Mercury MerCruiser MCM 454 Mag MPI Service Manual page 691

GENERAL SYSTEM DIAGNOSTICS
Discrete Input Circuit Check - Non-Scan - 7.4L MPI Only (1 of 5)
a
CIRCUIT DESCRIPTION:
Several discrete switch inputs are utilized by the fuel injection system to identify abnormal
conditions that may affect engine operation. A pull-up switch is currently used in conjunction
with the ECM to detect critical conditions to engine operation.
If a discrete switch changes states from its normal at-rest position, that is normally open to
closed (or closed to open), the ECM senses a change in voltage and responds by activating
the audio warning system.
TEST DESCRIPTION:
A problem with the discrete circuit system will have to be broken down into several sub-
steps. These will include the following:
Testing the audio warning buzzer.
Testing the individual switches.
Testing the wiring.
Be sure that all items above have been performed prior to replacement of the ECM.
DIAGNOSTIC AIDS:
Check engine oil and gear lube levels. Check transmission fluid for overheat condition.
An intermittent problem may be caused by a poor or corroded connection, rubbed through
wire connection, or a wire that is broken inside the insulation.
Index
Page 5E-98
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3 PNK
E DK BLU
31A DK GRN
D
C 3 PNK
585 TAN/WHT
B
A
121 WHT
b
BLK
a - Harness Connector
b - Gear Lube Switch
c - Oil Pressure Switch
d - Coolant Over Temperature & Discrete Switch Output
e - Not Used
f - Discrete Switch Input (J1-4 For Earlier Models And J1-6 For Earlier And J1-6
For Later Models)
SERVICE MANUAL NUMBER 23
d
112 GRY
J2-11
e
31 TAN J2-26
J1-4
931 BRN
or
f
J1-6
c
90-861326--1 MARCH 1999