Mercury MerCruiser MCM 454 Mag MPI Service Manual page 488

SERVICE MANUAL NUMBER 23
Distributor Module Mode
The following describes IC operation during cranking and when the engine starts running.
During cranking, the relay is in the de-energized position (see figure "Distributor Module
Mode"), preceding. This connects the pickup coil to the base of the transistor via the signal
converter. When the pickup coil applies a positive voltage to the transistor, the transistor
turns ON. When voltage is removed, the transistor turns OFF. When the transistor turns ON,
current flows through the primary winding of the ignition coil. When it turns OFF, the primary
current stops and a spark is developed at the spark plug. A small amount of advance is built
into the IC module via a timing circuit, in case the engine remains in the ignition module tim-
ing mode.
With the relay de-energized, a set of contacts (shown closed) would ground the IC line
signal.
ECM Control Mode
When the engine rpm reaches a predetermined value (for this example, 300 rpm), the ECM
considers the engine running and applies five volts on the bypass line to the IC module. This
energizes the relay and causes the contacts from the pickup coil and the grounding contacts
for the IC line to open. This connects the IC line to the base of the transistor, and bypasses
the ignition module timing control.
The IC system is now controlled by the IC signal from the ECM and the time at which the
spark occurs can be determined by a variable time circuit in the ECM.
Index
90-861326--1 MARCH 1999
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MULTI-PORT FUEL INJECTION DESCRIPTIONS AND SYSTEM OPERATION
Page 5B-29