Spark Management; High Energy Ignition With Ignition Control; Modes Of Operation; Distributor Module Mode - Mercury MerCruiser MCM 454 Mag MPI Service Manual

SERVICE MANUAL NUMBER 23

Spark Management

High Energy Ignition with Ignition Control (IC)
The Electronic Fuel Injection is controlled by an Engine Control Module (ECM). This module
is the nerve/decision center of the system. It uses all the information it gathers to manage
ignition spark, delivering increased fuel economy and maximum engine performance.
The system uses inputs from sensors to make decisions on the amount of spark advance
or retard allowed.
The system has been designed to control ignition advance and retard electronically by the
ECM.
In order for the ECM to properly calculate spark advance, it must always know at what speed
the engine is running. The engine speed signal is accomplished by a circuit within the distrib-
utor module which converts the pickup coil voltage to a square wave reference signal that
can be used by the ECM. This square wave engine speed reference signal is known as REF
HI. The ECM must also have something to compare the REF HI value against. Therefore,
an additional line is provided between the ECM and the distributor module that is known as
REF LO. These two lines, between the ECM and the distributor, provide a precise indication
of engine speed.
The two other lines between the ECM and distributor which control the Ignition Control (IC)
operation are known as the bypass and IC circuits.

Modes Of Operation

There are two modes of ignition system operation:

DISTRIBUTOR MODULE MODE

The ignition system operates independent of the ECM. The distributor module module in
the distributor maintains a base ignition timing and is able to advance timing to a total of 27
degrees. This mode is in control when a Code 42 is detected while engine is running and
will have a noticeable effect on engine operation.

ECM CONTROL MODE

The ECM control mode controls the ignition timing. The ECM calculates the desired ignition
timing based on information it gets from input sensors.
Index
90-861326--1 MARCH 1999
Downloaded from https://needmanual.com/
MULTI-PORT FUEL INJECTION DESCRIPTIONS AND SYSTEM OPERATION
Page 5B-27