DESCRIPTION
SERVICE MANUAL NUMBER 22
90-860074--1 FEBRUARY 2002 Page 5A-9
Electronic Control Module (ECM) and Sensors
General Description
The Mercury MerCruiser Electronic Diesel Injection (EDI) system is equipped with a
computer that provides the operator with state-of-the-art control of fuel delivery. Computers
use voltage to send and receive information.
Computers and Voltage Signals
Voltage is electrical pressure. Voltage does not flow in circuits. Instead, voltage causes
current. Current does the real work in electrical circuits. It is current, the flow of electrically
charged particles, that energizes solenoids, closes relays and lights lamps.
Besides causing currents in circuits, voltage can be used as a signal. Voltage signals can
send information by changing levels, changing waveform (shape) or changing the speed
at which the signal switches from one level to another. Computers use voltage signals to
communicate with one another. The different sections inside computers also use voltage
signals to communicate with each other.
There are two kinds of voltage signals, analog and digital. Both of these are used in
computer systems. It’s important to understand the difference between them and the
different ways they are used.
Analog Signals
An analog signal is continuously variable. This means that the signal can be any voltage
within a certain range. An analog signal usually gives information about a condition that
changes continuously over a certain range. For example, in a marine engine, information
about temperature is usually provided by an analog signal. There are two general types of
sensors that produce analog signals: the 3-wire and the 2-wire sensor.
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