Mercury 305 CID, Service Manual, Page: 324 / 967

THUNDERBOLT IV AND V IGNITION SYSTEM - 4B-15 90-823225--1 1096
General Description
The Thunderbolt V ignition system has several spark
control features that will be described following:
Idle Speed Spark Control
Acceleration Spark Advance
Mean-Best-Timing Spark Advance
Over-Speed Control (Rev-Limiter)
Knock Retard Spark Control
NOTE: The Ski models do not have the Mean Best
Timing feature.
Idle Speed Spark Control
The ignition module will control ignition timing to
maintain a calibrated idle speed. This is accom-
plished by making small spark advance adjustments.
This feature is only active within a certain RPM
range. This range may be slightly different from one
engine model to another. The approximate range is
400-700 RPM.
Acceleration Spark Advance
This feature is active during acceleration only. When
accelerating, the ignition module may add more
spark advance to the “Base Spark Timing Curve”.
The amount of spark advance added, is totally de-
pendant on how fast RPM increases (how fast the
throttle is moved). This feature is also active within a
certain RPM range. This range may be slightly differ-
ent from one engine model to another. The approxi-
mate RPM range for this feature is 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
searches for the optimal ignition timing. This is also
accomplished by small changes to the spark ad-
vance. At a given RPM, the module will try to add a
small amount of advance and wait to see if there is
an RPM change. If RPM increases, it will try to in-
crease timing more. The module will continue to ad-
vance 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. The approxi-
mate RPM range for this feature is 1200-4000 RPM.
Within this range, 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.
This feature has an RPM range that varies from mod-
el to model. The over-speed limit for a particular en-
gine is set slightly higher than the top end of the RPM
range for that model. For example, if the recom-
mended 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 approxi-
mately 4750 RPM for this example. At this point,
spark comes back on.
Knock Retard Spark Control
The knock control feature helps provide protection
from harmful detonation. Knock control is handled by
the Knock Control Module. This module receives a
signal from a sensor that is mounted on the engine
block. The knock control module works in conjunc-
tion with the ignition module to retard the timing if
spark knock is present.