CDI & Ignition Coil


A capacitor-discharge ignition (CDI) is built into the ECU. Most 4-stroke ECUs provide a simple induction-coil ignition. Although it would have been cheaper to construct an induction ignition, the TR280i definitely has a CDI.

Two-strokes benefit greatly from having a CDI because the rise-time of the spark event is so brief (which helps overcome plug fouling due to the rich and oily mixtures found in 2T engines).

The CDI uses a DC-to-DC converter to boost/invert 12-volt power to about -350 VDC to charge the energy storage capacitor which is internal to the ECU.

The K-Scan diagnostic software allows you to test the ignition, and create sparks at zero engine speed. I observed the primary of the ignition coil with an oscilloscope and also fired the CDI into both capacitive and resistive loads. Firing the ignition coil into a spark plug reveals a -225V pulse that lasts about 20 microseconds. It has a very fast rise time (less than 500 ns). Open circuit, the CDI capacitor charges to about -350 V.

This voltage is negative with respect to the chassis because the voltage required to eject electrons from the hotter, sharper center electrode is substantially lower (by about 30% according to some sources) than would be needed to eject them from the cooler, duller ground strap.

Ignition Coil

The ignition coil is marked “Japan 1G7107.” It has a primary resistance of 2.0 ohms and a secondary resistance of 5500 ohms. The spark plug cap is nominally 5000 ohms.

Incidentally, if you replace the spark plug cap, it must have an internal resistor. NGK part number TB05EMA (NGK 8636) is a suitable replacement. They are waterproof (intended for use on watercraft), inexpensive, reliable, and readily available.

Oscilloscope capture of CDI firing into a 2-ohm resistive load