Speed Density System - Mercury MerCruiser D2.8L D-Tronic Service Manual

SERVICE MANUAL NUMBER 22

Speed Density System

The Electronic Diesel Injection (EDI) 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 factors: the Engine rpm Sensor, Intake Air Sensor and the
Manifold Absolute Pressure Sensor.
SPEED
The Engine rpm Speed sensor signal comes from a three-wire magnetic pickup mounted
on the port side of the flywheel housing. The ECM uses this information to determine the
speed or rpm factor for fuel quantity and injection timing management.
DENSITY
The Intake Air Temperature (IAT) and the Manifold Absolute Pressure (MAP) sensors are
joined into one sensor assembly. These two sensors contribute to determining the air
density factor.
The IAT sensor circuit 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 circuit is a 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 or 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 applying voltage to the Fuel Quantity Actuator (rotary actuator).
As manifold pressure decreases or vacuum increases, the amount of fuel is decreased.
When no voltage is applied to the actuator, its return springs reduce the injected fuel quantity
to zero (none injected).
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 electrical inputs to the ECM which are used
for modification of the air/fuel mixture, as well as for other ECM control functions.
90-860074--1 FEBRUARY 2002
DESCRIPTION
Page 5A-17