TM 9-6115-730-24
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ELECTRONIC UNIT INJECTOR FUEL SYSTEM - Continued
The ECM controls the amount of fuel injected, by varying signals to the unit injectors. The unit injectors will inject
fuel only if the unit injector solenoid is energized. The ECM sends a 90 volt signal to the unit injector solenoid to
energize it. By controlling the timing and duration of the 90 volt signal, the ECM can control injection timing and
the amount of fuel injected. The ECM sets certain limits on the amount of fuel that can be injected. One limit
based on boost pressure controls the fuel/air ratio for emissions control purposes. When the ECM senses a higher
boost pressure (more air into cylinder), it increases the limit to allow more fuel into cylinder. A second limit is
based on the horsepower rating of the engine. (It is similar to the rack stops and torque spring on a mechanically-
governed engine and provides horsepower and torque curves for a specific engine rating.) Injection timing
depends on engine rpm, load, and other operation factors. The ECM knows where top center of cylinder number
one is from the signal provided by the engine Speed/Timing Sensor. It decides when injection should occur
relative to top center and provides the signal to the unit injector at the desired time.
Unit Injector Mechanism
The unit injector mechanism generates downward force to pressurize the fuel in the injector. At the precise time,
the unit injector injects fuel into the combustion chamber. The camshaft gear is driven by an idler gear which is
piloted in the cylinder block and bolted through the timing gear housing to the block. The idler gear is driven by
the crankshaft gear. Timing marks on the crankshaft gear, idler gear, and camshaft gear are aligned to define the
correct relationship between piston and valve movement. The camshaft has three cam lobes for each cylinder. Two
lobes operate the inlet and exhaust valves, and one operates the unit injector mechanism. Force is transmitted
from the unit injector lobe on the camshaft through a lifter to a pushrod. Force is transmitted from the pushrod
through the rocker arm assembly to the top of the unit injector.
ENGINE COOLING SYSTEM
The engine cooling system (Figure 4) consists of a radiator, hoses, thermostat, water pump, surge tank and
expansion bottle, a belt driven fan, and cooling jackets. The water pump forces coolant through passages (cooling
jackets) in the engine block and cylinder head and an engine oil cooler. The coolant absorbs heat from the engine
as it passes through these passages. A gear driven water pump located in the right hand side of the engine
supplies the coolant for the engine cooling system. The coolant is supplied to the engine oil cooler, cylinder head,
and cylinder liner. Coolant is pulled from the bottom of the radiator into the water pump by an impeller. The water
pump is gear driven at 1.17 times engine speed by an idler turned by the crankshaft gear. The water pump
impeller is an open-face, radial vane cast iron design. The coolant is pumped through an engine oil cooler and
into a supply manifold in the cylinder block. The supply manifold distributes coolant at each cylinder where coolant
flows around and cools the upper portion of the cylinder liner. Coolant flow from each liner enters the cylinder
head that is divided into single cylinder cooling sections. The coolant flow is split at each liner so that 60 percent
flows around the liner and the remainder bypasses the liner and flows directly to the cylinder head. In the cylinder
head coolant flows across the center of the cylinder and the injector seat boss. At the center of the cylinder, coolant
flows up around the injector sleeve over the exhaust port and exits into return manifold. The return manifold
collects the coolant from each cylinder and directs the flow to thermostat housing. With the thermostat closed,
coolant flows through the regulator, bypassing the radiator, and back to the water pump for recirculation. With the
thermostat open, the coolant is directed through the radiator and back to the water pump inlet. The full-flow
bypass coolant thermostat regulates the outlet temperature of the coolant. When the engine is cold the valve is
closed, and the coolant flows through the regulator, bypassing the radiator, to the water pump inlet for
recirculation. As the coolant temperature increases, the thermostat opens to direct some of the coolant to the
radiator, bypassing the remainder to the water pump inlet. At full operating temperature, the valve moves to the
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