One of the most common of electric scooter parts in need of replacement, the CDI module is as essential to the scooter ignition system as the spark plug. Thought to be an invention of the brilliant Nikola Tesla, the Capacitor Discharge Ignition (CDI) is the standard system used on most of today’s gas-powered motor scooters, as well as on many motorcycles, marine outboards and other small engines. A complete CDI module is made up of a transformer, the charging circuit, the rectifier, a capacitor, and the trigger circuit.
How the CDI Module Works
The transformer first raises the voltage to 400 to 600 volts. Moving along the charging circuit, the electric current charges the capacitor, with the rectifier preventing the capacitor from discharging before the ignition point. Receiving the triggering signal, the trigger circuit stops charging and allows the capacitor to discharge to the low inductance ignition coil. This increases the original 400-600 Volt capacitor discharge to as much as 40 kV at the secondary winding, jumping the spark plug gap, and igniting the gas/air mixture in the cylinder. The charging circuit is then reconnected and resumes charging the capacitor all over again.
AC or DC?
Most current scooters us an alternating current (AC) system; however the popular KYMCO scooter brand uses direct current (DC). As a result, electric scooter parts for KYMCO machines, including the CDI module, can sometimes be difficult to find.
It is always best to match the engine size to the CDI specifications. A CDI module made for a 50cc engine might work on a 100cc, 125cc, or 150cc engine, and some are designed to function over a range of engine sizes. Others however, will only work for a particular engine displacement. Always check the specifications to be sure of getting the correct scooter ignition parts.
2-stroke and 4-stroke Scooter Ignition
The 2-stroke and 4-stroke engines are completely different, and as a result, their engines’ ignition timing is totally different as well. You cannot use a CDI made for a 2-stroker on a 4-stroke scooter. Conversely, you cannot use a 4-stroke scooter’s CDI on a 2-stroke scooter.
On many current CDIs, the connectors are integral to the unit’s case, forming a plug that snaps directly on to the scooter’s wiring harness. On other models, the connectors may be located on wires, allowing some leeway on exactly where the module is mounted. The skillful mechanic may shorten or lengthen these wires as necessary. The most common CDI connector style uses two plugs side by side; one 4-pin plug, and a 2-pin plug. These connectors may be square, rectangular, or simple spades.
Note: Photo illustrations of many CDI connectors can be seen at the website listed at the bottom of this article.
Unrestricted or Restricted?
An unrestricted (or racing) CDI provides the current no matter how high the engine RPMs go. As the CDI controls the spark plug, the plug will continue to fire at high RPM. For reasons that should be obvious, these CDI units are made for racing use, and are not very suitable for regular highway use. A restricted CDI module however, will stop firing once a pre-set engine RPM is attained. Without current reaching the spark plug, the engine RPMs slow to below the pre-set limit and only then will the CDI resume providing current to the plug.
When replacing a CDI module, or any other scooter ignition or electric scooter parts, it is always a good idea to reference your owner’s manual. If the manual is lost or otherwise missing, many manufacturers and brands have downloadable manuals available on their websites.