3408E AND 3412E HEUI ENGINE CATERPILLAR

ENGINE OF 775D HIGH-WAY TRUCK.

3408E and 3412E HEUI engines.

Shown is a 3412E Engine equipped with Hydraulic Electronic Unit  injection (HEUI), which is used in the 773D and 775D Update  Off-highway Trucks.
These engines utilize the HEUI system for power, reliability and economy
with reduced sound levels and low emissions.
The engine power ratings for the "D" Series Update Trucks are:


769D/771D (3408E): 
 gross power--380  kW (510 hp) net
 power--362 kW (485 flywheel hp).

773D (3412E): 
gross power--509 kW (682 hp) net power--485 kW (650 flywheel hp)

775D (3412E): 
gross power--541 kW (725 hp) net power--517 kW (693 flywheel hp)

Engine Electronic Control System

Shown is the electronic control system component diagram for the 3408E and 3412E HEUI engines used in the "D" Series Update Off-highway Trucks.  Fuel injection is controlled by the engine Electronic Control Module (ECM).

Many electronic signals are sent to the engine ECM by sensors, switches and senders.  The engine ECM analyzes these signals and sends signals to various output components.  Output components can be relays, lamps, other controls or solenoids.  For example, based on the various input signals, the engine ECM determines when and for how long to energize the injector solenoids. When the injector solenoids are energized determines the timing of the engine. How long the solenoids are energized determines the engine speed.

Engine ECM (arrow)

Fuel injection and some other systems are controlled by the engine ECM (arrow) located on top of the engine.  Other systems controlled by the engine ECM include:
- Ether injection
- Engine start function
- Engine oil pre-lubrication
- Variable speed fan control

The engine ECM has two 40-pin connectors.  The connectors are identified as "J1" and "J2."  Be sure to identify which connector is the J1 or J2 connector before performing diagnostic tests.

The engine ECM is cooled by fuel.  Fuel flows from the fuel transfer pump through the ECM to the secondary fuel filters.

Occasionally, Caterpillar will make changes to the internal software (personality module) that controls the performance of the engine.  These changes can be performed by physically installing a new personality module, located below the ECM, or by using the WinFlash program that is part of the laptop software program Electronic Technician (ET).  ET is used to diagnose and program the electronic controls used in Off-highway Trucks.  If using the WinFlash program, a "flash" file must be obtained from Caterpillar and uploaded into the existing ECM personality module.

Input sensors

Shown are some of the sensors that provide input signals to the engine ECM.  The fuel temperature sensor (1) is located in the fuel lines before the fuel enters the fluid supply manifolds to the injectors.  The ECM uses the fuel temperature measurement to make corrections to the fuel rate and maintain power regardless of fuel temperature (within certain parameters).  This feature is called "Fuel Temperature Compensation."

The atmospheric pressure sensor (2) is located in the high pressure pump mounting bracket.  The engine ECM uses the atmospheric pressure sensor as a reference for calculating boost and air filter restriction and for derating the engine at high altitudes (maximum derate of 24%).  All pressure sensors (except injection actuation pressure) in the system measure absolute pressure and, therefore, require the atmospheric sensor to calculate gauge pressure.

All the pressure sensor output signals are matched to the atmospheric pressure sensor output signal during calibration.  Calibration can be accomplished using the ET service tool or by turning on the key start switch without starting the engine for five seconds.


The engine oil pressure sensor (3) is located in the high pressure hydraulic pump.  Engine oil pressure varies with engine speed.  If the engine oil pressure is less than 44 kPa (6.4 psi) at low idle to less than 250 kPa
(36 psi) at high idle, the engine ECM will log an event that requires a factory password to clear.

The engine oil temperature sensor (4) is located in the high pressure hydraulic pump.  The engine oil temperature sensor is used by the ECM to compensate for the effects of oil temperature on fuel injector timing and fuel delivery. This compensation provides consistent engine operation throughout a variety of operating temperatures.

Cold start protection with Cold Mode Timing is activated when the oil temperature decreases below a preset value of 60°C (140°F).  The engine ECM provides an elevated engine idle speed of 1000 rpm for 14 minutes when the engine oil temperature is below 60°C (140°F).

Increasing the low idle speed helps prevent incomplete combustion and overcooling.  To temporarily reduce the elevated idle speed, the operator can release the parking brake or depress the throttle momentarily, and the idle speed will decrease to LOW IDLE for 10 minutes.

The ether injection system uses the engine oil temperature sensor as its temperature reference.

The engine ECM will provide a "Pull-up Voltage" to the signal circuit of most sensors when the ECM senses an OPEN circuit.  Frequency sensors do not receive a Pull-up Voltage.  The signal circuit is usually Pin C of the 3-pin sensor connectors.  The Pull-up Voltage for most sensors is approximately 6.50 volts, but this value can vary with different electronic controls.  Generally, the Pull-up Voltage will be higher than the high value of a sensor's normal range.  For example, the normal range of a coolant temperature sensor is 0.4 to 4.6 Volts with temperatures between
-40°C and +120°C (-40°F and +248°F). The Pull-up Voltage of
6.50 Volts for this sensor is greater than the normal 4.6 Volts high value.


To test for Pull-up Voltage, use a digital multimeter set to "DC Voltage," and use the following procedure (key start switch must be ON):
1. Measure between Pin B (analog or digital return) and Pin C (signal) on the ECM side of a sensor        connector before it is disconnected.  The voltage that is associated with the current temperature or      pressure should be shown.
2. Disconnect the sensor connector while still measuring the voltage between Pins B and C.  If the          circuit between the ECM and the sensor connector is good, the multimeter will display the Pull-up     Voltage.

Secondary speed/timing sensor

Shown is the secondary speed/timing sensor (1).  The primary speed/timing sensor is located on the other side of the high pressure hydraulic pump nearest the ECM.  These sensors calculate engine speed for governing and crankshaft position for timing purposes.

The secondary speed/timing sensor allows continuous operation if the primary sensor fails.  A failure of the primary sensor will cause the ECM to automatically switch to the secondary sensor.  Also, the check engine alert indicator will turn on.

Also shown is the Injection Actuation Pressure (IAP) control valve (2). The IAP control valve receives Pulse Width Modulation (PWM) output signals from the ECM.  The IAP control valve is used to change the swashplate angle of the high pressure hydraulic pump.  Changing the swashplate angle of the pump allows the ECM to control the amount of pressure that is used to inject fuel into the engine.

Injection Actuation Pressure (IAP) is controlled between
5000 and 21360 kPa (725 and 3100 psi).  Fuel injection pressure is approximately seven times the IAP.  Therefore, fuel injection pressure is controlled between 35000 and 149500 kPa (5075 and 21700 psi).

Timing wheel

Shown is the timing wheel removed from the engine.  The timing wheel is an integral part of the drive gear for the pump.  The timing wheel has a total of 24 teeth. 23 teeth are large with small slots between them
(80/20 size).  The other tooth and slot (arrow) have equal dimensions (50/50 size).  The 50/50 size tooth/slot is used by the ECM to locate Top Dead Center (TDC) of the No. 1 cylinder for fuel timing.  The speed/timing sensors can identify this tooth because it creates a different signal than the other teeth.

The speed/timing sensors are positioned against the teeth.  The teeth and sensors generate a Pulse Width Modulated (PWM) output signal for the purpose of timing and a frequency modulated output signal for speed measurement.

A timing mark, identified by the letter "H" on the opposite face of the timing wheel, is used to time the wheel relative to the other timing gears and crankshaft TDC
The engine speed/timing sensor receives a regulated 12.5 ± 1.0 Volts from the engine ECM.  To check the output signal of the speed/timing sensor, connect a multimeter between Pins B and C of the speed/timing sensor connector.  Set the meter to read "Frequency."  The frequency output of the speed/timing sensor should be approximately:

* Cranking:  30 to 70 Hz
* Low Idle:  125 to 185 Hz
* High Idle:  420 to 460 Hz

A passive (two wire) engine speed sensor is positioned on top of the flywheel housing (see Slide No. 165).  The passive speed sensor uses the passing teeth of the flywheel to provide a frequency output.  The passive speed sensor sends the engine speed signal to the transmission/chassis ECM and the brake ECM.

The signal from the passive speed sensor is used for Automatic Retarder Control (ARC) engine control speed

The output signal of the passive speed sensor can also be checked by connecting a multimeter between the two pins of the speed sensor connector and setting the meter to read "Frequency."

NOTE: Disable the injectors with the crank without injection plug (see Slide No. 61) during the cranking test to prevent the engine from starting.  The cranking speed and frequency output signal will vary depending on weather and machine conditions (battery condition). When viewing engine speed in the ET status screen, cranking speed should be between 100 and 250 rpm.

Coolant temperature sensor.

The engine coolant temperature sensor (1) is located in the front right cylinder head.  This sensor is used by the ECM to control various functions.  The following systems or circuits use the coolant temperature sensor input signal to the ECM:

* The Caterpillar Monitoring System coolant temperature gauge.
* If the truck is equipped with the attachment variable speed fan, the coolant temperature sensor is         used as a reference for fan control as engine temperature varies.  The speed of the fan is a function     of the coolant temperature.  Below 88°C (190°F), the fan rotates slowly (100 to 300 rpm).  At 98°C     (208°F), the fan speed is maximum (800 to 1000 rpm).  Between those temperatures, fan speed is       modulated.  The fan speed control can be overridden by the service tool for testing purposes.
* Cold mode operation:  Back-up for the engine oil temperature sensor.
* Automatic ether injection:  Back-up for the engine oil temperature sensor

The turbocharger outlet pressure sensor (2) sends an input signal to the ECM.  The ECM compares the value of the turbo outlet pressure sensor with the value of the atmospheric pressure sensor and calculates boost pressure.

The function of the sensor is to enable the air/fuel ratio control which reduces smoke, emissions and maintains engine response during acceleration.  The system utilizes boost pressure, atmospheric pressure and engine speed to control the air/fuel ratio.  Engine fuel delivery is limited according to boost pressure and engine speed.

A failure of this sensor can cause the engine to derate as much as 60% when the ECM defaults to a zero boost condition

Coolant flow warning switch (arrow)

The coolant flow switch (arrow) sends an input signal to the engine ECM. The ECM provides the input signal to the Caterpillar Monitoring System, which informs the operator of the coolant flow status.

If the ECM detects a low coolant flow condition, a low coolant flow event will be logged.  A factory password is required to clear this event.

IAP pressure sensor

The Injection Actuation Pressure (IAP) sensor (arrow) is located in the right side fluid supply manifold.  The ECM uses this pressure measurement to control the displacement of the high pressure hydraulic pump.

The sensor has a range from 0 to 4.8 Volts, which corresponds to a pressure range of approximately 4000 to 33000 kPa (600 to 4800 psi). With the engine stopped, the default value when read with the service tool is 1800 kPa (260 psi).

The ECM will not activate the injectors to start the engine if the pressure reading is below 4500kPa (650 psi).  A fault will be generated if the actual IAP differs from the desired IAP by more than 1000 kPa (145 psi).

Injection Actuation Pressure (IAP) is normally controlled between 5000 and 21360 kPa (725 and 3100 psi).  Fuel injection pressure is
approximately seven times the IAP.  Therefore, fuel injection pressure is controlled between 35000 and 149500 kPa (5075 and 21700 psi).

The timing calibration connector (2) is located next to the ECM.  A timing calibration sensor (magnetic pickup) is installed in the flywheel housing and connected to the timing calibration connector.

Using the Caterpillar ET service tool, timing calibration is performed automatically for both speed/timing sensors.  The desired engine speed is set to 800 rpm. This step is performed to avoid instability and ensures that no backlash is present in the timing gears during the calibration process.

Timing calibration improves fuel injection accuracy by correcting for any slight tolerances between the crankshaft, timing gears and timing wheel.

Timing calibration is normally performed after the following procedures:

* ECM replacement
* Speed/timing sensor replacement
* Timing wheel replacement.

Turbocharger inlet pressure sensor (arrow)

The turbocharger inlet pressure sensor (arrow) is located in a tube between the air cleaners and the turbochargers.  The engine ECM uses the turbocharger inlet pressure sensor in combination with the atmospheric pressure sensor to determine air filter restriction.  The ECM provides the input signal to the Caterpillar Monitoring System, which informs the operator of the air filter restriction.

If air filter restriction exceeds 7.5 kPa (30 in. of water), an air filter restriction event will be logged, and the ECM will derate the fuel delivery (maximum derating of 20%) to prevent excessive exhaust temperatures. A factory password is required to clear this event

Crank without injection plug (arrow)

The crank without injection plug (arrow) can be used to disable the injectors for maintenance or diagnostic purposes. During normal operation, the plug has a jumper wire installed between Pins B and C.  To enable the crank without injection feature, the jumper wire must be repositioned between Pins C and A.

Engine oil quick fill connector

Shown is the left side of the engine compartment on a 773D/775D update truck. Engine oil can be added at the quick fill oil change connector (1).

A jumper cable with mating receptacle connectors can be connected at the auxiliary start receptacle (2).  The auxiliary start receptacle provides a safe way to connect to another machine or battery jumping source.

The alternator (3) and the air conditioning compressor (4) are also located on the left side of the engine

Engine start function

The engine start function is controlled by the engine ECM and the transmission/chassis ECM.  The engine ECM provides signals to the transmission/chassis ECM regarding the engine speed and the condition of the engine pre-lubrication system.  The transmission/chassis ECM will energize the starter relay only when:
* The shift lever is in NEUTRAL.
* The parking brake is ENGAGED.
* The engine speed is 0 rpm.
* The engine pre-lubrication cycle is complete or turned OFF.

If the truck is equipped with the engine oil pre-lubrication system, pre-lubrication is controlled by the engine ECM and the
transmission/chassis ECM.  The transmission/chassis ECM signals the engine ECM when to energize the pre-lubrication pump relay.  The engine ECM signals the transmission/chassis ECM to crank the engine when:
* Engine oil pressure is 3 kPa (.4 psi) or higher.
* The pre-lubrication pump (1) has run for 17 seconds. (If the system times out after 17 seconds, a         pre-lubrication fault is logged in the engine ECM).
* The engine has been running in the last two minutes.
* Engine oil temperature is above 50°C (122°F).
* Coolant temperature is above 50°C (122°F)

The engine oil pre-lubrication system can be bypassed to allow quick starts. To override the pre-lubrication system, turn the key start switch to the CRANK position for a minimum of two seconds.  The transmission/chassis ECM will begin the pre-lube cycle.  While the pre- lube cycle is active, turn the key start switch to the OFF position. Within 10 seconds, turn the key start switch back to the CRANK position.  The transmission/chassis ECM will engage the starter relay.

If the engine oil pre-lubrication system is bypassed using the above procedure, the engine ECM will log a pre-lube override event that requires a factory password to clear.

Shown to the left of the engine oil pre-lube pump is the secondary steering and parking brake release pump (2).  Both pumps are located behind the front bumper.

NOTE:  The ECAP and ET can enable or disable the pre-lubrication feature in the engine ECM

Ether start system

If the truck is equipped with an ether start system, the ECM will automatically inject ether from the ether cylinder (arrow) during cranking. The operator can also inject ether manually with the start aid switch in the cab (see Slide No. 35).

The engine ECM will energize the ether injection relay only if:
* Engine oil temperature is below 10°C (50°F).
* Coolant temperature is below 10°C (50°F).  Coolant temperature provides a back-up for oil temperature.
* Engine speed is below 1200 rpm.

The ECM turns ON the ether start valve for three seconds, then OFF for three seconds for a cycle time of six seconds.  This cycle time allows a maximum ether injection rate of ten cycles per minute and a maximum flow of 60 cc/minute (3.7 cu. in. per minute).

The manual start aid switch injects a single 6 cc (0.4 cu. in.) of ether each time the switch is depressed.

NOTE:  The manual start aid switch is a dealer installed option.

HEUI fuel injector solenoid

Shown is the top of a cylinder head with the valve cover removed.  The most important output signal from the engine ECM is to the Hydraulic Electronic Unit Injection (HEUI) injector solenoid (1).  Two injectors are located in each cylinder head.  The engine control analyzes all the input signals and sends an output signal to the injector solenoid to control engine timing and speed.

Engine timing is determined by controlling the start and end times that the injector solenoid is energized.  Engine speed is determined by controlling the duration that the injector solenoid is energized.

The jumper tube (2) directs Injection Actuation Pressure (IAP) supply oil from the fluid supply manifold to the injector.  If a low IAP is shown on the ET status screen, leakage probably exists between the fluid supply manifold and the injectors.

The correct bolt tightening sequence must be performed when installing an injector and the jumper tube.  Improper installation can cause leakage and poor performance.  Refer to the Service Manual for the correct tightening sequence.

Earlier fuel injectors have a drain passage for the IAP supply oil on top of the injector.  The drain passage is opened each time the injector solenoid is de-energized, and oil will exit through this passage.  Before cranking an engine with the valve cover removed to check for leaks, unplug the injector solenoid connectors and stay clear until the potential oil leak spray patterns are observed.

Hydraulic fuel injection

Actuation of the fuel injection system is accomplished using hydraulics, rather than the conventional camshaft actuation commonly found on other diesel fuel systems.

Hydraulic actuation offers several advantages compared to mechanical actuation, including the ability to make injection pressure independent of engine operating speed.  This capability is especially advantageous for engine response, cold starting, emissions and noise control.

During normal operating conditions, oil is pressurized between
5000 and 21360 kPa (725 and 3100 psi) by the high pressure hydraulic pump to actuate the injectors.  The level of hydraulic pressure is controlled by the ECM, which signals the IAP control valve to upstroke the hydraulic pump.  When the engine is running, high pressure oil is always available to all the injectors
Oil from the high pressure pump enters the IAP oil supply passage and flows through the jumper tube to the injector.  Oil used by the injector is released below the valve covers and drains back to the sump through the pushrod compartments.

Low pressure engine oil enters the fluid supply manifold and flows through the lube oil passage to the valve rocker assembly.

Low pressure fuel enters the fluid supply manifold and flows to the inlet of the injector through a drilled passage in the fluid supply manifold.  The injectors receive four times the amount of fuel needed for injection.  The extra fuel is used for cooling. Any excess fuel not injected returns to the fuel tank.

The fuel supply to the injector is sealed from the combustion chamber and the area below the valve cover by upper and lower o-ring seals between the cylinder head injector sleeve and the injector.

Combustion chamber gases are prevented from entering the fuel supply passage by a metal-to-metal contact between the cylinder head injector sleeve and the injector.

The cylinder head injector sleeve is threaded into the cylinder head.  A metal washer is used to seal the lower end of the adapter to prevent leakage between the cooling system and the combustion chamber.

NOTE:  In this illustration and those that follow, the colors used to identify the various pressures in the systems are:

Red - Supply oil/coolant pressure
Green - Drain or tank oil/coolant Red and White Stripes - Reduced supply oil pressure
Brown - Lubrication or cooling pressure
Orange - Pilot or load sensing signal pressure
Blue - Blocked oil
Yellow - Moving components
Purple - Air pressure

HEUI injector

Shown is the current style injector with the oil drain passage venting the return oil downward.  This injector is a modification from the previous design which vented the oil upward.  These injectors are interchangeable. However, the current injector reduces the tendency of the engine to discharge oil mist from the breather.

The quantity of fuel delivered is controlled by varying the time the solenoid is energized.  This period of time (called "duration") is calculated by the ECM to ensure delivery of the correct amount of fuel.

When the ECM energizes the solenoid, the poppet valve moves and Injection Actuation Pressure (IAP) supply oil can flow to the intensifier piston.  The IAP oil moves the intensifier piston and the plunger down. The downward movement of the plunger pressurizes the fuel and the nozzle valve lifts, allowing fuel to enter the cylinder.  The time at which fuel leaves the tip is called the "start of injection

The rate at which fuel is injected is controlled by the Injection Actuation Pressure (IAP).  A high IAP pushes the piston and plunger faster, causing a higher flow rate through the nozzle tip.

The ratio between the intensifier piston and the plunger is seven to one. The IAP is controlled between 5000 and 21360 kPa (725 and 3100 psi). Therefore, fuel injection pressure is controlled between
35000 and 149500 kPa (5075 and 21700 psi).

When the ECM ends injection, the poppet seats and drains the area above the intensifier piston.  Downward travel of the piston and plunger reverses, filling the barrel for the next injection sequence.

As pressure drops below the plunger, the valve closing pressure, which is approximately 21000 kPa (3000 psi), closes the reverse flow check valve. This pressure is retained in the nozzle for the next cycle.

The spring chamber below the intensifier piston contains a mixture of oil and fuel.  The IAP oil that leaks past the seals flows into the spring chamber and is used for lubrication.  High pressure fuel can also leak past the plunger into the spring chamber and mixes with the oil.  During each injection cycle, the oil and fuel mixture is ejected into the low pressure fuel.

Cooling System

The 769D/771D update trucks use a folded core radiator, and the 773D/775D update trucks use a conventional radiator core.  The coolant level is checked at the radiator top tank.  Remove the radiator cap (1) to check the coolant level.

A pressure relief valve (2) prevents the cooling system from becoming over pressurized.  Cooling system pressure should be between
55 and 110 kPa (8 and 16 psi). Raising the pressure raises the boiling point. If the pressure is inadequate, the coolant will boil over and the engine will overheat

Cooling system water pump

The cooling system water pump (1) is located on the right front of the engine.  The pump draws coolant from the two bypass tubes (2) until the temperature regulators (thermostats) open.  The thermostats are located in the two housings (3).  When the thermostats are open, coolant flows through the radiator to the water pump inlet.

If the cooling system temperature increases above 107°C (226°F), the engine ECM will log an event that requires a factory password to clear.

3412E engine

Shown is a 3412E engine used in the 773D and 775D update trucks. Coolant flows from the water pump past the coolant flow warning switch (see Slide No. 58).  The coolant flow switch sends an input signal to the engine ECM.  The engine ECM provides the input signal to the Caterpillar Monitoring System, which informs the operator of the coolant flow status.

If the engine ECM detects a low coolant flow condition, a low coolant flow event will be logged. A factory password is required to clear this event.

Coolant also flows through the engine aftercooler (1) and the three oil coolers located on the engine.  The three oil coolers located on the right side of the engine are:
* Engine (2)
* Hoist and brake (3)
* Transmission and torque converter (4)

Coolant flows from the oil coolers to the engine cylinder block. Coolant flows through the engine block and the cylinder heads.  From the cylinder heads, the coolant returns to the temperature regulators and either goes directly to the water pump through the bypass tubes or to the radiator (depending on the temperature of the coolant)

Cooling system circuit

Shown is the cooling system circuit. Coolant flows from the water pump, through the aftercooler and oil coolers, to the engine block.  Coolant flows through the engine block and the cylinder heads.  From the cylinder heads, the coolant returns to the temperature regulators (thermostats) and either goes directly to the water pump through the bypass tubes or to the radiator (depending on the temperature of the coolant)

Lubrication System

The engine oil pump is located in the engine oil pan.  The pump draws oil from the oil pan through a screen. The relief valve for the lubrication system is located on the pump.

Oil flows from the pump through an engine oil cooler bypass valve (1) to the engine oil cooler (2).  The bypass valve for the engine oil cooler permits oil flow to the system during cold starts when the oil is thick or if the cooler is plugged.

Oil flows from the engine oil cooler through the engine oil filters (3) and enters the engine cylinder block to clean, cool and lubricate the internal components and the turbochargers.

Engine oil samples can be taken at the Scheduled Oil Sampling (S•O•S) tap (4) located on the front of the oil filter base.

Inlet supply tube

After the lubricating oil flows through the cooler and the oil filters, the oil flows from the left side of the engine front cover, through the inlet supply tube (1), to the high pressure hydraulic pump (2). The pump is a variable displacement, piston-type pump used to pressurize the HEUI injectors for fuel injection.

Priming the pump with oil after replacement is extremely important to prevent slipper pad overheating.  Pump failure or damage will occur due to lack of lubrication if the case is not filled with oil during replacement.

The priming port (3) is located adjacent to the inlet tube and is the rearmost of the two plugs.  The front plug is the case drain passage and is vented over the pump drive gears.  Therefore, the front plug should not be used for priming.  Remove the plug from the priming port and fill the compartment with oil.

The engine oil pressure sensor (4) and the engine oil temperature sensor (5) are located on the high pressure hydraulic pump.

If the engine oil pressure is less than 44 kPa (6.4 psi) at LOW IDLE or less than 250 kPa (36 psi) at HIGH IDLE, the engine ECM will log an event that requires a factory password to clear.

During normal operation, oil pressure should be a minimum of 220 kPa (32 psi) at LOW IDLE and a minimum of 275 kPa (40 psi) at FULL LOAD

Reverse flow check valves

Shown is the rear of the high pressure hydraulic pump. Oil flows through two reverse flow check valves (1) to the two fluid supply manifolds. The reverse flow check valves prevent high pressure surges from the injectors returning to the pump or the other injector bank.

Also shown is the Injection Actuation Pressure (IAP) control valve (2). The IAP control valve receives Pulse Width Modulation (PWM) output signals from the ECM.  The IAP control valve is used to control the swashplate angle of the high pressure hydraulic pump.  Changing the swashplate angle of the pump allows the ECM to control the amount of pressure that is used to inject fuel into the engine.

Injection Actuation Pressure (IAP) is controlled between
5000 and 21360 kPa (725 and 3100 psi).  Fuel injection pressure is approximately seven times the IAP.  Therefore, the fuel injection pressure is controlled between 35000 and 149500 kPa (5075 and 21700 psi)

Engine oil system operation

The engine oil pump draws oil from the oil pan through a screen.

Oil flows from the pump, through an engine oil cooler bypass valve, to the engine oil cooler.  The bypass valve for the engine oil cooler permits oil flow to the system during cold starts when the oil is thick or if the cooler is plugged.

Oil flows from the engine oil cooler, through the oil filters and enters the engine cylinder block to clean, cool and lubricate the internal components and the turbochargers.

Lubrication oil also flows to the high pressure hydraulic pump.  The pump is a variable displacement, piston-type pump used to pressurize the HEUI injectors for fuel injection.

Oil flows from the high pressure hydraulic pump through two reverse flow check valves to the two fluid supply manifolds.  The reverse flow check valves prevent high pressure surges from the injectors returning to the pump or the other injector bank.

Fuel System

The fuel tank is located on the left side of the truck.  The fuel level sight gauge (1) is used to check the fuel level.  A fuel level sender is located on the fuel level sight gauge. The fuel level sender provides input signals to the Caterpillar Monitoring System, which informs the operator of the fuel level.

Open the drain valve (2) daily to remove condensation from the fuel tank

Primary fuel filter

Fuel is pulled from the fuel tank through the primary fuel filter (1) by the fuel transfer pump located on the rear of the high pressure oil pump.  The primary fuel filter has a 30 micron rating.  A fuel priming pump (2) is located on top of the primary fuel filter.  The fuel priming pump is used to fill the filters after they are changed.

A water separator cup is located on the bottom of the primary fuel filter. Condensation should be drained daily from the water separator cup through the drain valve (3).

NOTE:  If the fuel system requires priming, it may be necessary to block the fuel return line during priming to force the fuel into the injectors.

Fuel transfer pump

The fuel transfer pump (1) contains a bypass valve to protect the fuel system components from excessive pressure.  The bypass valve setting is higher than the setting of the fuel pressure regulator.  The setting of the bypass valve is 716 ± 35 kPa (104 ± 5 psi).  Fuel flows from the transfer pump, through the engine ECM, to the secondary fuel filters (2).  The secondary fuel filters have a 2 micron rating.

Fuel flows from the secondary fuel filters, through the fluid supply manifolds, to the Hydraulic Electronic Unit Injection (HEUI) fuel injectors.  The injectors receive four times the amount of fuel needed for injection.  The extra fuel is used for cooling.  Any excess fuel not injected flows through the fuel pressure regulator and returns to the fuel tank.

The fuel temperature sensor (3) is located in the fuel lines before the fuel enters the fluid supply manifolds to the injectors.  The ECM uses the fuel temperature measurement to make corrections to the fuel rate and maintain power regardless of fuel temperature (within certain parameters).  This feature is called "Fuel Temperature Compensation."

Fuel pressure regulator (arrow)

Fuel from the fluid supply manifolds flows through the fuel pressure regulator (arrow) before returning to the fuel tank.  Fuel pressure is controlled by the fuel pressure regulator.

During normal operation, fuel pressure should be a minimum of 310 kPa (45 psi) at LOW IDLE and between 310 and 415 kPa (45 and 60 psi) at FULL LOAD

Low pressure fuel system circuit

Fuel is pulled from the fuel tank through the primary fuel filter by the fuel transfer pump located on the rear of the high pressure oil pump.  A fuel priming pump is located on top of the primary fuel filter.  The fuel priming pump is used to fill the filters after they are changed.

A water separator cup is located on the bottom of the primary fuel filter. Condensation should be drained daily from the water separator cup.

Fuel flows from the transfer pump through the engine ECM to the secondary fuel filters.

Fuel flows from the secondary fuel filters, through the fluid supply manifolds, to the Hydraulic Electronic Unit Injection (HEUI) fuel injectors.  The injectors receive four times the amount of fuel needed for injection.  The extra fuel is used for cooling.  Any excess fuel not injected flows through the fuel pressure regulator and returns to the fuel tank.  Fuel pressure is controlled by the fuel pressure regulator

Air Induction and Exhaust System

Shown are the air intake system components.  The engine receives clean air through the two air filters located on the right front platform.  Located above the air filter housings are the precleaner bowls (1).  Check the level of dirt accumulation in the precleaner bowls.  Empty the precleaner bowls when the dirt level reaches the full mark.

Two filter elements are installed in the filter housings.  The large element is the primary element (2) and the small element is the secondary element (3).

Any restriction caused by plugged filters can be checked at the air filter restriction alert indicator on the Caterpillar Monitoring System message center (see Slide No. 36).  The alert indicator lights when the filter restriction is approximately 7.5 kPa (30 in. of water).

A turbocharger inlet pressure sensor (see Slide No. 60) is located in a tube between the air cleaners and the turbochargers.  The engine ECM uses the turbocharger inlet pressure sensor in combination with the atmospheric pressure sensor to determine air filter restriction

3412E Two turbochargers

Shown is the 3412E engine used in the 773D and 775D update trucks. The 3412E engine has two turbochargers (one on each side of the engine). The 3408E engine, used in the 769D and 771D update trucks, has only one turbocharger mounted at the top rear of the engine.

The clean air from the filters enters the turbochargers (1).  The turbochargers compress the air with the compressor wheel, which is connected to and driven by the turbine wheel. The compressed air from the turbochargers flows to the aftercooler (2).  After the air is cooled by the aftercooler, the air flows to the cylinders and combines with the fuel for combustion.

The turbochargers are driven by the exhaust gasses from the cylinders. The exhaust gasses enter the turbochargers and turn the turbine wheels. Exhaust gasses then flow through the exhaust piping, and the muffler.

If the "D" Series update trucks are equipped with an exhaust heated body, a switch on the dash will initiate the body up sound reduction feature (see Slide No. 35).  When the switch is depressed, engine HIGH IDLE is reduced to 1800 rpm when the body is raised.  The body up switch, located on the rear frame, provides an input signal to the transmission/chassis ECM that tells the control when the body is UP.  The transmission/chassis ECM then signals the engine ECM to reduce the engine speed until the body is DOWN

3412E exhaust system

Shown is the exhaust system for a 3412E engine with twin turbochargers.

The turbochargers are driven by the exhaust gasses from the cylinders. The exhaust gasses enter the turbochargers and turn the turbine wheels, which are connected to the compressor wheels.  Exhaust gasses then flow through the exhaust piping and the muffler.

The engine receives clean air through the two air filters located on the right front platform.  The clean air from the filters enters the turbochargers.  The turbochargers compress the air with the compressor wheel, which is connected to and driven by the turbine wheel.  The compressed air from the turbochargers flows to the aftercooler.  After the air is cooled by the aftercooler, the air flows to the cylinders and combines with the fuel for combustion

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