BOLT 6V0371 - Caterpillar

The Diagnostic Process
System Operation
The 3176 Diesel Truck Engine was designed from the beginning for electronic control. The injection pump, fuel lines and nozzles used in traditional Caterpillar engines have been replaced with an electronic unit injector in each cylinder. A solenoid on each injector controls the amount of fuel delivered by the injector. An Electronic Control Module (ECM) sends a signal to each injector solenoid, to provide complete control of the engine.Electronic Controls
The 3176 Diesel Truck Engine electronic system consists of two main components: the Electronic Control Module (ECM) and the Personality Module. The ECM is the computer which controls the 3176 engine. The Personality Module is the software which controls how the computer behaves (the personality module stores the operating maps that define horsepower, torque curves, rpm, etc.). The two must be used together: neither can do anything by itself.The ECM determines a "desired rpm" based on the throttle signal, vehicle speed signal (only while in cruise control), PTO switches (only while in PTO) and certain diagnostic codes. The ECM then maintains the desired engine rpm by sensing actual engine rpm and deciding how much fuel to inject in order to achieve the desired rpm.Fuel Injection
The ECM controls the amount of fuel injected, by varying signals to the injectors. The injectors will inject fuel ONLY if the injector solenoid is energized. The ECM sends a 100 volt signal to the solenoid to energize it. By controlling the timing and duration of the 100 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. "FRC Fuel Pos" is a limit based on boost pressure to control the fuel-air ratio, for emissions control purposes. When the ECM senses a higher boost pressure (more air into cylinder), it increases the "FRC Fuel Pos" limit (allows more fuel into cylinder). "Rated Fuel Pos" is a limit based on the horsepower rating of the engine. It is similar to the rack stops and torque spring on a mechanically governed engine. It provides horsepower and torque curves for specific engine family and rating. All of these limits are programmed at the factory into the Personality Module and are not programmable in the field.Injection timing depends on engine rpm, load and other operation factors. The ECM knows where top-dead-center on cylinder number one is from the signal provided by the engine Speed/Timing Sensor. It decides when injection should occur relative to top-dead-center and provides the signal to the injector at the desired time.Under certain conditions, the 3176 Engine may intentionally operate on just three cylinders. This feature is called "Three Cylinder Cutout" and occurs when the engine is operated under no load, at high rpm (when signals to the injectors are of very short duration). Under these conditions, the ECM does not inject fuel into cylinders 4, 5, and 6. The accompanying change in sound and feel of the engine should not be mis-diagnosed as an engine