SLEEVE 3T0918 - Caterpillar

System OperationThe 3126B HEUI Truck Engines use a Hydraulic Electronic Unit Injector fuel system. The injection pump, fuel lines and nozzles required for fuel injection in mechanical Caterpillar engines have been replaced with a hydraulic electronic unit injector in each cylinder. Some mechanical drive components, cam lobes, lifter, push rod, and rocker arms required for fuel injection in mechanical or Electronic Unit Injector Caterpillar engines have been replaced with a high pressure oil system.A solenoid on each injector controls the amount of fuel delivered by the injector. A gear-driven axial piston pump raises the normal engine operating oil pressure to injector actuation pressure levels required by the injectors. An Engine Control Module (ECM) sends a signal to the injection actuation pressure control valve to control injection pressure, and another signal to each injector solenoid to inject fuel. Electronic ControlsThe 3126B HEUI Truck Engines electronic system consists of the Engine Control Module (ECM), Engine Sensors, Injection Actuation Pressure Control Valve, and Vehicle Interface. The ECM is the computer which controls the 3126B HEUI engine. The Personality Module in the ECM contains the software which controls how the ECM behaves (the personality module stores the operating maps that define power rating, torque curves, rpm, etc). Engine GovernorThe Electronic Control system on the 3126B HEUI engine serves as the engine governor. The Electronic Control system determines when and how much fuel to deliver to the cylinders, as well as injection pressure based on the actual and desired conditions at any given time during starting and operation. Basic GovernorThe basic governor operation considers the desired and actual conditions, and then takes action to best accommodate the desired conditions.The desired conditions are typically the position of the accelerator pedal, desired vehicle speed when in cruise control, or desired engine rpm when in PTO control. The actual conditions are based on current operating conditions such as coolant temperature, load conditions, etc. Timing ConsiderationsOnce the ECM has determined how much fuel is required, it must next determine when to inject the fuel. Injection timing is determined by the ECM after considering input from the coolant temperature sensor, intake manifold air temperature sensor, and boost pressure sensor. The ECM knows the cylinder position for timing because of the signal from the Engine Speed/Timing Sensors. The ECM adjusts timing for best engine performance, fuel economy and white smoke control. Actual and Desired Timing cannot be viewed with an Electronic Service Tool.The ECM knows where top center on cylinder #1 is from the signal provided by the engine Speed/Timing Sensors. The ECM decides when injection should occur relative to top center and provides the signal to the injector at the desired time. Fuel InjectionThe ECM controls the amount of fuel injected by varying signals to the injectors. The injectors will pump fuel only if the injector solenoid is energized. The ECM sends a high voltage signal to energize the solenoid. The energized solenoid lifts the poppet off its seat, closing the oil path to drain and opening