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c d i

Discussion in 'Technical and Troubleshooting Torque' started by pragnasty, May 23, 2005.

  1. what is a cdi?
    is it the computer control module for your bike? (like an ecm for the car)

    also what are pickups and their function?
    i think i need a new cdi module but i have a very uncommon bike and i dont know any one that tests them. but do you? :?:

    my mechanic and i have come to the conclusion that i might need a new one but wanted to test it to make sure it was that before i go spending $300+ .
    i am losing power under load. at random rpm. its not fuel related, drained and cleaned carbs, no leak in lines. taken air filter out, made no difference. basically its a flat spot at any rpm, and is continual through all gears. therefore we think its electrical......
    any thoughts????
    much appreciated :cry: :cry: :cry:

  2. How about asking around to see if there's someone else with the same/ similar bike around here?
    Swapping the units is usually a matter of 10 mins (2 small bolts and a plug to undo). Quickly jerry rig the 2. unit with duct-tape to the bike and go for a spin. if all is fine, you KNOW it's the problem.
  3. Never heard of 'CDI'. Can you ask for an explanation from your mechanic?

    Could be an issue with the timing advance. A blocked or leaking vacuum pipe could cause this. You said no leaks, but have you removed them and tested for continuity. Sometimes oil residue can clog them.

    Only electrical fault I would suspect is the high tension coil pack. This also comes with a condensor that may need to be replaced. Neither should be that expensive and normally are delivered as a pair.
    Both these are way cheaper than $300, so worth investigating first.

    Best of luck.
  4. CDI is an acronym for "Capacitor Discharge Ignition".

    Basically, with the old kettering system a set of points shorted out one side of a transformer. This transformer (known as "the coil") has a large turns ratio, in other words, put a small voltage on one side (12V) and get a large voltage out the other side 20-50kV, enough to jump the gap of the spark plug and ignite the petrol/air mix in the cylinder.

    The CDI is an electronic equivalent of the points in a distributor, and by the magic known only to a few wizards and annointed ones, effectively increased the voltage presented to the 12V, thus giving more energy across the spark gap. (There's something I bet you never thought you would see: "energy" and "across" used in the same sentence as "more").

    They generally die from moisture getting in, transistor breakdown, capacitors "drying out", or dry solder joints.

    [edited because I am an idiot].
  5. What model bike is it and what year?
  6. cb250fy
    1998 model (grey import)
  7. Is the engine a MC14E?? if so than perhaps a CDI unit out of a MC19 or MC22 (CBR250) could do the job??

    I can test a Kawasaki CDI unit (I think I have the right gear and litreature) I also access to a part number for a Suzuki tester (apparently this device will test any Suzuki CDI unit)
  8. it is a MC14E. :wink:

    but.... already tried a MC22 CDI

    the connections are similar but not the same. sadly the electricals are not the same as the CBR"S. only the engines.
    have been trying several batches of fuel as well, and that too hasnt resolved the problem. if it was fuel, if i added an octane boost product to the fuel , would that be an indicater?

    i fitted an aftermarket headlight to the bike which draws a higher curent than the older headlight. had to change the 10amp fuse to a 15amp, to stop it from shorting.
    Could this have affected the circuitry on the bike?

    (sorry for so many questions)
  9. can I suggest going to a mechanic with a scope. he should be able to tell you without replacing it. most autoelecs would have a scope and in all honesty a good autoelec is easier to come by than a good mechanic. This kinda of problem can be dealt with by a capable autoelec.

    IMO: I doubt that your cdi unit will be at fault. but it is possible. I would be looking at coil/lead/plug breakdown first(secondary). then check the primart on the scope and they should be able to figure it out. And seeing that your mechanic hasn't done this i would be checking your mixtures too, but this can also be roughly checked on a scope too. :D
  10. scope as in Stethoscope ??

    im not to handy with the spanners. :oops:
  11. my bad, an oscilloscope. It is a tool which can give a graphic display voltage over time. They are a lot like the machines in the hospital that measure ur heart beat that end up with a flat line when you die(seen them yeah). Automotive scopes operate at a different voltage ranges to the scopes you find in hospitals and have many different features, but are in essence the same thing.

    good fun and very handy if you know how to use one.
  12. Gee I was going to say that days ago but assumed that CDI's had disappeared in the late 80's but obviously not on bikes.

    I can remember when Electronics Australia & Electronics Today International (both long dead electronics magazines) had kits you could build for CDI systems for your car or bike.

    Damn it, I'm showing my age again :roll:
  13. Just about every state has an automotive electronic place, that can test and repair cdi units, when my one shyte itself on my old fj1200, I was faced with $900 for a new one with my courier discount. I managed to get it repaired in 2 days, way way cheaper, cheaper than I could get one from the wrecking yard for. Taxi depots are a good place to ask around, they get electronic management systems repaired at these places.
  14. wow. These were the best things about in the 70's. I'm suprised they put them on a 98 model.

    Do youthink may your mechanic could have used a generic term and actually just means you ignition module?
  15. well he would have fooled me.
    im taking it to a honda dealership first thing mundy morning and see what the y have to say. hopefullly theyve got experience in grey imports like mine.
    if that fails ill take Sobils suggestion and head to a autoelectrician

    PS: has everyone had the same problem with finding an autelectrcian for their bikes? no one wants to work on them at all. the last sparky i talked to said "Only if were sitting round scratching our arse". fair enough every bikes different and parts are hard to source... but i mean whats so different ? ive wired up many a car, (this is my first bike) and its not that bloody hard......................+negative -postive .......

  16. Capacitive Discharge Ingition (CDI) Operation.


    Posted by JAW - Petrol engines require a spark at the right time to fire the air/fuel mix which in turn pushes a piston (or spins a rotor or wobbles a wobbleboard or...) There are several ways to make a spark - kettering points charged coil, magneto, flint and a stone - but I'm talking CDIs.

    Lets get straight to it. Have a look at the diagram. This is all you'll ever need for your motorbike to run be it 2 stroke or four stroke. Nothing else. That's right. No battery, no regulator, no rectifier - all that junk is for lights and other boring stuff.

    The basic CDI circuit. Notice that there are a couple of coils (or windings). These literally are wire wrapped around iron. What does wire wrapped arond iron do?

    Quick in-a-nutshell physics lesson: When you have a coil with electricity flowing thorugh it this creates a magnetic field. And vice versa - if you have a magnetic field near a coil it will make electricity flow in it. Straight away you can see what the Exciter Coil is doing then. That grey blob underneath it is actually a magnet on the rotor in your engine and it spins around the exciter coil making electricity.

    So the first thing that happens is that the engine, spinning a magnet, makes the power for the CDI. How much power? In fact, the faster the engine spins the more voltage is produced to feed the CDI. This may be in the order of 100VAC at peak revs.

    Pulse sensor - another coil, but only a small one. It too has a magnet spinning near it - but the magnet only passes by the coil once per revolution and it's a real small magnet. This means that most of the time there is not electricity flowing in the pulse sensor - but when the magnet passes by the coil you get a tiny spike of electricity. This tells the CDI to make a spark as the magnet is cunningly positioned so that the spike hits the CDI at just the right time

    There are another 2 windings being The Coil. As in the ignition coil, the one the spark plug connects to. Expanded in-a-nutshell physics lesson: the more turns in a winding, the more voltage; less windings, more current. The "coil" is actually 2 windings, the left hand side only a few turns and the right hand side with lots of turns.

    If you put voltage in the few turns side, this makes a magnetic field (remember, electricity = magnetic field) and then the other coil with lots of turns gets electricity in it because of the magnetic field from the first one. Because of the many turns the voltage is much higher but the current much lower. Confused?

    Essentially the coil takes a low voltage and turns it into a high voltage. This is to create a spark, which is actually just electricity managing to travel through air (_everything_ will conduct electricity if the voltage is high enough; a compressed air/fuel mix requires at least 10,000 volts).

    The CDI itself is really just an amplifying switch with time delay. It sits there storing up electricity in a capacitor (ie the "C" in "CDI"), then the pulse sensor spikes the input, the CDI waits a small amount of time and then puts the burst of electiricity it stored into the coil. That small amount of time represents the advance curve of an engine, ie at idle an engine may need 10 degrees advance but at full revs it may need 40 degrees advance. By delaying the signal and having the pulse sensor in the right place, the CDI can make sure the engine is getting the right advance curve.

    Rounding it off, the CDI is connected to ground (so that electricity can flow) and an Engine Kill Switch will short out the CDI resulting in no spark and thus the engine will stop.

  17. Yeah it's funny isn't it. I built a new harness for my old bike and its pretty easy stuff.

    it only gets a bit confusing when you blow a regulator or an ignition module. Even then, once you figure it out it's pretty easy.
  18. Ahhh, now it all falls together. Your riding a lawn mower ;-)

    Haven't played with these ignition systems before, so I'll just put a sheepish look on my face and nod sagely a lot....

    Best of luck.