Actron OBD II AutoScanner CP9135 Instructions Manual page 35

What does the computer control
The main control areas of the vehicle computer are fuel delivery, idle speed,
spark advance, and emission controls. Some on-board computers may also
control the transmission, brakes, and suspension systems as well.
What has not changed
A computer-controlled engine is very similar to the older, non-computerized
engine. It is still an internal combustion engine with pistons, spark plugs,
valves, and camshaft(s). The ignition, charging, starting, and exhaust systems
are very similar as well. You test and repair these systems just as before.
The technical manuals for these components show you how to perform the
tests. Additionally, compression gauges, vacuum pumps, engine analyzers,
and timing lights will continue to be useful.
The engine computer control system
The vehicle's on-board computer is the "heart" of the system. It is sealed in
an enclosure and connected to the rest of the engine by a wiring harness.
The computer is located, in most cases, in the passenger compartment,
behind the dashboard or in the "kick panel" position, although some
manufacturers locate the computer control module in the engine compartment
area. Most computers can withstand a lot of vibration and are built to live in
a rugged environment.
The computer is programmed by the manufacturer. The program is a complex
list of lookup tables and instructions telling the computer how to control the
engine based on various driving conditions. To do its job, the computer uses
sensors to know what is happening and then provides instructions back to a
network of switches and actuators throughout the vehicle.
Sensors (computer inputs)
Sensors are devices which measure operating conditions and translate them
into signals the computer can understand. Some examples of sensors:
thermistors (for temperature readings), potentiometers (like a throttle position
sensor), and signal generators (such as an 02 sensor).
Relays and actuators
Relays and Actuators are electric devices energized by the computer to
perform a specific function. A relay is an electromagnetic device (or switch)
for remote or automatic control that is actuated by the computer or another
device. Actuators might include solenoids (such as fuel injector valves) and
small motors (such as the Idle Speed Control). Not all of the computer's
outgoing 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
About OBD II
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