Chapter 7 Computer Control Fundamentals 119
The engine control computer in various makes and
models of vehicles is called by different terms, according to
individual manufacturer preferences. Some of these terms
include:
p
On-board computer.
p
Microprocessor.
p
Controller.
p
Engine control module (ECM).
p
Electronic control unit (ECU).
p
Electronic engine control (EEC).
p
Powertrain control module (PCM).
p
Vehicle control module (VCM).
Since there are over two dozen motor vehicle manu-
facturers, the number of names for the computer can cause
confusion. In this book, the engine-control computer is
always called the ECM for engine control module. Calling
engine control computers ECMs makes it easier to separate
them from other vehicle computers and the computers used
in diagnostic equipment.
Multiple ECMs
Almost all vehicles have more than one control. There
may be separate control modules for the transmission, anti-
lock brakes, air conditioner, and body electrical circuits.
These control modules communicate with each other
and act as a single unit when tasks in different parts of the
vehicle must be coordinated.
Note: The computers that control engine
operation on most vehicles should not be
confused with those operating the anti-lock
brake system (ABS), cruise control, air bags, anti-
theft system, climate control system, or suspension.
Main Control Modules
In most vehicles, one computer has the job of manag-
ing the data flows of the other vehicle control modules. The
job may be assigned to the vehicle control module (VCM),
body control module (BCM), or instrument cluster module
(ICM). The control module also monitors and sometimes
controls other vehicle systems besides the engine and
transmission. Examples are anti-lock brakes, stability con-
trol systems, and audio/video systems.
Having one computer control all information passing
between control modules means that some other comput-
ers or components can be eliminated. Other advantages
include centralized control of many vehicle systems, reduc-
ing the possibility of crossed signals and interference.
Thirty-Two-Bit ECM
The latest ECMs are 32 bit. This means that they can
process a piece of information consisting of 32 numbers
and letters as a single piece of data. Older ECMs could
process only 16 numbers and letters as a single data piece.
A 32-bit computer works much faster than older models,
interfaces better with CAN busses, and can store as many
as 4 billion numbers. The 32-bit ECM controls all of the
electrical devices on a vehicle, either directly or through
other ECMs attached through a CAN system. CAN systems
are discussed later. Figure 7-3 shows a 32-bit ECM installed
on the vehicle firewall.
ECM Communications
Signals from the input sensors and to the output
devices are analog signals, or signals composed of variable,
nondiscreet voltage levels. The analog signals to and from
the sensors vary from millivolts to battery voltage, depend-
ing on the system, and are usually referred to as reference
voltages. Computers are digital devices working with a
series of on and off, or discreet, signals. This information is
referred to as serial data or the data stream.
The ECM operates output devices by turning them on
and off. Output device operation is controlled by varying
the length of time the device is operated, either by apply-
ing voltage or supplying ground. The length of time over
which the signal or ground is provided is called the pulse
width, Figure 7-4. The ECM also provides external com-
munications to other on-board computers and diagnostic
equipment, which are discussed in the next section.
Multiplexing and Data Busses
As explained earlier, the ECMs in most vehicles com-
municate with each other. They transfer information via a
path called a data bus. The data bus is simply a roadway
for data transfer between the ECMs and other control
modules.
Figure 7-3. The ECM in this vehicle operates all of the vehicle
systems, either directly or by controlling messages to other vehi-
cle computers. Fins are cast into the body for heat removal.