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The network of sensors has the job of converting
information the computer needs into electrical
signals the computer can understand. Signals
running from sensors to the PCM are referred to as
inputs. Sensors monitor the key parameters
shown in the table.
NOTE: Not all engines use every sensor listed.
Relays and Actuators
Relays and Actuators are electric devices energized by the computer to allow com-
mands to perform a specific function. Relays can be called switches (such as the
coolant fan switch). Actuators might include solenoids (such as fuel injector valves)
and small motors (such as the Idle Speed Control). Not all of the computers outgo-
ing signals are routed to relays and actuators. Sometimes information is sent to
other system computers like transmission, brakes, ignition modules, and trip
computers. These signals are also called outputs.
How the computer controls fuel delivery
Engine operation and emissions performance depend upon precise fuel delivery and
ignition control. Early computer systems controlled fuel by electronically adjusting the
carburetor metering and jet systems. Soon, however, this was replaced by the more
precise fuel delivery of fuel injection.
In an electronically carbureted system, the computer simply controls fuel flow based
on how far the throttle is opened by the driver. The computer knows how much air
can flow through the carburetor at various throttle openings, and adds the appropriate
amount of fuel to the mixture at the carburetor.
Fuel injection is some what more sophisticated in the way it delivers fuel. The
computer still adds an appropriate amount of fuel to the entering air, but now it uses
fuel injectors (either in a throttle body or at each intake port). Fuel injectors are far
more precise than carburetor jets, and create a much finer fuel mist for better
combustion and increased efficiency. In addition, most fuel injection systems have
ways of measuring exactly how much air is entering the engine, and can calculate the
proper air/fuel ratio using lookup tables. Computers no longer have to estimate how
much air the engine is using.
In many modern systems, the computer also uses information provided by sensors
to give it an idea of how well it is doing its job, and how to do it better. Sensors can tell
the computer how warm the engine is, how rich or lean the fuel mixture is, and
whether accessories (like the air conditioner) are running. This feedback information
allows the computer to fine tune the air/fuel mixture, keeping the engine operating at
its peak.
What the Computer needs to know:
Engine operating condition. Sensors used are: coolant temperature, throttle
position, manifold pressure (vacuum), air flow and RPM.
Air intake. Sensors used are: mass air flow, manifold absolute pressure, manifold
air temperature and RPM.
Air/fuel mixture status. Sensors used are: oxygen sensor(s).
Engine Temperature
Throttle position
Incoming Air Temperature
Air Fuel Ratio, in percentage (%)
Intake Manifold Vacuum
Engine RPM
Volume of Incoming air