1980 CM400a electrical issues
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  1. #1
    Junior Member mwgalen's Avatar
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    1980 CM400a electrical issues

    HI all,
    Im Marcus. Have wrenched on a few bikes and a few cars over the years and have found my way to this Honda just a few weeks ago.
    When I first saw it, it ran (although a little rough) enough for me to run around the block. I brought it home and did a couple of laps of the neighborhood and parked it in my carport for the night. The next day, with the plan of riding to work, I tried to kick start it and got nothing. I left it to deal with later. I picked up a battery on my way home and connected it up. The lights all come on and the starter cranks. The motor turns over, but I get no spark.

    The wiring is a mess and so I have been working through the harness looking for shorts or grounds with little luck. I am throwing myself on your mercy hoping someone can help me diagnose this thing and/or direct me to where one might pick up a Hondamatic CDI equivalant?

    Thanks for any help in advance!

  2. #2
    Senior Member Bill H's Avatar
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    The 400as are the ,IMHO, the worst ones to have elec problems.You will need a full service manual. It could anything from the stator, cdi ,change switch,or change switch rely,for starters. There is a sticky in the 400 elec section you should read before starting on it.
    I had the same problem on a new to me cb 400 a and it was a bad stator. i would check everything else before looking at the cdi. If turns over with the starter its probably not the change switch or rely. I would start at the stator.

    Bill

  3. #3
    Junior Member mwgalen's Avatar
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    Hey Bill,
    Thanks for replying. I did a resistance test on the yellows coming from the stater and it seemed to be all still within tolerance. I dont remember the numbers at the moment. I will check again and post with actual data.
    While looking through everything I was worried that there was a kill switch engaged somewhere that I couldn't find. Is there an easy way to eliminate that? Would the starter crank if a kill switch was engaged?
    Thanks again!

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  5. #4
    Senior Member Sonreir's Avatar
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    First up, one of the more confusing aspects of working on a 1978+ Honda twin versus working on the earlier ones is the ignition system. For the CM/CB/400/450, a CDI ignition system is used that is separate from the charging system. That is, the battery doesn't matter for the spark because the ignition system runs directly from the stator. The battery and charging system is just for the lights and starter motor.

    With that information out of the way, it's time to start testing the ignition system.

    Start with resistance readings for the ignition coils itself:
    *Resistance between the yellow and green wires should be about one Ohm or less.


    Now for the wires coming from the stator:
    *Resistance should be about 135 Ohms between the pink and green wires
    *White and blue should be about 85 Ohms
    *Brown and light blue should be 207 Ohms

    If all of those readings check out, it's probably time for a new CDI unit.

  6. #5
    Member knauff13's Avatar
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    The only kill switch I know of is on the right handlebar above the starter button. Did you make sure it's on "run"? I apologize if that comes across as a stupid question... I've done it, lol. The starter does crank and never fires if this is set to "off," and there are two off positions.
    Ben Knauff
    1978 CB400A Hawk Hondamatic

  7. #6
    Junior Member mwgalen's Avatar
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    Quote Originally Posted by Sonreir View Post
    First up, one of the more confusing aspects of working on a 1978+ Honda twin versus working on the earlier ones is the ignition system. For the CM/CB/400/450, a CDI ignition system is used that is separate from the charging system. That is, the battery doesn't matter for the spark because the ignition system runs directly from the stator. The battery and charging system is just for the lights and starter motor.

    With that information out of the way, it's time to start testing the ignition system.

    Start with resistance readings for the ignition coils itself:
    *Resistance between the yellow and green wires should be about one Ohm or less.


    Now for the wires coming from the stator:
    *Resistance should be about 135 Ohms between the pink and green wires
    *White and blue should be about 85 Ohms
    *Brown and light blue should be 207 Ohms

    If all of those readings check out, it's probably time for a new CDI unit.


    Alright, so the coil reads around .7-.8 Ohm across the green and yellow

    Between the pink and green at the stater is 16 ohms
    Between the Blue and white at the stater is 5.4 ohms

    *The only wires I have coming out of the left side of the case are Pink, Green, Orange, Brown, Blue and White . Im not seeing the Light Blue on the bike or on the diagram I have. There is a Light Blue parraleling the Blue and White connecting the CDI to the Change Relay. Is that what you are refering to?
    Also, am I testing all of these on the Stater side or the CDI side? I assumed the Stater and the numbers are from there.
    Thoughts?

    Thanks for the help


    Hey Ben,
    Not a stupid question at all and yes I checked. Ive definitely made that mistake before.
    Last edited by mwgalen; 06-05-2019 at 08:08 PM.

  8. #7
    Super Moderator J-T's Avatar
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    Have you actually checked for spark or are you just saying the engine doesn't fire?

    The CDI unit is located below the gas tank. It has a single black wire with a white stripe, should go to a bullet connector. Disconnect that wire. That eliminates the kill switch and the ignition switch along with any short to ground. If the ignition system is working you will get spark when the engine turns over regardless of the positions of the ignition and kill switches.
    JT
    81 CM400T Under re-construction
    82 GL500

  9. #8
    Junior Member mwgalen's Avatar
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    Quote Originally Posted by J-T View Post
    Have you actually checked for spark or are you just saying the engine doesn't fire?

    The CDI unit is located below the gas tank. It has a single black wire with a white stripe, should go to a bullet connector. Disconnect that wire. That eliminates the kill switch and the ignition switch along with any short to ground. If the ignition system is working you will get spark when the engine turns over regardless of the positions of the ignition and kill switches.
    Definitely does not have spark. Pulled a plug and did the ground test. I also read about the black/white wire trick else where in the forum and have already tried that. Still no spark. Does that eliminate a short to ground anywhere in the system?
    Thanks for the help

  10. #9
    Sensei 66Sprint's Avatar
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    NO.... Removing the black/white wire connection simply eliminates any other "kill" grounding from having effect to prevent the pulse from reaching the coil as long as it is disconnected.......
    The Yellow wire to the coil could be finding an intervening ground, so check it.....Green coil wire IS a ground.....
    You DO realize that neither plug will fire if BOTH plugs can't simultaneously find (jump to/arc to) ground or other common base connection...right?
    Input wires from the magneto could also still be shorted to ground.......Or an open pulser/pick-up coil could be the problem.....
    Last edited by 66Sprint; 06-06-2019 at 02:46 AM.
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  11. #10
    Junior Member mwgalen's Avatar
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    Quote Originally Posted by 66Sprint View Post
    NO.... Removing the black/white wire connection simply eliminates any other "kill" grounding from having effect to prevent the pulse from reaching the coil as long as it is disconnected.......
    The Yellow wire to the coil could be finding an intervening ground, so check it.....Green coil wire IS a ground.....
    You DO realize that neither plug will fire if BOTH plugs can't simultaneously find (jump to/arc to) ground or other common base connection...right?
    Input wires from the magneto could also still be shorted to ground.......Or an open pulser/pick-up coil could be the problem.....
    Hello,
    That is what I figured. The next question is, is there a systematic way to check for an intervening ground somewhere else on the bike?

    I am aware that both plugs must be grounded together, but as I understand it they ground through the engine and so pulling a plug and grounding it to the engine should still allow it to spark, yes?

    THanks for the help!

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