Results Of Incorrect Operation - Mercury MerCruiser MCM 454 Mag MPI Service Manual

MULTI-PORT FUEL INJECTION DESCRIPTIONS AND SYSTEM OPERATION

Results of Incorrect Operation

Open IC Line from the ECM to the Distributor Module - While the engine is cranking, the
ECM expects to see the IC signal pulled to virtually zero because it is grounded in the distrib-
utor module. Since the IC line is open, it cannot be grounded by the module and the IC signal
will be able to rise and fall, or do what is called toggling. The ECM recognizes the toggling
as an abnormal condition, and will not apply bypass voltage to the distributor module when
the engine reaches run rpm.
Since bypass voltage is not applied to the relay, it remains open and the engine continues
to run on the pickup coil triggering in the ignition module timing mode.
If this condition occurs while the engine is running, the engine will stop, but it will restart and
run in the ignition module timing mode with reduced power.
Grounded IC Line - During cranking, the IC voltage is at virtually zero so the ECM does
not recognize a problem. When engine rpm reaches the value for the run condition, the ECM
applies bypass voltage to the distributor module. Bypass voltage on the module switches
the distributor power transistor to the IC line. Because the IC line is grounded, it will have
no voltage applied so it cannot operate the power transistor to enter the IC mode.
If the IC line becomes grounded while the engine is being operated, the engine will stop and
will be difficult to restart.
An open or ground in the IC or bypass will cause the engine to run on the distributor module
timing. This will cause reduced performance, poor fuel economy and erratic idle.
Grounded or Open Bypass Line - While the engine is cranking, the IC line will be grounded
and the ECM will not notice anything abnormal. When run rpm is reached, the ECM applies
bypass voltage to the bypass line but because of the ground or open, it will not be able to
energize the relay. Therefore, the relay will stay de-energized and the IC line will remain
grounded.
When the ECM sees the IC line not toggling, it will not enter the IC mode. Since the relay
is de-energized, the engine will continue to run in the ignition module timing mode.
If this condition occurs while the engine is running, the engine will simply operate in the
ignition module timing mode.
Open or Grounded REF HI Line - This line provides the ECM with engine speed informa-
tion. If this line is open or grounded, the ECM will not know that the engine is cranking or
running and will not run.
Open or Grounded REF LO Line - This wire is grounded in the ignition module and
provides a reference ground from the ignition module to the ECM. The ECM compares
reference ground with reference high voltage. If this circuit is open, or grounded at any other
location than through the module, it may cause poor performance.
Index
Page 5B-32
Downloaded from https://needmanual.com/
SERVICE MANUAL NUMBER 23
90-861326--1 MARCH 1999