Mercury 3.0 Liter Service Manual download pdf (Page 35)

90-898303 2-30

Capacitor Charging #4, #5 & #6 CDMs

The flywheel rotates the permanent magnets past the capacitor charging coils

causing the coils to produce AC voltage (260-320 volts). The opposite voltage

pulse is then conducted to the CAPACITOR DISCHARGE MODULES (CDM),

where it is rectified (DC) and stored in a capacitor. The stator voltage return

path is through the ground wire one of the other CDMs and back through that

CDM’s charging coil wire to the capacitor charging coils.

NOTE: The CDM contains a zener diode (not shown for clarity). The zener

diode regulates the capacitor voltage to 300 volts, preventing overcharging of

the capacitor (and possible failure) if the SCR does not receive a trigger pulse.

#1 Cylinder Trigger Circuit

The TRIGGER assembly (also mounted under the flywheel) has three coils,

one for two cylinders - one on each bank. These coils are mounted adjacent to

the flywheel center hub. The center hub of the flywheel contains a permanent

magnet with two north-south transitions.

As the flywheel rotates, the magnet north-south transitions pass the trigger

coils. This causes the trigger coils to produce a voltage pulse which is sent to

the control module. The control module shapes the signal before sending it

onto the capacitor discharge module (CDM). A positive voltage pulse will

activate the electronic switch (SCR) inside the capacitor discharge module

(CDM). The switch discharges the capacitor voltage through the coil primary

windings. The return voltage pulse exits the CDM through the ground wire and

returns through the control module.

Spark timing is advanced or retarded by the movement of the trigger assembly

attached to the throttle/spark arm.

Ignition Coil Circuit

As the capacitor voltage flows through the primary windings of the ignition coil,

a voltage is induced into the ignition coil secondary windings. This secondary

voltage rises to the level required to jump the spark plug gap and return to

ground. This secondary voltage can, if necessary, reach approximately 40,000

volts. To complete the secondary voltage path, the released voltage enters the

ground circuit of CDM module.

Stop Circuit

To stop the engine, the stop switch is closed allowing the capacitor charge

current from the stator to drain directly to ground.

Notes

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Mercury 3.0 Liter Service Manual Page 35