Mercury MerCruiser MCM 454 Mag MPI Service Manual page 678

SERVICE MANUAL NUMBER 23
Chart A-7 (2 of 6): Ignition System Check
DIAGNOSTIC AIDS:
An intermittent may be caused by a poor connection, rubbed through wire insulation or a
wire broken inside the insulation. Check for the following items:
TEST DESCRIPTION:
Number(s) below refer to the step number(s) on the diagnostic table:
2. Two wires are checked to ensure that an open is not present in a spark plug wire.
4. A spark indicates the problem must be in the distributor cap, rotor, or coil output wire.
6. Normally, there should be battery voltage at the "C" and "+" terminals. Low voltage would
8. Checks for a shorted module or grounded circuit from the ignition coil to the module. The
11. Applying a voltage (1.35 -1.50 volts) to the module terminal "P" should turn the module
12. This should turn the module "OFF" and cause a spark. If no spark occurs, the fault is
Index
90-861326--1 MARCH 1999
Downloaded from https://needmanual.com/
Poor connection or damaged harness. Inspect the ECM harness and connectors for
improper mating, broken locks, improperly formed or damaged terminals, poor terminal
to wire connection, and damaged harness
The "tach" needs to be disconnected while testing the ignition system. You will also need
a place to check coil trigger voltage. By disconnecting the "5-wire harness connector"
(WHITE and PINK wires), this will give you a test terminal to check coil trigger voltage as
needed in several steps. After "tach" is disconnected, try starting the engine. If the
engine starts, check for a short to ground in the boat "tach" circuit.
indicate an open or a high resistance circuit from the distributor to the coil or ignition
switch. If "C" terminal voltage was low, but "+" terminal voltage is 10 volts or more, circuit
from "C" terminal to ignition coil is open or primary winding of the ignition coil is open.
distributor module should be turned "OFF," so normal voltage should be about 12 volts.
If the module is turned "ON," the voltage would be low, but above 1 volt. This could cause
the ignition coil to fail from excessive heat. With an open ignition coil primary winding,
a small amount of voltage will leak through the module from the "batt" to the "tach"
terminal.
"ON" and the tach voltage should drop to about 7-9 volts. This test will determine
whether the module or coil is faulty or if the pick-up coil is not generating the proper sig-
nal to turn the module "ON". This test can be performed by using a DC test battery with
a rating of 1.5 volts (Such as M, C, or D cell). The battery must be a known good battery
with a voltage of over 1.35 volts.
most likely in the ignition coil because most module problems would have been found
before this point in the procedure.
GENERAL SYSTEM DIAGNOSTICS
Page 5E-85