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Author Topic: Electronic Tachometers  (Read 1519 times)

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Offline tsflstb

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Electronic Tachometers
« on: February 28, 2006, 11:11:13 AM »
I'm thinking of using this electronic tach from Summit.  It's pretty small, cheap, and goes to 11K RPM.

http://static.summitracing.com/global/images/instructions/sum-g2870.pdf

Installation instruction #5...Connect green wire to negative terminal on coil.  I'm not really up to speed on the ignition system, but I've heard about the "wasted spark".  Would connecting this to just one coil (instead of both) on my 400F give me the correct RPM?  Would I get 1/2X RPM, 2X RPM, or a giant ball of flames?
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Offline n9viw

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Re: Electronic Tachometers
« Reply #1 on: February 28, 2006, 11:23:38 AM »
You would get0.5x RPM, since the coil would be firing only twice per crank revolution, instead of 4x.
I've considered this, and wonder if an electronic tach COULD be connected to both coils, if a person used a diode in the leads going from the green wire to each of the coils' negative sides. That would keep the voltage drop from one set of points from affecting the wrong coil, but the tach would "see" both coils close twice per revolution, which would "look" like one set of points closing four times.
What's the worst that could happen- it doesn't work? Then the bike doesn't run. Oh well! Take it off and say, "it DEFINITELY won't work". :D

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Offline cb(r)

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Re: Electronic Tachometers
« Reply #2 on: February 28, 2006, 11:41:23 AM »
guys I thought that to run a digital tach you need to use your mechanical cable in conjunction to a special adaptor on the digital tach for this idea  to work?  I  could be wrong.  I am definitely not someone with alot of electrical ability.
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Offline tsflstb

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Re: Electronic Tachometers
« Reply #3 on: February 28, 2006, 11:56:20 AM »
Quote from: n9viw on February 28, 2006, 11:23:38 AM
You would get0.5x RPM, since the coil would be firing only twice per crank revolution, instead of 4x.

I thought the tach just took one firing pulse to be 1 revolution?  They say this tach works on 4, 6, and 8 cylinder engines - how does it know how many pulses to look for?  Excuse my dumb-assedness if there's an obvious answer, which there probably is...
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Offline tsflstb

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Re: Electronic Tachometers
« Reply #4 on: February 28, 2006, 12:01:25 PM »
Quote from: cb(r) on February 28, 2006, 11:41:23 AM
guys I thought that to run a digital tach you need to use your mechanical cable in conjunction to a special adaptor on the digital tach for this idea  to work?  I  could be wrong.  I am definitely not someone with alot of electrical ability.
I'd like to see something like that.  I guess you could have a set of contacts that spin with the tach cable and replicate the coil...
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Offline cb(r)

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Re: Electronic Tachometers
« Reply #5 on: February 28, 2006, 12:14:43 PM »
these guys have one www.dakotadigital.com but I do not know if it is only for their gauges or what.  very pricey gauges.  If I can find it I had a diagram that some guy was making these adaptors homemade or for d.i.yers.
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Offline Steve F

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Re: Electronic Tachometers
« Reply #6 on: February 28, 2006, 01:20:40 PM »
Quote from: n9viw on February 28, 2006, 11:23:38 AM
You would get0.5x RPM, since the coil would be firing only twice per crank revolution, instead of 4x.
I've considered this, and wonder if an electronic tach COULD be connected to both coils, if a person used a diode in the leads going from the green wire to each of the coils' negative sides. That would keep the voltage drop from one set of points from affecting the wrong coil, but the tach would "see" both coils close twice per revolution, which would "look" like one set of points closing four times.
What's the worst that could happen- it doesn't work? Then the bike doesn't run. Oh well! Take it off and say, "it DEFINITELY won't work". :D


If the points are like on the 750's, I'm thinking that the ignition system works the other way....firing ONCE for every revolution since the points are working off the crankshaft.  Does the 400 have the points on the camshaft (firing once for every 2 crank revolutions)?
So, if the points on your 400 are on the crankshaft, you'll be reading twice (2x) the actual rpm with the typical electronic tach as they are usually designed to receive signals off a distributor which runs at half the crankshaft rpm's. 
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Offline tsflstb

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Re: Electronic Tachometers
« Reply #7 on: February 28, 2006, 01:32:53 PM »
The points rotate with the crank.  So if I understand this correctly, the tach gets 2X the RPM it expects. 

Back to my original question, would connecting the tach to only one coil give it 1/2 the pulses (from one set of points)
    ---->  2X(because it's from the crankshaft)*1/2X(because only one coil)=1X the actual RPM= 8)

Looks good on paper.

I emailed the guys at Dakota Digital about the adapter.  They're having someone look into it.  I'm sure they could do it, but I'm a little scared of the cost.
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Offline cb(r)

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Re: Electronic Tachometers
« Reply #8 on: February 28, 2006, 01:48:10 PM »
ya the cost drove me away from the whole unit.
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Offline MRieck

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Re: Electronic Tachometers
« Reply #9 on: February 28, 2006, 02:18:57 PM »
The Daytona electronic tach (by Shindy) will not work with points. It's not a bad unit either.
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