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SECU-3 is an internal combustion engine control unit. It is being developed as an open source project (drawings, schematic diagrams, source code etc. are open and freely available for all). Anyone can take part in the project, and can access all the information without any registrations. SECU-3 system controls the ignition, fuel injection and various other actuators of the internal combustion engine (ICE) and vehicle. In particular, it is capable of controlling the carburetor choke using a stepper motor (auto choke), thus controlling RPM when engine is warming up. SECU-3 manages AFR on the carburetor engines (similar to AXTEC AFR systems), idle cut-off valve and wide open throttle mode valve in carburetor systems, controls electric fuel pump and gas valves in closed loop mode according to the feedback from the oxygen sensor. The SECU-3 system provides unique opportunities for reassigning the I/O pins of the mainboard for custom uses in engine tuning. It also provides smooth speed control of the engine electric cooling fan. The system includes its own software which allows editing all major settings and fuel and ignition maps in real time (when the engine is running), and switching between 2 or 4 sets of maps. SECU-3 system has many other advanced features (listed below). Currently, there are five modifications of the unit: The device is developed using the 8-bit AVR microcontroller ATMega644, with 64kB memory (ROM), 4kB random access memory (RAM), and operates at a clock frequency of 20 MHz. It includes analog and digital inputs, separate chip for preprocessing signal from the knock sensor (KS) (except SECU-3 'Lite' and 'Micro' units), a signal conditioner for VR start-pulse sensor (except SECU-3 Micro unit), a signal conditioner for the VR crankshaft position sensor (CKP), the interface with a computer, and the outputs for actuators control. Structural diagram of the system with SECU-3T unit: Structural diagram of the system with SECU-3L unit is shown on the following picture: Structural diagram of the system with SECU-3 Micro unit: Example of wiring diagram of the SECU-3T unit for controlling of simultaneous or semi-sequential fuel injection on the 4-cylinder engine is shown on the picture below. Hi-z injectors and stepper IAC valve are used. On the right side of picture we can see external connector functions which should be remapped to specified values. It is done in the SECU-3 Manager software.