Speed Density System - Mercury MerCruiser MCM 454 Mag MPI Service Manual

SERVICE MANUAL NUMBER 23

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 manage-
ment portion of its operation. That is, three specific signals to the ECM establish the engine
speed and air density factors.
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 deter-
mine the "speed" or rpm factor for fuel and ignition management.
DENSITY
Two sensors contribute to the density factor, the Intake 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 resistance 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 engine loads. These pressure
changes are supplied to the ECM in the form of electrical signals.
As intake manifold pressure increases (vacuum decreases), 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, de-
livered by the fuel injection system.
The remaining sensors and switches provide electrical inputs to the ECM which are used
for modification of the air/fuel mixture, as well as for other ECM control functions, such as
Idle Air Control (IAC).
Index
90-861326--1 MARCH 1999
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MULTI-PORT FUEL INJECTION DESCRIPTIONS AND SYSTEM OPERATION
Page 5B-19