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Convert cable tach to electonic? - IT WORKS

Discussion in 'XJ Modifications' started by bill, Jul 12, 2009.

  1. bill

    bill Active Member

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    Is this possible - The XS 400 has a cable type tach - I have an XJ650 cluster. I can swap the tachs out but can I us the electronic somehow?
     
  2. Polock

    Polock Well-Known Member

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    Re: Convert cable tach to electonic?

    isn't the 400 a two cylinder ? that might be a problem
     
  3. bill

    bill Active Member

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    Re: Convert cable tach to electonic?

    yup it's a 2. The current Tach is driven off the cams. I was wondering if there was a device to plug into there that would provide a signal to an electronic tach. My fall back it is install the mechanical tack into the 650 housing. I looked at ti last night I need to cut the mount from the existing cluster but it is doable.
     
  4. xulf13

    xulf13 Member

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    Re: Convert cable tach to electonic?

    Bill, is the coil on that 400XS a double firing one? meaning, does it fire twice per cylinder? if so then you can get your signal from the negative of the coil without issues.
     
  5. bill

    bill Active Member

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    Re: Convert cable tach to electonic?

    nope 2 cylinders - 2 pickups - 2 coils - 1 fire.
     
  6. bill

    bill Active Member

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    Re: Convert cable tach to electonic? - New question---

    OK in reading this stuff I'm asking the wrong question. I think I can just use the Electronic tach by tapping into the coil right?

    The 650 has 4 cyl but the tach is on one coil that fires once per engine revolution (granted it fires 2 plugs but that is not relevant?) So tapping one of my 400's coils I should be able to use it as is...
     
  7. xulf13

    xulf13 Member

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    Re: Convert cable tach to electonic? - New question---

    I think so. You won't lose anything by trying, might get a wrong reading if not, but won't blow anything up. But if I'm not mistaken the 650 fires twice per engine revolution. Since each coil fires 2 cylinders, it sends spark on the intake stroke and the exhaust stroke. that might matter because it fires twice as fast so that the tach uses the signal and divides it by two. if you use that tach then it might read a lower RPM by half maybe than what it really is. But I'm just guestimating.
     
  8. bill

    bill Active Member

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    Re: Convert cable tach to electonic? - New question---

    Actually I have been thinking this out. The 650 fires 2 plugs once each engine revolution. The 2 coils fire 180 degrees out from one another. The 4 stroke cycle is 2 revolutions. The Twin does the same but only 1 plug is firing at a time but still firing once a revolution. It's like looking at only cylinders 1 and 2 on the 650.

    So it should work fine - I'm gonna try it by this weekend for sure. Have some reassembly to do before I fire her again
     
  9. johntc

    johntc Member

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    Re: Convert cable tach to electonic? - New question---

    Right. On the 4 cyl you could hook up to either coil and you would get one pulse (spark) per engine revolution since each spark plug is fired on both the compression and exhaust strokes.
    On the 400 each coil only fires every 2nd revolution so your tach will read only 1/2 what it's actually turning.
    If you were to hook up to both coils you might have a problem with cross firing unless you rigged up some diodes.
    How come everything that sounds like it should be so simple have to be so difficult (and so much fun)?
     
  10. bill

    bill Active Member

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    Re: Convert cable tach to electonic? - New question---

    JT, I think the only way that would happen would be if the CDI counted revolutions so to speak.

    I have 2 pickups 180 deg apart and 2 separate coils just like the 4 cyl so I believe the 400 is wasted spark also unless you add fancy logic to the CDI and why bother?
     
  11. johntc

    johntc Member

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    Re: Convert cable tach to electonic? - New question---

    Could be, let us know how it works.
     
  12. bill

    bill Active Member

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    Ok I couldn't stand it so no pipes etc I cobbled the tach into one of the coil primarys and fired her up. It worked great - I would certainly know if it was half or double but it sure looked reasonable.

    I'm fired up now - got to wire it in properly but I'm a happy boy.
     
  13. xulf13

    xulf13 Member

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    Cool beans. I'm glad it worked :)
     

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