DC-CDI ignition troubleshooting

Many small bikes of today and tomorrow are now using the DC-cdi technology, this is for reliability and mantaining good spark every now and then, but then at somehow and at somepoint, the system fails. How does it fails?

Firstly, we must understand how the system works from start to finish.

IGNITION SYSTEM

     We'll now get into the ignition system itself. The outline simple enough. You start inside the left engine side cover, where two

PICKUPS
     sit near the rotor and produce a pulse of electric current when the pistons near TDC (Top Dead Center) which is fed to the

DC-CDI BOX
     (Capacitive Discharge Ignition, variously called the "ignitor box", "brain box", etc.). This box has two basic functions. It adjusts the signal from the pickups based on engine speed to change the timing as needed, and a capacitor thats being charged by an internal HV generator in accordance with RPM switches an SCR dumping energy of the CAPACITOR to the primary winding of the

IGNITION COILS
     so that the secondary winding is "induced" to spit out a high voltage shot to the

SPARK PLUG WIRES and on to the

SPARK PLUG CAPS which in turn sends it to the

SPARK PLUGS
     where all this voltage (maybe somewhere between 10kv to 40kv-that's thousands of volts) has enough pressure to make a small current jump the air gap between the electrodes at the bottom of the plug, thereby making a spark. This spark ignites the fuel mixture which, at that point is sitting all around the plug tip in a compressed state, ready to go BANG.

     So that is basically what happens But things aren't quite this simple, and so we'll take each element and talk about it in more depth. We'll also note the problems we've run into, the tests that can be made, and the repairs/fixes we know about. Please understand I'm not a trained engineer just an electronics guy. My comments are based on what I've learned from reading, talking with people, and personal experience with the system.

    

PICKUPS

     Pickups start the process by giving the CDI a signal to the SCR trigerring pulse shaping circuit. Pickups are themselves little coils, but of a different kind, since they have permanent magnets inside the winding. So a magnetic field is present all the time and doesn't need a current flowing through the winding to create it.

     Then on the rotor (flywheel) we have a small metal plate which passes very close to the pickup as the rotor turns around. How does this non-magnetic "protrusion" (as Yamaha calls it) induce a shot of current in this coil winding? Remember that while this winding sits wrapped around a permanent magnet, current does not flow because there is no movement of the permanent magnetic field relative to the winding. Both are stationary relative to each other.

     But now something called "reluctance" comes into play which can be likened to resistance in a conductor. Magnetic fields "run" less strongly through the air (greater "reluctance") than they do through metal which provides a more friendly path. So when this "protrusion" cuts into the magnetic field radiating from the permanent magnet in the pickup coil, this magnetic field can "run" through this metal better (than through the air) and is momentarily strengthened. As the magnetic field "fattens up" a little, it moves relative to the coil winding sitting around it. This movement is sensed by the coil winding, and a slight current flow, sufficient to trigger the CDI, is generated.

    

CDI's

The Heart of the ignition, the so called timing mechanism..At the moment of ignition, this energy is transferred from the capacitor to the ignition coil. Because the combustion chamber is under pressure at the moment of ignition, the coil has to transform the capacitor voltage up to several thousand volts in order to generate a spark. This requires a coil with a winding ratio of approximately 500:1 and good magnetic coupling between the primary and secondary windings. The inductance of the coil can be low as the CDI coil does not store energy.

COILS

     As mentioned..it does not store energy..once the capacitor is dumped on its primary, secondary winding because of the ratio between primary and secondary is high...energy will be then transferred to the spark plug to arc it producing hot sparks.

SPARK PLUG WIRES

     These are simply large, heavily insulated wires capable of carrying the high voltage current to the plug caps with a minimum of resistance.  This is encased in a rubber insulator--the whole thing being about 7mm in diameter.

     But there is more to these wires than meets the eye. As we now know when you run current through a conductor, a magnetic field around the conductor is created. In the case of these wires, you have high voltage present and current flowing thousands of times a minute, and so a magnetic field, as well as the presence of high voltage potential in the air (electro field) radiates in and out from the wires when the engine is running. These effects in turn create RFI (Radio Frequency Interference) which impacts am radio reception, and EMI (Electromagnetic Interference) which can raise hell with electronics, particularly those found in today's sophisticated engine management systems. 

SPARK PLUG CAPS

     In a motorcycle system, these caps serve two purposes.

--They connect the plug wires to the spark plugs.

--They also add about 5k ohms of resistance to this path to suppress RFI and EMI effects.

     But how does adding 5K of resistance cut down on these effects? Well, it was not too easy to find someone who really knew, but here is the explanation I found.

     As we have seen when volts in the kilovolt (thousands of volts) range start to push from the center electrode in the plug tip, gas atoms become conductors and the "infinite" resistance of the plug gap simply goes away. What is needed is some resistance before the air gap to cut down the amount of current flow, that is, the amps. With less current flow, the magnetic and the electro fields generated are smaller and less powerful, and so produce less interference for radios and electronics. That, I am told, is why the 5K ohms are there and how they do their job.

SPARK PLUGS

     Last, but not least, we have the spark plug. A few quick observations:

--There are "resistor´ plugs that have resistance built into them, probably for the same reason that we find resistance in plug caps.

--Tips of the plugs can vary in design. ) as the standard. These have the familiar nickel alloy center and ground electrodes. But new materials are coming on line (platinum, iridium) which permit smaller electrodes and are said burn cleaner and last longer. They also cost a lot more. Whether, on the low compression and low revving engine, they make much difference, is a question. You might try a set to see if you feel a little more power, get a little better gas mileage, etc. But if you'd rather spend your money on something else, OEM plugs will run your bike just fine, and should last a good 10K miles.

--Plugs have different heat ranges. "Cold" plugs dissipate more heat into the cylinder head (which is also hot, but always cooler than the plug tip) than do "hot" plugs. The lower the number, the hotter the plug.

     The basic design tradeoff is this: you want a plug that runs hot enough to burn off carbon and oil deposits. But you want a plug that will run cool enough so as not to burn up its electrodes (or suffer from something called "oxide fouling"), and thus have a short and unhappy life. Or worse yet, start to glow and cause "pre-ignition" (pinging) which can damage the engine. The proper heat range will encourage the plug to keep itself nice and clean, but also allow it to last a long time.

     A good operating range for the tip of a plug might be between 450 and 650 degrees C (roughly 850-1200 F). Starting with an understanding of the operating temperature range of the engine (which can differ significantly between engines), engineers specify a plug that will run within the desired heat range.

     One phenomenon that some riders experience is this: a cylinder fails to fires, or fires badly with the cap on the plug. But when the cap is pulled just slightly off the plug and the cylinder lights up and runs. Why would this happen? Well, my best guess is that there is a problem somewhere in the system that is causing a weak spark. By adding an additional air gap, the coil is forced to reach a bit higher voltage before the two jumps can occur, thus making for a strong, hotter spark. The basic problem still needs to be found.

     When riders start to foul plugs, a typical reaction is to try a hotter plug to burn off the built up carbon. This really isn't the way to go, in my opinion. You shouldn't be trying to solve fuel mixture or ignition problems by changing heat ranges. Get your carburetion and ignition right, and run the right plug.

TROUBLES, TESTING, AND COMMENTS

GENERAL

     When you suspect ignition problems, first be sure that your battery is hot, etc. (mentioned above), An inexpensive digital multimeter is a must to work on ignition systems, so if you don't have one, bite the bullet and get one.

A NOTE ON COILS     

CDI coils is not the same as TCI coils...primary winding of the CDI coils is never in related to positive line of the battery so theres no voltage whatsoever you can detect on it when no ignition trigerring occur.

PICKUPS

 Pickups are generally very reliable. But they do live in a harsh environment which includes very hot oil while the engine is running, and great temperature swings. The wires connecting them to the CDI also have to run in this environment during their travel inside and through the side cover. So occasionally pickups, or their connecting wires, may have a problem.

     You can perform this resistance test by finding the connectors for these wires (under the seat). There should be some resistance. If there isn't, it may mean that the wires have become frayed/damaged inside the side over, and are shorting out on the engine casing.

     One point of slight interest is that these pickups trigger a spark once every time the flywheel rotates, whereas the engine (being 4 cycle) only needs a spark every other time. What happens to this extra spark? Well, it occurs when the piston is reaching TDC (top dead center) on the exhaust/intake stroke. The exhaust gasses have just been pushed out of the cylinder and the new fuel/air mix is just starting to enter. So there is really nothing around to explode, and the spark just happens, but produces no result and does no harm.

    

CDI's

     What can happen to CDI? A good question....CDI fails only when you supply it more than 16 volts.or more or have a defective voltage regulator that clips stator voltage down to a minimal level enough to supply the cdi..nominally is 14.4 volts and less...CDI also since its totally enclosed with epoxy resins..this is to ensure moisture and vibration plus shocked proof when at extrme working condition of the bike..in such cases...due to the fact we are highly enthusiastic with bikes...we buy this buy those aftermarket parts without knowing...IS THIS REALLY FOR MY BIKE or other model? definitely, some of them may cause our ignition system to fail.CDI will not fail if not altered...will be there for years will service you for years.

SPARK PLUG WIRES

     In my experience OEM spark plug wires don't cause problems, and you mainly inspect the insulation to look for cracks or other signs of breakdown. If you can feel a kick from one while the engine is running that's a sign that there is a leak somewhere, caused by a crack, or maybe dirt on the outside of the wire. As noted earlier, the wire itself should be good for a very long time.

     Spark plug wires can be replaced. What you would use for a replacement? I'm not so sure. My local Yamaha dealer sends people to the auto store and tells them to buy "hard wire" cables. That means with metal, rather than carbon conductors.

    

     Spark plug wires screw into the spark plug caps, and also into most of the coils. (This is the old fashioned method, but it seems to work O.K. for us.) While there is a rubber dust seal around these joints, sometimes corrosion can start to develop in the tips of the wires. A quick and sometimes effective cure for spark problems is to cut 1/4 inch or so off the ends of the wires to access some fresh strands, and screw the plug cap/coil back in.

SPARK PLUG CAPS

     You can measure the resistance in your plug caps very easily with your multimeter set to ohms. Should be in the 5K range. Beyond that you can inspect for physical damage-cracks, etc.--and make sure that the cap grips the plug tightly. Plug caps can wear out eventually (all that voltage pounding through that resistor, I suppose) but are easily replaced, preferably with ones of similar resistance.

SPARK PLUGS

     First you can check the numbers to make sure they are the right ones. Then you can to see that they are gapped correctly (.06-.07), and don't show excessive electrode wear. Then you can make sure they are clean both inside and out. And if you want to, you can touch your ohm meter to both ends of the center electrode to assure continuity. I've seen one or two plugs that failed this test. Resistor plugs may have an air additional gap inside them and so would not check out this way.

Removing And Installing Plugs

     When removing a plug, the best thing to do is partially back it out, and then blow out the well (or clean it some other way) to remove any crud that has accumulated down there. That way dirt doesn't drop into the cylinder when the plug comes out.

     Plug bodies are steel and cylinder heads are aluminum. With two different metals corrosion can occur. When refitting plugs, make sure the plug threads are clean. I usually go down into the plug hole (if I can reach it) with a Q Tip soaked in carb fluid, and try to clean up the walls a little as well. If you want to do what the pros do, before screwing the plug back in, smear a SMALL dab of anti-seize grease (available from your auto parts store) onto the threads which will lube them, cut down possible corrosion, and make removal easier, while not affecting the ground connection.

     To tighten plugs you can use a torque wrench or tighten by feel. For a new plug, the washer will crush. Used plugs obviously have flattened washers. I'd say tighten a plug until it is good and snug, but be very careful not to over-tighten.

Cleaning Plugs

     A dirty plug in otherwise good condition can be cleaned and reused. Since I sometimes do tuning work on my bikes which involves plug reading, my plugs are going in and out quite a bit.  I assure this by spraying carb cleaner and using direct air to make sure the plug is totally free of grit, inside, under the gasket, and on the threads.

     The next best way is to go after the plug with carb cleaner, and little brushes, tooth picks, whatever it takes to get down around the insulator.

      The basic problem with dirt in and on a plug is that it can form an alternative path for the spark voltage to travel to ground. The current takes the easy road and avoids the plug gap.

     For the same reason, a gas fouled plug won't fire. If a plug gets wet from a flooded cylinder--even if it is brand new, or otherwise pretty clean, it will likely need cleaning before it will fire again.

    

Reading Color

     Looking at plug tips is helpful in learning about what is going on inside the cylinder. If you are running rich, the center of your plug (electrode and insulator) will be somewhere between dark and black with carbon. If your plug is misfiring due to faulty ignition it will also be black and possibly wet. Many manuals have color pictures showing what the different conditions look like. Where it gets harder is knowing when you are dead on with your carburetion. Reading plugs used to be more reliable, but now, due to gasoline additives and leaned out bikes, plugs may not show small differences. In general, whitish to light gray to light tan should be good, with mid chocolate being on the rich side but O.K. Bone white may indicate a lean condition, but many plugs will look that way when there is really no sign of lean running or overheating. Note that the color around the edge of the plug body is usually dark and indicates conditions present mainly when using the choke and on start up.

Extra Plugs

     I generally carry a set of extra plugs just for the heck of it. One situation where they might come in handy is if you have to go over a high altitude pass. Altitude will make your engine run rich, and the higher you go (especially if you have been running a bit rich at sea level), the more likely you are to foul plugs.

CONCLUSION

     Hopefully this article has given you a better understanding of the basics of the CDI ignition system. As you can now see, when a cylinder quits firing and you have checked out your fuel system (running problems are more usually carb related, than ignition related) and figure it is ignition, there is no one-shot, easy, quick fix that will work every time. You have to start testing, checking and certifying components until you find the problem and/or the defective part.

An inexpensive multimeter is a must for this work.

     As usual, nothing helps like some good "peer review" so comments and corrections are encouraged and appreciated.

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