SERVICE MANUAL NUMBER 24
IGNITION SYSTEM
Carbureted Engines - Thunderbolt V Ignition
Spark Control Features
IDLE SPEED SPARK CONTROL
The Ignition Module controls ignition timing to maintain a calibrated idle speed by making
small spark advance adjustments. This feature is only active between 400-700 rpm.
ACCELERATION SPARK ADVANCE
When accelerating, the Ignition Module may add more spark advance to the “Base Spark
Timing Curve.” The amount of spark advance added depends on how fast rpm increases.
This feature is also active between 1200-4000 rpm. Within this range, the module can add
approximately 10 degrees of spark advance to the base spark timing curve.
MEAN-BEST-TIMING (MBT) SPARK ADVANCE
During light load cruising, the Ignition Module maintains optimal ignition timing by making
small spark advance adjustments. At a given rpm, the module will add a small amount of
advance and wait to see if there is an rpm change. If rpm increases, it will increase timing
more. The module will continue to advance timing until it no longer gets an increase in rpm.
Conversely, if it senses an rpm drop, it will start to retard some of the spark timing. Between
1200-4000
rpm the Ignition Module can add approximately 10-15 degrees of spark advance
to the base spark timing curve.
NOTE: The Audio Warning System is also connected into the Ignition Module circuit. If the
audio warning system becomes activated by the closing of one of the audio warning system
switches, the MBT feature is deactivated.
OVER-SPEED CONTROL
The Ignition Module will prevent the engine speed from exceeding a preset limit by stopping
the spark. The over-speed limit is set slightly higher than the top end of the rpm range. For
example, if the recommended range is 4600-5000 rpm, the over-speed limit would be set
at 5100 rpm. When rpm reaches this limit, spark is turned-off until engine rpm drops down
to a RESET rpm, which would be approximately 4750 rpm for this example. At this point,
spark comes back on.
90-861327--1
OCTOBER 1999
Page 4B-11
| Categories | Mercury MerCruiser Manuals |
|---|---|
| Tags | Mercury MerCruiser 305 CID, Mercury MerCruiser 350 CID |
| Model Year | 1998, 1999, 2000, 2001 |
| Download File |
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| Document File Type | |
| Copyright | Attribution Non-commercial |
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