Re: Help Understanding a CD Ignition
CD ignition means Capacitor Discharge Ignition. It functions by having the stator generate a high voltage to charge a capacitor(s) inside the CD box. It generally has one capacitor for each cylinder and one coil for each cylinder. A low power trigger coil for each cylinder generates a small electrical voltage that signals a transistor to "Dump" the capacitor into the coil. This rapid build up of voltage and the subsequent abrupt drop causes the coil to generate a very high secondary voltage and fire the sparkplug.
The stator generates a very high A/C primary voltage (at least 20 times the 12 volt system) that charges all the capacitors. To do this, it may have multiple separate windings. The ignition A/C is not rectified to D/C. The positive A/C sine wave will charge one capacitor and negative sine wave will charge the other. This happens when the flywheel is turning at or above cranking speed, so if you turn the flywheel by hand with the key on, you will not get spark. On the other hand, the ignition is self energising. Any time the key is on, and the flywheel is turning rapidly, the ignition is energised and you must do something to stop the engine. Thus: Even with a completely dead starting battery, if you are able to pull start (rope) the engine fast enough, it will fire and run. Generally, The CD boxes are shorted to ground through the ignition switch to prevent spark.
Since there are no points and the flywheel magnets and stator are constantly producing power, There is no "dwell" time. The capacitors have MORE time to charge. However, they will discharge much more rapidly (almost instantaneously) than, and build up a greater resultant magnetic field in the coil primary than a points system. The internal capacitors are the same general type but much higher in capacity than the external capacitor in points systems. The capacitor across the points functions only to absorb voltage surges as the points open and close, thereby preventing arcing and extending the points set life.
The advantages of CD ignition are:
1. Less moving parts so timing is more precise. Elimination of the rotor gap inside the distributor so less "noise" interference. Shorter, lower resistance secondary wires (on an outboard).
2. No points to wear or go out of adjustment; primary voltage can be much higher since there are no points to pit or burn.
3. Higher secondary voltage because primary magnetic field build up and collapse is faster and greater. Thus: Plugs get a "hotter" spark and are less prone to misfire
4. Cheaper for the manufacturer to make. (Easier and cheaper to buy the components than to make and machine a precision distributor.)
As said above, without special equipment, you can not troubleshoot the CD boxes. By switching with a known good one, you CAN eliminate it as a cause of no spark. You CAN test the stator for open circuits or shorts, and resistance, and you can test the coils. You can also test the triggers for open, shorts, and resistance.
