Re: switch box
The switchboxes (aka "power packs) act as a sophisticated "electronic switch" / "routing device" to send voltage to the ign coils.
The flywheel has magnets fastened to its inner surface and they ride very closely around the "Stator", which mounts under the flywheel to the upper bearing cap.
When these magnets pass the "Stator", voltage is generated thru windings permanently embedded in the stator. The switchboxes must 'accept' this voltage and send it to the proper coil at the correct time.
They do this by taking input from the 'trigger' which is also mounted under the flywheel, and rests in the very center, inside the I.D. of the stator.
The trigger is also affected by the magnets and the position of the trigger determines when the switchboxes will send voltage to a coil, and which cylinder will be the one to fire next.
The throttle arm has linkage connecting it to both trigger and carbs, so that when the arm moves to increase throttle, the trigger moves in synch with the carbs. When the trigger moves in the "up throttle" direction, it advances timing, and vice-versa. Maximum spark advance (timing) is deterimined by a timing scale/pointer and adjusted via a mechanical stop.
The ignition coils have primary and secondary windings, and thru the "transformer effect", amplify the voltage sent to them, to provide a very high voltage to fire the spark plugs.
Anyway, that's a very basic description, there's of course lots of fine details but hopefully this is what you were looking for.
If you're having issues with your ignition system, let us know what symptoms you're having and maybe we can help.
Be sure to list your Serial # in your post.
If you're curious, you can look up parts diagrams for your motor here (they're broken down first by horsepower then serial #).
http://www.mercruiserparts.com/SelectModelType3.asp?class_id=2
HTH........ed
