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COMPUTER BASICS
What does the Engine Control Computer do?
EEC-IV and MCU
This section explains the EEC-IV engine
computer control system, the types of
sensors and how the computer controls
fuel delivery, idle speed, spark timing
and emission devices. The MCU system
is described later, but this entire section
must still be read for complete under-
standing.
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 mechanical
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 mechani-
cal parts making the engine more
reliable.
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 pro-
grammed by factory engineers. The
program is a complex list of instructions
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 can understand. You can think
of sensors as “high tech” senders the
devices found in older vehicles for
gauges and dashboard message lights
