0387044 JOHNSON BODY ASSY.,Upper

Ignition System
Earlier engines were built with a magneto. Newer engines use the CAT Digital Ignition System. For instructions on retrofitting the ignition system, refer to Special Instruction, REHS0476, "Installation Instructions for the CAT Digital Ignition System for G3408 and G3412 Engines".Digital Ignition System
The digital ignition system includes a timing disc, a speed sensor (timing), a timing control, and wiring harnesses. The sensor monitors the rotation of the timing disc in order to sense a reference position of the crankshaft. The sensor sends a signal to the timing control. The timing control calculates the correct ignition timing. The timing control initiates the ignition by providing a pulse to the transformer of the proper cylinder.Magneto
The magneto generates the electric current that is necessary for spark ignition. The current from the magneto travels through the wiring harness to the ignition transformer. The transformer increases the voltage. The transformer creates an electrical impulse that arcs across the electrodes of the spark plug.Inlet Air and Fuel System
The engines are available with these optional systems for the inlet air:
Naturally aspirated
Turbocharged
Turbocharged with separate circuit aftercoolingInlet air is filtered by an air cleaner.For turbocharged engines, the inlet air is compressed by the turbocharger. The turbocharger is driven by the engine exhaust. For engines with separate circuit aftercooling, the air is cooled in an aftercooler. The air becomes more dense from compression and cooling. This provides more oxygen for combustion.Air is mixed with fuel in the carburetor before combustion. The throttle is controlled by a governor. The governor efficiently maintains the engine rpm that is selected by the operator.The valves and the valve mechanism control the flow of air and exhaust gases in the cylinder during engine operation. Each cylinder has two inlet valves and two exhaust valves. The camshaft actuates the rocker arms and valves through mechanical lifters and pushrods.Lubrication System
The engine lubrication oil is circulated by a gear-driven pump. The engine oil is cooled and the engine oil is filtered. A bypass valve provides unrestricted flow of lubrication oil to the engine parts if the engine oil filters become plugged.Cooling System
The jacket water coolant is circulated by a gear-driven centrifugal pump. A 90 °C (194 °F) temperature regulator regulates the temperature of the coolant. Heat can be removed from the coolant by a radiator or a heat exchanger.For engines with separate circuit aftercooling, the air is cooled by a heat exchanger. The water that cools the air in the heat exchanger is in a circuit that is separate from the jacket water. The aftercooler coolant is pumped by an auxiliary water pump. The separate circuit is required to operate at 32 °C (90 °F) or 54 °C (130 °F). The operation temperature depends on the engine rating and the application.For engines with air-to-air aftercooling, the aftercooler is supplied by the customer. This aftercooler is similar to a radiator. The inlet air is cooled by a fan that moves the ambient air through the radiator.Engine Service Life
Engine efficiency and maximum utilization of engine performance depend on adherence to proper operation and maintenance recommendations. This includes the use of recommended lubricants, fuels, and coolants.For the engine maintenance that is required, refer to this Operation and Maintenance Manual, "Maintenance Interval Schedule" (Maintenance Section).