The computer, or electronic control unit (ECU), is the "brain" of the fuel injection system. The sensors and wiring harness serve as the "nervous system": checking temperatures, positions, and other considerations for proper injection system operation. Fig below shows how electric current is fed to the computer from various sensors and how the computer feeds current to the injectors. A car's computer is actually a preprogrammed microcomputer (preset, miniaturized electronic circuit). It has microscopic electronic circuits which are formed inside integrated circuits (ICs).
An integrated circuit is an electronic chip or circuit manufactured by photographically reducing a circuit and placing it on a special semiconductor (transistor type) material. This enables the computer to have literally hundreds or thousands of transistors, resistors, capacitors, and similar components in a very small space. Different circuits are provided in the computer for performing different functions.
Engine sensors send flow of information to electronic control module (on-board computer) in form of small electric currents. The module, acting on signal received, feeds current that operates injectors.
On-board computer contains thousands of miniaturized circuits.
The picture is showing a photo of the inside of an automobile computer. Note the very small components, especially the integrated circuits.
The on-board computer is about the size of a car radio and is often placed in the passenger compartment. Since computers are sensitive to vibration, extreme temperature change, and moisture, they are sometimes located behind or under the dash panel, Fig.3.7. This places them away from engine heat, moisture, and the elements in the engine compartment.
There are four basic parts or sections to a car's computer: input/output devices, central processing unit, power supply, and memories.
Input/output Devices
The input/output devices are electronic circuits that convert signals from sensors into digital (on/off or computer) signals for use in the central processing unit (brain or calculator section) of the computer. The devices (circuits) can also change computer language into electrical signals to operate system components.
On-board computer is usually behind instrument panel (dash). In this location, it is shielded from damaging engine heat and vibration. Some computers are mounted on air cleaner or elsewhere in engine or passenger compartment. (Cadillac)
Central Processing Unit
The central processing unit performs mathematical functions or logic functions to deliver the correct air-fuel ratio and to operate other system devices. It uses digital signals from the input devices to determine what is going on during vehicle operation and what should be done to increase efficiency.
Power Supply
The power supply in a car's computer prevents voltage fluctuations that could affect computer operation. A computer relies on very smooth dc current, mainly from the car battery. The power supply simply regulates input voltage to other parts of the computer.
Computer Memories
Most computers have three basic types of memory circuits: read only memory, random access memory, and programmable read only memory.
The read only memory (ROM) is programmed data that can only be analyzed by the computer itself. It is information used by the computer in performing the various functions. The ROM program cannot be changed. If the battery or voltage supply is disconnected from the computer, the data in the ROM will remain in the computer.
The random access memory (RAM) is temporary information held in the computer. It is like a "note pad" of inputs and outputs. Data such as self-diagnosis codes can be pulled out of RAM. If battery voltage is removed, all information is erased from RAM.
The programmable read only memory (PROM) has information on the particular make and model car. It has data about engine size, vehicle weight, transmission type, rear axle ratio, etc. As you will learn in the chapter on fuel injection service, the PROM is normally removed and reused when replacing the central processing unit (computer). If voltage is disconnected from the PROM, it will retain its information.
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