CDI ignition systems,trouble shooting,how do they work?

Discussion in 'Electrical' started by duivendyk, May 20, 2008.

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  1. duivendyk

    duivendyk Guest

    There appears to be a lot of misunderstanding and confusion about the capacitor discharge ignition systems used today on most small IC engines. The reason for the popularity of CDI systems is that they are easier to miniaturize than the old fashioned but rugged magneto setups as found for instance on the B&S lawn mowers. They also dispense with breaker contacts which will wear out,making a maintenance-free ignition system possible (at least in theory)

    CDI is a successor to the magneto based ignition systems evolved a century ago by Bosch of Germany (Their logo still shows a magneto). It was a self-contained system,no batteries were required and it was used exclusively until these became common with the advent of electric starters). Magnetos were big and expensive and lost out to cheaper battery ignition,but held their own in motorcycles due to good high speed performance,They were standard in the smaller 2 strokes as flywheel magnetos/generators until CDI systems became competitive with the advent of solid state electronics.

    In a CDI ignition system,a storage capacitor is charged to a high voltage (a few hundred volts) by the flywheel charging coil and, at the proper moment quickly discharged into the primary winding of the ignition coil,which is actually a stepup pulse-transformer. The secondary output is a short very high amplitude pulse which fires the spark plug. All these events take place during each engine revolution. The charging coil is usually located inside the flywheel,if there is room for it, but if the flywheel doubles as a cooling fan it has to be located outside. The spark timing is controlled by a small trigger coil located in close proximity to the flywheel which generates a low voltage pulse (actually two, a positive and a negative one), as the fly wheel magnet passes by. The circuitry in the CDI unit uses this pulse to close a solid state switch (usually a thyrister) to discharge the storage capacitor into the ignition coil primary and generate the high-voltage pulse at the secondary to fire the plug. This capacitor has previously been charged to a high voltage with the high energy pulse induced in the charging coil by the flywheel magnets. All these events take place every revolution.

    Both the pick up coil and the power coil have two wires connected to them, one side of each goes to ground (usually coded black),they may be grounded internally in the magneto or connected together and brought out as a single wire and grounded outside or connected to a ground connection on the CDI unit. The two live outputs (charger coil output and trigger) are connected to the CDI unit. They can be identified by disconnecting them from the CDI and checking the resistance to ground with an Ohm meter. The trigger coil has low resistance,the charger coil higher (a few hundred Ohms typically).Both outputs are ac. signals!!,the charger output typically in the range of 100 V ac. DC measurements of this signal are MEANINGLESS and deceiving !,all they measure is the degree of asymmetry of the ac output,not the actual amplitude.Even simple multimeters have an ac position. The charger output is an unhealthy signal that could kill you under the right circumstances,sit on a wet saddle and grab the bare "white lead" for kicks. while riding around. This is not all that likely of course,but the output of the charger coil should be treated with respect.

    I have never set eyes on one of these Chinese 2 stroke engines,so as far as color coding goes bear with me,I have concluded that the "tormentor" "white wire" is connected to the charge coil output and the "blue wire" is connected to the trigger coil. Shorting either one to ground will disable the ignition and stop the engine,in the first case the storage cap does not get charged,in the second it does not get discharged. Opening up the connections between these outputs and the CDI unit accomplishes the same thing, I would prefer shorting to ground esp., for the low level signal from the trigger output,a long lead would be connected to the CDI trigger input and a spurious signal could conceivably blow up the input device. Shorting them to ground does not hurt anything and is safer. I would use grounding connections going back to the engine ground and weather proof the switch and I would leave the output of the charging coil alone. It is probably possible to power other devices from the output of the charging coil, I think it could power a sophisticated high voltage low current dc circuit to minimise the drain on the coil feeding an efficient power convertor to convert this high voltage (50 V plus) to 12 or 6 volts power at higher currents.

    Trouble shooting hints. Caution ,if testing for spark ALLWAYS have a plug connected with the shell grounded,never ever test with an open plug lead, you may destroy the ignition coil.

    1) Disconnect leads going to kill switch (shorting mode hookup!) and try starting engine (Figure out how to stop it in case it runs).

    2) The charge coil and trigger coil are fairly robust devices and not likely to fail,measure resistance to ground (disconnect from CDI),if shorted or open circuit,they are defective or leads bad.

    3) Ignition coil,disconnect from CDI if possible,measure resistance to ground of primary,should be a few ohms,measure secondary resistance to ground, should be a few thousand ohms. Caution: High resistance may be due to high resistance HT lead to plug! Not easy to check unless you can get at coil output directly.

    4) CDI unit,probably implicated if other tests OK

    I hope that this post has shed some light on the mysteries of CDI ignition systems

  2. stude13

    stude13 Active Member

    thank you very much!
  3. azbill

    azbill Active Member

    excellent post !!! :D:D:D
    very helpful :)
  4. Very Informative....
  5. Maxvision

    Maxvision New Member

    Just rebuilt an engine

    Not sure if there's a "trigger switch" in MB ignitions. Never saw any sensor or electronics in the engine case other than the magneto coil. I'm guessing (notice I said guessing) that once the capacitor reaches a certain charge a transistor or some other microprocessor will cause it to discharge.

    I'm guessing this because after I put my rebuilt engine on the bike I could not get it to start to save my life. Spark plug was sparking just fine, I checked everything else three times and just couldn't figure it out.

    A few days later I had an epiphany. What if I put the magneto magnet on backwards. The keyway on the magnet is offset to the poles of the magnet. Maybe putting it on backwards cause the sparkplug to spark somewhere other than close to top dead center of the piston.

    I pulled the magnet off, turned it around and put it back on. EUREKA! Bike fired right up and runs like a dream.
  6. Rich S

    Rich S New Member

    What bike do you have? I am working on a 1983 Yamaha xt 200 which also has a CDI unit. I have tried everything but it doesn't start. I feel that my bike doesn't start because the spark doesn't fire at top dead center. Did your bike also have the cdi unit. I am desperate to find the answer. Thanks for help.
  7. Fabian

    Fabian Well-Known Member

    It was an excellent post.