Mercury Mariner 8/9.9HP 4-Stroke Factory Service Manual, Page 40Get this manual
Theory of Operation The ignition system uses CDI (Capacitor Discharge Ignition)This system provides quick voltage buildup and strong spark required for high power and high performance enginesThe CDI ignition system does not incorporate mechanically operated points, therefor making this CDI unit virtually maintenance freeAs the flywheel rotates, electrical power (alternating current) is produced by the capacitor charging coilThis power is rectified by diodes so that direct current voltage is utilized by the ignition systemWhen the silicone controlled rectifier (SCR) is off, the D.Cvoltage is stored by the capacitorAs the flywheel rotates 360 from the point at which charging of the system began, the trigger coil produces voltage which is rectified and applied to the (SCR) gateThis gate signal then turns on the (SCR) and the stored voltage is shorted to ground thru the primary winding of the ignition coilThe voltage discharged to the primary winding of the ignition coil causes surge of high voltage to be induced in the secondary winding of the ignition coilThis induced voltage of sufficient amplitude causes the spark plugs to fireThe voltage stored by the capacitor holds the (SCR) on until the ignition coil current is dischargedThen once the capacitor has discharged, the (SCR) turns off and the ignition cycle is repeatedActuating the stop switch shorts the power being produced by the charge coil to groundThis prevents charging of the capacitor and thus stops spark plug firing