Repairing or Replacing the KDX CDI
Here is the info you need to repair your CDI (capacitive discharge ignition) but if you need a new replacement then I can sell you one for $75. Click here to read about it.
When I owned the international version of
(the SR) its CDI, ignition coil, and stator coil all bit the dust
after I failed to tighten up the screws sufficiently on the stator
plate upon reassembly. A local technician replaced the SCR and rewound
the stator coil, and I bought (for temporary use) a generic ignition
coil that has 1.5 ohms on the primary. It worked but the spark was
read that due to moisture condensation it is common for the stator coil
to lose good contact with ground (chassis common), so it is important
to keep those contact places clean. I also imported a used CDI and
ignition coil (both not working. The coil primary only had .2 ohms
whereas it should have 1 ohm) from a normal KDX and I took the CDI
apart and replaced its SCR and then it worked. I didn't even know that
these CDIs are repairable
but now I see that they utilize common components and no type of brainy
integrated circuit. So from my experience I want everyone to know that
CDIs can be repaired. The CDI from my KDX 200SR (Int'l model) is the
same except for a 1.4K thermistor instead of a 1.7K thermistor that the
normal KDX has. (the
1.4K retards the ignition timing more).
Wht/Red and Blk/Wht is the CDI input from the stator coil and the output to the kill switch. (The two CDI wires go to the same spot on the circuit and so are interchangeable.) The two black CDI wires are the ground wires, one from the magneto and one to the ignition coil. The Blk/Ylw wire is the CDI output wire to the ignition coil.
The capacitor actually is unlabeled. A technician told me it is 1.5uf and then I put it in a circuit to test its value and verified that it is 1.5uf. This drawing shows the copper conductor paths as if you had X-ray vision. Actually they are on the back side of the board and unseen when looking at the parts side of the board. The part that has the letters K A G is the SCR. There is an unused space for a resistor to connect the gate (G) and cathode (K) which you would need to fill with a 12K resistor on the standard CDI if you replace the 1.7K thermistor with a 2K thermistor. The resistors are color coded and do not list numeric values. The codes are 16: brown/blue/black, 33: orange/orange/black, 390: orange/white/brown, 620: blue/red/brown, 1.8K: brown/grey/red.
Circuit operation: The positive voltage from the stator coil charges the 1.5uf capacitor which remains high at the capacitor/SCR connection until the SCR turns on. This event happens when the cathode (K) of the SCR is sufficiently more negative than the gate (G). (when it reaches the required gate turn-on voltage, usually 2.5v). When that happens (a certain time period after the charge input changes from positive to negative) depends on the RC (not Ricky Carmichael. Resistor/Capacitor) time constant and the amount of negative voltage which depends on the stator coil and pull-down resistor (16 ohm in this case. if burned then the timing would be more advanced). The pull-down resistor is responsible for the negative half of the signal being so small (4 volts). Without it the negative amplitude would be 340 volts also. The sudden transition from +340v to -4v is translated across the 1.5uf capacitor to the primary of the ignition coil, which voltage is further amplified to deliver 10's of thousands of volts to the spark plug.
This test circuit doesn't allow the CDI to put out enough voltage to power an ignition coil, so you have to use an oscilloscope to view the voltage spikes coming out of it. If you don't use the resistor/diode input combo then you'll fry the 16 ohm resistor inside of it. The 330 and 220 are resistors. The 330 is a 25 watt resistor and much bigger than a normal resistor.
Click here to read about how I modified the CDI for a different timing curve to better match my engine and pipe.
More of my pages: SR mods, motocross mods, KIPS servicing, tire changing guide, alternative healing methods