oil filter base, to open and allows an unrestricted oil flow through the engine.

As oil temperature increases, oil viscosity and pressure decreases and the bypass valves close. Oil now flows through the oil cooler and oil filter to deliver filtered oil to the engine components.

A restriction in the oil filter element will cause the filter bypass valve to open. This sends the flow of oil to the manifold in the cylinder block.

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 valve rocker arm shafts, camshaft journals, and the camshaft idler (drive) gears.

Oil spray orifices in the oil jet tubes inside the cylinder block spray oil on the underside of the pistons. This cools the pistons and provides lubricant 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.

Cooling System

1. Cylinder head. 2. Aftercooler. 3. Water temperature regulator housing. 4. Coolant outlet from radiator. 5. Air compressor. 6. Bypass water line. 7. Coolant outlet to aftercooler. 8. Water manifold. 9. Coolant outlet to water manifold. 10. Cylinder block. 11. Oil cooler. 12. Coolant inlet to water pump. 13. Water pump. 14. Shunt line. 15. Radiator. 16. Vent line. 17. Upper chamber. 18. Bleed tube. 19. Lower chamber.

This engine has a pressurized cooling system. Coolant is circulated by a belt driven, centrifugal type water pump. A water temperature regulator, located at the left front of the cylinder head, restricts coolant flow through the radiator until the coolant reaches operating temperature.

The water pump has two outlets. One outlet directs coolant through a passage in the water temperature regulator housing to the aftercooler, to lower the temperature of inlet air in the inlet manifold. The other water outlet directs coolant through a water manifold in the side of the cylinder block, to cool engine lubricating oil. Coolant flows from the water manifold around the cylinder liners, into the cylinder head and around the precombustion chambers. Both streams of water join at the water temperature regulator.

The water cooled air compressor receives coolant through a tube that is connected to the water manifold in the side of the cylinder block. Coolant returns through a tube from the air compressor head to the diesel engine cylinder head.

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

When the coolant reaches the temperature required to open the temperature regulator, coolant is then directed to the radiator lower chamber.

The upper chamber of the radiator receives any air in the lower chamber by way of the bleed tube. Any air in the engine is vented to the radiator upper chamber thru the vent line. The shunt line from the upper chamber to the inlet of the water pump provides for rapid filling of the system.

A pressure relief cap assembly is used to control the pressure in the cooling system, and prevents loss of coolant through the radiator overflow tube.

Pressurizing the cooling system serves two purposes. First, it permits safe operation at coolant temperatures higher than the normal boiling point, providing a margin of cooling for intermittent peak loads. Secondly, it prevents cavitation in the water pump, and reduces the possibility of air or steam pockets forming in the coolant passages. Proper operation of the pressure relief cap assembly is essential. A pressure relief cap allows pressure (and some water, if the cooling system is too full) to escape when the pressure in the cooling system exceeds the capacity of the pressure cap. Loss of pressure will cause steam to form when coolant temperature is above the normal boiling point.

Cooling System Components

Water Pump

The centrifugal-type water pump has two seals, one prevents leakage of water and the other prevents leakage of lubricant.

An opening in the bottom of the pump housing allows any leakage at the water seal or the rear bearing oil seal to escape.


The fan is driven by three V-belts, from a pulley on the crankshaft. Belt tension is adjusted by moving the clamp assembly which includes the fan mounting and pulley.

Water Temperature Regulator

The water temperature regulator restricts the flow of coolant through the radiator, until the coolant reaches operating temperature; approximately 165° F (74° C).

Basic Block

Vibration Damper

The twisting of the crankshaft, due to the regular power impacts along its length, is called twisting (torsional) vibration. The vibration damper is installed on the front end of the crankshaft. It is used for reduction of torsional vibrations and stops the vibration from building up to amounts that could cause damage.

The damper is made of a weight (1) in a metal case (3). The small space (2) between the case and weight is filled with a thick fluid. The fluid permits the weight to move in the case to cause a reduction of vibrations of the crankshaft.

1. Solid cast iron weight. 2. Space between weight and case. 3. Case.

Electrical System

The electrical system has three separate circuits: the charging circuit, the starting circuit and the low amperage circuit. Some of the electrical system components are used in more than one circuit. The battery (batteries), circuit breaker, ammeter, cables and wires from the battery are all common in each of the circuits.

The charging circuit is in operation when the engine is running. An alternator makes electricity for the charging circuit. A voltage regulator in the circuit controls the electrical output to keep the battery at full charge.

The starting circuit is in operation only when the start switch is activated.

The low amperage circuit and the charging circuit are both connected to the same side of the ammeter. The starting circuit connects to the opposite side of the ammeter.

System Components


The alternator is a three phase self-rectifying charging unit. The alternator is driven from an accessory drive pulley by a V-type belt.

The only part in the alternator assembly which has movement is the rotor. The rotor is held in position by ball bearings at both ends.

1. Brushes. 2. Stator. 3. Fan. 4. Slip rings. 5. Collar. 6. Bearings. 7. Bearings. 8. Rotor.

Alternator Regulator

The regulator controls the alternator output according to the needs of the battery and the other components in the electrical system.

1. Plug. 2. Connector.

Starting Motor

The starting motor is used to turn the engine flywheel fast enough to get the engine running.

1. Field. 2. Solenoid. 3. Clutch. 4. Pinion. 5. Commutator. 6. Brush assembly. 7. Armature.

The starting motor has a solenoid. When the start switch is activated, electricity from the electrical system will cause the solenoid to move the starter pinion to engage with the ring gear on the flywheel of the engine. The starter pinion will engage with the ring gear before the electric contacts in the solenoid close the circuit between the battery and the starting motor. When the start switch is released, the starter pinion will move away from the ring gear of the flywheel.

Magnetic Switch

A magnetic switch (relay) is used sometimes for the starter solenoid or glow plug circuit. Its operation electrically, is the same as the solenoid. Its function is to reduce the low current load on the start switch and control low current to the starter solenoid or high current to the glow plugs.

A solenoid is a magnetic switch that uses low current to close a high current circuit. The solenoid has an electro-magnet with a core (6) which moves. There are contacts (4) on the end of core (6). The contacts are held in the open position by spring (5) that pushes core (6) from the magnetic center of coil (1). Low current will energize coil (1) and make a magnetic field. The magnetic field pulls core (6) to the center of coil (1) and the contacts close.

1. Coil. 2. Switch terminal. 3. Battery terminal. 4. Contacts. 5. Spring. 6. Core. 7. Component terminal.

Shutoff Solenoid

The rack shutoff solenoid, when energized, moves to override the governor action. This moves the governor and fuel rack to the fuel off position. The solenoid can be energized by any one of several sources. The most usual is the manually operated momentary switch activated by the operator.


Wiring Diagrams

1. Series parallel switch. 2. Ammeter. 3. Light load. 4. 12V Relay. 5. Regulator. 6. 24V Starting motor. 7. Customer furnished momentary switch. 8. Customer furnished key switch. 9. Alternator. 10. Batteries. 11. Rack shut off solenoid. 12. Heat start switch. 13. Lights. 14. Glow plug magnetic switch. 15. Glow plugs.

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