Digital Signals
SWITCH TYPES
Switched inputs (also known as discretes) to the
computer can cause one bit to change, resulting in in-
formation being communicated to the computer.
Switched inputs can come in two types: they are
“pull-up” and “pull-down” types. Both types will be
discussed.
Digital signals are also variable, but not continuously.
They can only be represented by distinct voltages
within a range. For example, 1 V, 2 V or 3 V would be
allowed, but 1.27 V or 2.65 V would not. Digital sig-
nals are especially useful when the information can
only refer to two conditions - “YES” and “NO,” “ON”
and “OFF,” or “High” and “Low.” This would be called
a digital binary signal. A digital binary signal is limited
to two voltage levels. One level is a positive voltage,
the other is no voltage (zero volts). As you can see
in the following figure, a digital binary signal is a
square wave.
With a pull-up type switch, the ECM will sense a volt-
age when the switch is CLOSED. With the pull-down
switch, the ECM recognizes the voltage when the
switch is OPEN.
Discretes can also be used to inform a computer of
FREQUENCY information.
PULSE COUNTERS
For the computer to determine frequency information
from a switched input, the computer must measure
the time between voltage pulses. As a number of
pulses are recorded in a set amount of time, the com-
puter can calculate the frequency. The meaning of
the frequency number can have any number of
meanings to the computer.
HI
ON
YES
LO
OFF
NO
Digital Binary Signal
An example of a pulse counter type of input is the dis-
tributor reference pulse input. The computer can
count a train of pulses, a given number of pulses per
engine revolution, and determine the RPM of the en-
gine.
The computer uses digital signals in a code that con-
tains only ones and zeros. The high voltage of the
digital signal represents a one (1), and no voltage
represents a zero (0). Each zero and each one is
called a bit of information, or just a “bit.” Eight bits to-
gether are called a “word.” A word, therefore, con-
tains some combination of eight binary code bits:
eight ones, eight zeros, five ones and three zeros,
and so on.
Engine Control Module (ECM)
The Engine Control Module (ECM) is the control cen-
ter of the fuel injection system. It constantly monitors
information from various sensors, and controls the
systems that affect engine performance.
Binary code is used inside a computer and between
a computer and any electronic device that under-
stands the code. By stringing together thousands of
bits, computers can communicate and store an infi-
nite variety of information. To a computer that under-
standsbinary,11001011mightmeanthatitshouldre-
set engine RPM at a lower level. Although the
computer uses 8-bit digital codes internally and when
talking to another computer, each bit can have a
meaning.
The ECM also performs a diagnostic function check
of the system. It can recognize operational problems
and store a code or codes which identify the problem
areas to aid the technician in making repairs.
72801
Electronic Control Module (ECM)
Index
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ELECTRONIC FUEL INJECTION (MULTI-PORT AND THROTTLE BODY) - 5C-11
| Categories | Mercury MerCruiser Manuals |
|---|---|
| Tags | Mercury MerCruiser 454 CID, Mercury MerCruiser 502 CID |
| Model Year | 1993, 1994, 1995, 1996, 1997 |
| Download File |
|
| Document File Type | |
| Copyright | Attribution Non-commercial |
(4 votes, average: 4.5 out of 5)
Marine readers have rated 1993 1997 Mercury-MerCruiser GM V8 454 CID 7.4L and 502 CID 8.2L Marine Engines Service Manual Number 16 4.5 out of 5.0 based on 4 product reviews. Hugely helpful service manual! Perfect. Thank you!
Manuals are all Important for Technician
This book is invaluable if you do your own repairs. Great book.
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