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 Post subject: Re: 3cyl. Tach !
PostPosted: Sun Nov 06, 2011 2:29 pm 
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doesn't the coil saturate with 12 volts and then get dumped to ground to discharge the coil's secondary?

you'd get a little added bump from the inductor absorbing the magnetic field as it collapses.

from trying to get the dash tachometer to work with a 12 volt pulse from my megasquirt controller i know that you can't make it happen unless you use a relay coil to get that little extra bump. 12 volts alone won't trigger the tach.

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 Post subject: Re: 3cyl. Tach !
PostPosted: Sun Nov 06, 2011 3:33 pm 
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Yeah, though without scoping that line, it's hard to know how big that signal should be.

I've just tried to duplicate JBs setup and can't get the tach to respond. I only have one, and don't want to take the risk of blowing it up, so I think I am going to wait for his resistor value suggestion.

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 Post subject: Re: 3cyl. Tach !
PostPosted: Mon Nov 14, 2011 1:01 am 
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Sorry for the delay - work and life and laziness got in the way.


Your two resistors, 27K and 9.1K in parallel gives 6.806K.

In a previous post, a tach went from 4 cyl and 6.25K to 3 cyl and 11.49K, or 1.838x

Using the same scaling will get you pretty darn close I expect.

so 6.806K x 1.838 = 12.5K total.

There isn't a standard 5% 12.5K resistor, but there is a 12.4K 1% you could use instead of your 27K and 9.1K resistor, OR, lift one leg of the 9.1K resistor and tack a 24K 5% across the existing 27K resistor. This would give you 12.7K, close enough.

Use a pulse generator like my setup to check your work.

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 Post subject: Re: 3cyl. Tach !
PostPosted: Tue May 08, 2012 8:47 pm 
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Got sidetracked for awhile but am about ready to get back into this. I'll try your values, JBD, thank you.

But I recall that I wasn't able to get my tach to respond using your setup and I didn't want to risk damaging mine. Can you please go over your connections, signal source and characteristics?

Thanks.

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 Post subject: Re: 3cyl. Tach !
PostPosted: Wed May 09, 2012 11:06 pm 
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Well, I found a 3cyl W/Tach Speedometer since I've screwed my 4cyl. lol anyway I was about to use the Resistor's but finally found a vert with a cluster, paid 25$ !

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 Post subject: Re: 3cyl. Tach !
PostPosted: Mon Jul 09, 2012 6:32 pm 
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I never could duplicate the test setup, so I decided to gamble.

Got her done. Took an ugly instrument cluster with a severely damaged clear panel (got it very inexpensively), cleaned it inside and out, sanded and polished the clear to perfection, changed the resistors to a 12.4K (two 6.2K parts in series) part and stuck it in. Turns out that I did not have to lower the steering column to accomplish this. I did run into one issue, and that was the length of the speedometer cable. The original speedo is in the center of the cluster, and the new one is a few inches to the left. That matters with the original routing- too tight. Of course, I found this out AFTER everything was installed and I was attempting to hook the cable back up to the transmission. I rerouted the cable and all is well.

Image

Beauty!

Works perfectly. Turns out that the ratio of 1.838 is correct. So take your 4 cyl tach, determine what the original resistance value is, multiply that by 1.838 and install the new value. Then stick the cluster in your 3-banger and off you go.

Thanks for the help. Cheers!

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