Repairing the KDX CDIThe following drawing is the CDI schematic for a normal KDX. Also following are the parts layout on the circuit board, board layout inside the CDI box, simple testing circuit, and voltage waveforms at three places. The parts most likely to fail, in order of liklihood, are the SCR, 1.5uf capacitor, diodes, and 16 ohm resistor. I would say to always replace the SCR (Radio Shack sells a good 4A/600v SCR which I used with success), and if the ignition coil is reading less than 1 ohm on the primary side then also replace the 1.5uf capacitor (which was allowing too much current to pass to the coil). Test all the diodes and the 16 ohm resistor and replace as necessary. To get to the parts you'll need to cut the top and sides off of the plastic container. (yes, essentially butchering it. You can replpace the box with the one listed below). Be careful because the bottom of the parts board is almost butting up to the top of the container. I have read that soaking the exposed silicone (which encases all the parts and is mixed with white stones) in acetone for 3 days will dissolve it or at least loosen it up a lot. I haven’t tried that since I can’t find any acetone here. You can dig out the silicine/stones with a jewelers screwdriver but go slowly so as not to damage components (especially the large yellow capacitor). My parts layout shows the large capacitor to the side but that’s just to show all the parts since in reality it lays on top of the SCR and some resistors. The exact replacement for the main capacitor is a 1.5uf/250v metallized polyester cap (#23MA515 $1.27 size 32x9x16mm) from Mouser Electronics. A better cap for handling current spikes is this metallized polypropylene 1.5uf/250v capacitor from DigiKey (#PF2155-ND $3.40 size 23x14x21mm) although it is fatter. Since the cap needs to handle high current spikes you can't use just any capacitor with the same value/voltage rating. Also, from my experimenting I found a 1uf cap causes less spark current, and a 2uf cap causes the engine to cut out at high revs. from www.mouser.com: #511-TS820-600T $1.31 (8A/600v sensitive gate SCR) #23MA515 $1.27 (1.5uf/250v capacitor) #660-CF1/2L160J $.05 (16 ohm, 1/2W resistor) #30BJ250-XXX $.22 (in place of XXX put the value of resistor wanted) #284-HS25-330 $3.49 (330 ohm, 25W resistor for test circuit) #281-220 $.13 (220 ohm, 1W resistor for test circuit) #625-1N4936 $.05 (1A/400v diode) #80-T350C475K025 $.57 (4.7uf capacitor in case you damage it) #334-NTC202 $.66 (2K thermistor in case you damage it) #400-5032 $2.11 (plastic box 2.7” x 1.7” x 1.2”) #400-7032 $1.52 (lid for plastic box)For the SR CDI you can replace the 1.4K thermistor with this one (same value) from www.digikey.com #PNT114-ND for $1.06 in case you damage the original when digging out the silicone. This thermistor changes value when the motor warms up in order to make the whole timing curve less advanced. I actually prefer to replace it with a resistor (680 ohm resistor for normal KDX: Mouser #30BJ250-680 $.22, or 430 ohm resistor for SR KDX: Mouser #30BJ250-430 $.22) and 1K thermistor (Mouser #334-NTC102 $.66) in series to keep the timing curve from changing too much. I don't like feeling the power lessen significantly after 5 minutes of riding. Before finishing, make sure the thermistor is surrounded with a lot of silicone sealant to insulate it from the air temperature. That way it will mostly respond to the motor and just a tad to the climate. From England (www.electrexworld.co.uk/) you can order an ignition coil (HT55 or HT80) for $45, a stator coil (C31) for $45, and a CDI (CD3210) for $148 that has 6 selectable ignition curves. From RickyStator you can order a stator coil for '82- '88 KDX200's for $40. Stock OEM parts are: Spark plug cap #21130-1006 for $15, Ignition coil #21121-1170 $60, stator (plate and both coils) #21003-1192 $298, CDI #21119-1257 $225.
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 22K 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 better timing curve. More of my pages: SR mods, motocross mods, KIPS servicing, tire changing guide, alternative healing methods |
