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Modern automotive engine management systems use a
complex network of electronic components to keep the
engine operating at peak efficiency with minimal emis-
sions, while at the same time delivering an acceptable
level of performance and driveability. Precise, rapid
electronic communications between the system com-
ponents is the key to maintaining this delicate balance.
Troubleshooting electronic engine control systems re-
quires a familiarity with automotive electrical and elec-
tronic systems and the use of computer control systems
on late-model vehicles. Following is a short review of
these principles to illustrate that there are more similar-
ities than differences among the different control sys-
tems.
The components used in an electronic control system
are extremely delicate and can easily be damaged by
improper handling or testing. Be aware, your body is a
conductor that is capable of producing a static electri-
cal charge that can destroy a sensor designed to oper-
ate on a low voltage signal. Wear a ground strap or dis-
charge any static electricity by touching a ground before
picking up an electronic component. Also, use only
equipment designed for electronic testing. Avoid using
analog test meters, which are typically designed for
working with high current circuits, to troubleshoot elec-
tronic systems and components.
COMPUTER CONTROL PRINCIPLES
The operation of any computer system is divided into
four basic functions: input, processing, storage, and
output. A computer operates by converting input volt-
age signals to other voltage signals that represent com-
binations of numbers. The numbers represent informa-
tion about quantities measured by sensors. These
include temperature, speed, distance, position, pres-
sure, and other factors of engine and vehicle operation.
The computer processes the voltage signals by com-
puting the numbers they represent and then delivering
output commands to actuators that control engine and
vehicle operation. Additionally, a computer stores
processed information and its own operating instruc-
tions in the form of other numerical information. There
are four basic functions of any computer system, figure
5-1.
1. Input—Input information provided as voltage
signals from system sensors is received by the
computer, or powertrain control module (PCM).
2. Processing—The PCM processes, or computes, the
input voltage signals into combinations of binary
numbers (1 and 0), or bits. It then compares the
processed information to instructions and other data
in its program to make logical decisions and send
output commands to other devices in the system.
3. Storage—Every computer has an electronic mem-
ory to store its operating instructions, or program.
Some computer systems also require that some
input signals be stored for later reference. Many
systems can “remember” previous operating con-
ditions and adapt their output commands to
“learned” characteristics of vehicle operation. All
systems have some kind of information storage ca-
pability, or memory.
4. Output—After receiving and processing input in-
formation, the computer sends output commands
to other devices to control engine operation. The
output devices are actuators such as solenoids
and relays. The computer may also send output in-
formation to display devices such as electronic in-
strument panels.
STORAGE
COMPUTER
PROGRAMS AND
MEMORY
PROCESSING
CENTRAL
PROCESSING
UNIT (CPU) OR
MICROPROCESSOR
OUTPUT
TO
ACTUATORS
INPUT
FROM
SENSORS
Fig. 5-1.
All computers perform four basic functions: input,
processing, storage, and output.
Chapter
E
COMPUTERIZED ENGINE CONTROL
DIAGNOSIS AND REPAIR (INCLUDING OBD II)
Summary of Contents for ASE-A8
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