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About Diagnostic Systems
HOW DOES A COMPUTER SYSTEM WORK ON A VEHICLE?
Chapter 2 12
The on-board computer receives information (inputs) from
sensors and switches located throughout the engine. These
devices monitor critical engine conditions (coolant
temperature, engine speed, engine load, throttle position,
air/fuel ratio etc.). The computer compares the actual values
received from these sensors with the reference values that are
programmed in it’s memory, and makes corrections as needed
so that the sensor values always match the pre-programmed
reference values for that particular driving condition.
Since vehicle operating conditions are constantly changing, the
computer continuously makes adjustments or corrections
(especially to the air/fuel mixture and spark timing) to keep all
the engine systems operating within the pre-programmed
reference values.
NOTE:
The computer does not make the adjustments or
corrections directly. It commands other devices such as the fuel
injectors, idle air control, EGR valve or Ignition Module to
perform these functions. These devices are called Actuators
because they initiate an action in response to the commands of
the computer.
How a Special Program in the Computer Detects and
Reports Problems in the System
•
Beginning in 1988 California’s Air Resources Board (CARB),
and later, the Federal Government's Environmental
Protection Agency (EPA), required vehicle manufacturers
to include a self diagnostic program capable of identifying
an emissions-related fault in a system in their On-board
Computers. The first generation of On-board
Diagnostics came to be known as OBD I.
NOTE:
Most manufacturers (including Ford) began
installing computers with On-Board Diagnostics on some
of their vehicles as early as 1981.
•
OBD I is a set of self-testing or self-diagnosing instructions
that are programmed into the vehicle’s on-board computer.
•
The program is specifically designed to detect failures in
the sensors, actuators, switches and wiring of the various
vehicle emissions-related systems (fuel injection system,
ignition system, EGR system, catalytic converter, etc.). If
the computer detects a failure in any one of these
components or systems, it alerts the driver by
illuminating a light on the dash (the light will illuminate
only if it is an emissions-related problem).
2
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