Electronic Control Module (Ecm) And Sensors; General Description; Computers And Voltage Signals; Analog Signals - MerCruiser 305 CID (5.0L) Service Manual

FUEL INJECTION DESCRIPTIONS AND SYSTEM OPERATION

Electronic Control Module (ECM) and Sensors

General Description

The MerCruiser Electronic Fuel Injection system is equipped with a computer that provides
the operator with state-of-the-art control of fuel and spark 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 cur-
rent. 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 comput-
er 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, 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.
THREE-WIRE SENSORS (MAP AND TP)
The following figure shows a schematic representation of a 3-wire sensor. All 3-wire sensors
have a reference voltage, a ground and a variable "wiper." The lead coming off of the wiper
will be the signal to the Engine Control Module (ECM). As this wiper position changes, the
signal voltage returned to the computer also changes.
3-Wire Sensor
Index
Page 5D-12
a - Typical Sensor
b - ECM
c - Voltage Out
d - Signal Input
e - Sensor Ground
a
SERVICE MANUAL NUMBER 24
b
c
d
90-861327 OCTOBER 1999