Tach is correct, what have I done wrong?

Discussion in 'Electrical' started by Nightowl, Mar 18, 2012.

  1. Nightowl

    Nightowl Account Removed

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    First the bike is running great. But here is my puzzle. I got one of those mini tachs for Christmas and after reading several of articles on the mini tachs I assumed it was a 2:1 ration and I would need a single fire adapter since my 2005 FLSTC is single fire ignition. So being basically cheap, and also part of the enjoyment of a motorcycle is doing things for yourself, I read an internet article on how to build a single fire adapter from parts purchased very cheaply at Radio Shack. So I wired up it all up and started the bike and the tach read zero. No problem I thought and rewired it without the adapter and sure enough at idle it read 500 rpms and I know my idle speed is 1000 to 1100. Since I can do the math (at least I can multiply by 2) I thought this is cool and will order a “real” single fire adapter from Barron’s. But first it was time for a ride.
    Well a few miles down the road the rpm read out doubled and by all my calculations was reading the engine speed correctly. Now sometimes it reads at ½ the rpms at start up and in a mile or so starts reading rpms correctly. Sometimes it is correct from the get go. Do you think there is some circuitry in there that can tell what single it is getting? Not a major deal but I am curious as to what the wisdom of the forum has to say. Thanks.
     
  2. R_W_B

    R_W_B Senior Member

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    Nightowl when you say you "assumed" it was a dual fire tach, is that a known fact ?

    I.e. did you ask the reseller or manufacturer if you needed a single fire adaptor ?

    I would be communicating with the folks you bought it from. Depending on how much you paid for it and it's quality some of those tachs vibrate back and forth so much it's hard to read them acturately.

    Also do you have a wiring diagram that came with it I could see ? I'm wondering what it said on the coil hook up.
     
  3. Nightowl

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    RWB: Well it is a cheap tach no question, but there is absolutly no needle bounce. It is wired just as the diagram shows. You wire the tach input to the negative side of one of the coils and then the power and light are wired to a switched source. The question is not does it work right, but why does it sometimes start at 1/2 of rpms and then correct itself. Like I said not a big deal to me (as long as there is no long term danger to my system) just more curious as to why than worried about a fix. One more thing when I turn on the ingintion switch the needle goes all the way to the max (8000 rpm) and the returns to zero until the bike is started and then it read either correct or 1/2 of actual rpms.
     
  4. Hoople

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    Just curious.. Are you running the exact genuine HD plugs and wires the book calls for? It almost sounds like a secondary signal level "threshold" for the ion sense circuit.
    Since there is no cam sensor, ignition phasing (not firing on TDC of exhaust stroke) is performed through ion sensing.
     
  5. R_W_B

    R_W_B Senior Member

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    Don't think it would matter, but you might try switching the tach to the other secondary coil terminal. Cobra tachs say to hook to the front jug coil terminal.

    You lost me on that one Hoop. He said his is single fire, why would there be a fire on TDC of the exhaust on a single fire ?
     
  6. R_W_B

    R_W_B Senior Member

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    Nightowl, whatever let us know how this turns out when you get you SF adaptor. If I remember correctly Barons will give them to you for free so I guess you just wanted to experiment with the homemade version.

    This one has got my interest up as to what's going on. I've heard of some Ignition modules (most of the newer bikes incorporate that in the ECM) that if the battery voltage drops below a certain level on start up it breaks the power to the coil. Which thereby results in collapsing the coil field and gives a spark inpulse. However I would not imagine this to be happening after the engine has fired and is idling.
     
  7. Nightowl

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    RWB: Thanks for your interest. Baron's includes the SF adapter free in their Tach Kits, but cost about $30 to purchase. My tach is from a company called Mid-USA and was a gift. I am not going to install an adapter at this point since everything is working and the bike is running great. The tach reading has always corrected it self in a few minutes. I am mostly curious as to why, and of course don't want to be causing any damage in the long run. Hoople, thanks for your imput, you are extremly knowledgable about the electronics, However your response was way above my understanding level. All of the ingintion components are stock HD as far as I can tell. Thanks guys. If anything changes I will let you know.
     
  8. R_W_B

    R_W_B Senior Member

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    {-Do you think there is some circuitry in there that can tell what single it is getting?-}

    I would just try to call Barons and ask them this. They are one of the better tach makers out there for their higher priced versions. If you don't tell them you got it as a gift you can get more info from them. Just tell them you don't have the S/N or order # handy. Although it's hard to imagine how the tach could be that smart when it doesn't appear to have any wired in CKP abilities so how is it going to know if the impulse occurs every rev or every other rev.

    I don't see where it would harm the system but Hoople would be better qualified to comment there. Just for giggles you could put a jumper wire on your tach coil hookup thereby hooking the tach up to "both" secondarys (the front and rear coils) and see if the rpms "stay" at the 1/2 reading from then on instead of adjusting.
     
  9. Nightowl

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    It is not a Barrons Tach. It is one of those Made in Taiwan. I have a call into the guy who sold it to my son-in-law over ebay (bought as a christmas present). He was a stand up guy and is looking into what he can find out and promised a call back. I also called Mid-USA and am waiting to hear back from them as well. Will keep you posted.

    I agree that Hoople is about as good it as it gets with the electrical stuff, If I was just smart enough to understand. Like the southern politician once said "I can explain it to you, but I can't understand it for you."

    Thanks,