Speed Density System - Mercury 305 CID Service Manual

ECM FUNCTION
The ECM supplies 5 or 12 volts to power various sen-
sors or switches. This is done through resistances in
the ECM which are so high in value that a test light
will not light when connected to the circuit. In some
cases, even an ordinary shop voltmeter will not give
an accurate reading because its resistance is too low.
Therefore, the use of a 10 megohm input imped-
ance digital voltmeter is required to assure accu-
rate voltage readings.
MEMORY
There are three types of memory storage within the
ECM: ROM, RAM and EEPROM.
ROM
Read Only Memory (ROM) is a permanent memory
that is physically soldered to the circuit boards within
the ECM. The ROM contains the overall control pro-
grams. Once the ROM is programmed, it cannot be
changed. The ROM memory is non-erasable, and
does not need power to be retained.
RAM
Random Access Memory (RAM) is the microproces-
sor "scratch pad." The processor can write into, or
read from, this memory as needed. This memory is
erasable and needs a constant supply of voltage to
be retained.
EEPROM
Electronic Erasable Programmable Read Only
Memory (EEPROM) is the portion of the ECM that
contains the different engine calibration information
that is specific to each marine application.

Speed Density System

The Electronic Fuel Injection system is a speed and
air density system. The system is based on "speed/
density" fuel management.
Three specific data sensors provide the ECM with the
basic information for the fuel management portion of
its operation. That is, three specific signals to the
ECM establish the engine speed and air density fac-
tors.
Index
5E-12 - ELECTRONIC FUEL INJECTION (MULTI-PORT AND THROTTLE BODY)
SPEED
The engine speed signal comes from the distributor's
High Energy Ignition (HEI) module to the ECM on the
distributor reference high circuit. The ECM uses this
information to determine the "speed" or RPM factor
for fuel and ignition management.
DENSITY
Two sensors contribute to the density factor, the In-
take Air Temperature (IAT) [Multi-Port models only]
and the Manifold Absolute Pressure (MAP) sensors.
The IAT sensor is a 2-wire sensor that measures the
temperature of the air entering the intake manifold.
The IAT sensor is a thermistor that changes its resis-
tance depending on the air temperature. When the
temperature is low, the resistance is high, and when
the temperature is high, the resistance is low.
The Manifold Absolute Pressure (MAP) sensor is a
3-wire sensor that monitors the changes in intake
manifold pressure which results from changes in en-
gine loads. These pressure changes are supplied to
the ECM in the form of electrical signals.
As intake manifold pressure increases (vacuum de-
creases), the air density in the intake manifold also
increases, and additional fuel is required.
The MAP sensor sends this pressure information to
the ECM, and the ECM increases the amount of fuel
injected by increasing the injector pulse width. As
manifold pressure decreases (vacuum increases),
the amount of fuel is decreased.
These three inputs MAP, IAT and RPM are the major
determinants of the air/fuel mixture, delivered by the
fuel injection system.
The remaining sensors and switches provide electri-
cal inputs to the ECM which are used for modification
of the air/fuel mixture, as well as for other ECM con-
trol functions, such as Idle Air Control (IAC).
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