Ignition Description - Mercury 15XD Service Manual

IGNITION

Ignition Description

The ignition system is an alternator driven capacitor discharge system. Major compo-
nents of the ignition system are the flywheel, stator, trigger coil, switch box, 2 ignition coils
and 2 spark plugs.
The flywheel has permanent magnets mounted in both the outer rim and the center hub.
The BLACK stator assembly is mounted below the flywheel and has a low speed (LS) and
a high speed (HS) capacitor charging coil. Low speed coil provides primary voltage to the
switch box from idle to approximately 2500 RPM. The high speed coil provides primary
voltage from 2000 RPM to the maximum RPM the outboard is capable of achieving.
The RED stator assembly is mounted below the flywheel and has only one capacitor
charging coil.
As the flywheel rotates, the magnets mounted in the flywheel outer rim pass the charging
coils creating voltage. This voltage charges the capacitor located in the switch box.
As the flywheel continues to rotate, the magnets in the center hub pass the trigger coil
creating AC voltage. This voltage turns on one of the two electronic switches (SCR) in the
switch box. A positive voltage pulse turns on the SCR switch associated with cylinder #1;
a negative voltage pulse turns on the SCR switch associated with cylinder #2.
The SCR switch discharges the stored capacitor voltage into the primary side of the re-
spective ignition coil. The ignition coil multiplies this voltage to a value high enough to
jump the spark plug gap -- 32000 volts for standard coils; 40000 volts for high energy coils.
This sequence occurs once per engine revolution for each cylinder
Spark timing on electronically advanced models is controlled internally by the switch box
with a fixed trigger.
Spark timing on mechanically advanced models is changed (advanced/retarded) by rotat-
ing the trigger coil which changes the trigger coil position in relation to the magnets in the
center hub of the flywheel.
The stop switch (or ignition switch) shorts the output of the stator to ground to stop the
engine on all models.
Page 2A-6 90-826883R2 JUNE 1998