Actron GM Code Scanner CP9001 - Manual (Page 27)

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COMPUTER BASICS
What does the Engine Control Computer do?
This section further explains the engine
computer control system, the types of
sensors and how the computer controls
fuel delivery, idle speed and timing.
The following is an introduction to
computer controlled engine systems.
Additional information may be found in
books dealing with this subject
available at your local library or auto
parts store. The more you know about
the computer system, the better and
faster you can troubleshoot and fix
problems.
Why Computers?
Computer controls were installed in
vehicles to meet Federal Government
regulations for lower emissions and
better fuel economy. This all began in
the early 1980’s when purely mechani-
cal control systems just were not good
enough anymore. A computer could be
programmed to precisely control the
engine under various operating
conditions and eliminate some
mechanical parts making the engine
more reliable.
Note that vehicle service manuals refer
to the computer as either the ECM
(Engine Control Module) or PCM
(Powertrain Control Module).
What the computer controls
The main control areas of the computer
are:
• Fuel delivery
• Idle speed
• Spark advance timing
• Emission devices (EGR valve,
carbon cannister, etc.)
The changes made to the basic engine
to allow a computer to control these
tasks are the only differences between
an older engine and a computerized
one. A little later we will discuss just
how the computer handles these tasks.
What has NOT changed?
A computer controlled engine is
basically the same as earlier types. It is
still an internal combustion engine with
pistons, spark plugs, valves and cams.
The ignition, charging, starting, and
exhaust systems are almost the same,
as well. You test and repair these
systems the same way as before,
using familiar tools. The instruction
manuals for these tools show you how
to perform the tests. Your compression
gauge, vacuum pump, dwell-tach
meter, engine analyzer, timing light,
etc., are still valuable!
The Engine Computer Control
System
The computer module is the “heart” of
the system. It is sealed in a metal box
and linked to the rest of the system by
a wiring harness. The computer
module is located in the passenger
compartment, usually behind the
dashboard or front kick panels. This
protects the electronics from moisture,
extreme temperatures and excess
vibration, which are common in the
engine compartment.
The computer is permanently
programmed by factory engineers. The
program is a complex list of instruc-
tions telling the computer how to
control the engine under various
driving conditions. To do its job, the
computer needs to know what is
happening and then it needs devices to
control things.
Sensors give the computer
information
The computer can only work with
electrical signals The job of the sensor
is to take something the computer
needs to know, such as engine
temperature, and convert it to an
electrical signal which the computer