# Auto Tach For Model Engine CDI



## kcmillin (Apr 17, 2011)

I am brainstorming ideas on a base for my my TI4. I am thinking a Dyno type setup with a hand throttle, key switch, and tachometer.

My question is can I connect and after market tachometer to an S&S model IC CDI unit? On a car you would connect the signal wire to the positive terminal on the coil. Could I just solder a wire to the positive side of the primary winding? Or is there going to be a problem with resistance and all the other electronic mumbo jumbo I don't understand fully. 

Is there another way of connecting a tach to a small engine (This is for demonstration, I already have handheld tachs, but would like a nice analog tach for the display)

Kel


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## RICHARDDV (Apr 17, 2011)

i worked on air cooled engines for sears for the last 15 years and we used an electronic induction tach that was sold as an accessory for our customers lawn mowers. It kept up with the op hours,service intervals as well as rpms. It connected simply by wrapping the black wire lead around the hightension lead and grounding the white wire. It sold for about $20.00 usd.   richard


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## Metal Butcher (Apr 18, 2011)

Kel, there is a good supply of inexpensive tachometers available on the internet and E-Bay.

Examples: http://cgi.ebay.com/ebaymotors/DIGI...5121231QQptZMotorcyclesQ5fPartsQ5fAccessories


http://cgi.ebay.com/ebaymotors/DIGI...6010883QQptZMotorcyclesQ5fPartsQ5fAccessories

I added one to my atv, about $15 at the time.

-MB


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## kcmillin (Apr 18, 2011)

Thanks for the Link MB. That is a pretty cool device, and cheap too! I will defiantly keep this as an option, but will it work with a 4 cylinder?

 Although the digital is cool, I am more interested in an analog tach, they are easier to read, and you can watch the needle swoop, with a digital, all you get is flashing numbers. Is there a way to use this device in conjunction with an automotive tach.

Kel


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## Metal Butcher (Apr 18, 2011)

kcmillin  said:
			
		

> I am brainstorming ideas on a base for my my TI4. I am thinking a Dyno type setup with a hand throttle, key switch, and tachometer.
> My question is can I connect and after market tachometer to an S&S model IC CDI unit? On a car you would connect the signal wire to the positive terminal on the coil. Could I just solder a wire to the positive side of the primary winding? Or is there going to be a problem with resistance and all the other electronic mumbo jumbo I don't understand fully.
> 
> Is there another way of connecting a tach to a small engine (This is for demonstration, I already have handheld tachs, but would like a nice analog tach for the display)



Kel, As long as your set up doesn't produce a waste spark and fires only once, an electronic anolog (dial) tach could work. With a digital tach it doesn't matter how many cylinders the engine has since it hooks up to only one cylinder at or near the plug. If the tach is switchable for both 2 & 4 stroke applications, then it will work in 2-stroke mode if you have a waste spark due to a crank mounted set of points or sensor.

It might be a lot easier to try using the inexpensive digital hour meter/tach. Its self contained power source and single wire hook-up allows it to be switched between other engines you may build or have. If it dosn't quite work out you can allays stick it on your lawn mower to monitor oil change intervals or time spent on this mindless task. I understand what you are trying to accomplish with the set-up your describing, and hopefully you'll get the information you need to make a decision. 

You could try asking Roy Sholl to find out if you can-and-how to hook up a tach to his system, model ignition is his specialty so he would be the one who would have the right answers.

This link below might shed some light on how an automotive tach is hooked up.

http://www.ehow.com/how_4962742_hook-up-tachometer.html

-MB


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## nfk (Apr 18, 2011)

Kel, 
It should work out of the box, assuming the coil is a 12V coil.
Take into account that the tacho will eat a few mA from the CDI and if the latest is not strong enough it will affect the spark.
A few years ago i made a small circuit to overcome this trouble. It was made for my motorcycle which had spark troubles after installing the tacho.
The circuit used a small opto-coupler, 2 resistors and a transistor.
I`ll try to find it and post it here if you think it fits your application.

Norberto


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## Metal Butcher (Apr 18, 2011)

Kel, this is a nice one that goes up to 25,000 RPM. I might buy this one myself. Great way to keep track of the run time on an engine, and to monitor the run time on a full tank of fuel.

http://cgi.ebay.com/ebaymotors/Hond...r_Truck_Parts_Accessories&hash=item19c464d5ac

This one goes to 99,999 RPM!? 

http://cgi.ebay.com/TACH-HOUR-METER...463?pt=LH_DefaultDomain_0&hash=item19c4a3dc27

-MB


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## kcmillin (Apr 18, 2011)

Thanks For the links MB. Those tachs appear to be the same thing, but their specs are different. One of them says it works with multi cylinder engines, and the other does not. Although their physical appearance is identical.

I am sure that these tachs are no doubt functional, but there is just something about watching a needle swoop across the numbers that is very appealing.

I just sent Roy an e-mail asking about this, I am awaiting reply.

Maybe there is a different way to connect a tach, not using the spark. Like an optical sensor, or even gear driven, like on some motorcycles.

Kel


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## Metal Butcher (Apr 18, 2011)

kcmillin  said:
			
		

> Thanks For the links MB. Those tachs appear to be the same thing, but their specs are different. One of them says it works with multi cylinder engines, and the other does not. Although their physical appearance is identical.
> 
> I am sure that these tachs are no doubt functional, but there is just something about watching a needle swoop across the numbers that is very appealing.
> 
> ...



Kel, I understand what you mean about the traditional needle. And it would be nice to find one that would not only work, but with a 1" or smaller face. I'm pulling for you, but I haven't had any luck so far. 

Its nice to know that there is the digital option. it Just depends on the purpose or goal of the individual builder.

Most of the commercially available model engine ignition systems are not 12 volt and don't need to be. This is going to be a problem when trying to add a powered accessory like a tachometer.

-MB


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## picclock (Apr 20, 2011)

For an old fashioned type tach probably the simplest way is to stick a magnet on the shaft with a pickup coil and measure the output with your analogue gauge. This can be done simply and accurately with very few components. 

Alternately, if you ignition system uses points, you can trigger a monostable which will can output a smoothed voltage proportional to speed. The monostable system is used by most cars with analogue dials.

A simpler, and slightly more devious way, which will appeal if you are not into electronics is to obtain an old car speedo and change the dial, connecting the speedo cable reduction drive to the engine.

If you need further info on these methods just let me know.

Best Regards

picclock


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## kcmillin (Apr 20, 2011)

picclock, thanks for the ideas. I like the pickup coil idea. How exactly does that work? Would I need large components? Would the gauge be measuring voltage?

Also, I got an e-mail back from Roy at S&S. He said I could try to connect the tach signal wire to the white signal wire on the hall sensor. This is no guarantee though, and might result in a blown hall sensor. But I have a few extra now, so it is no big deal. I might give it a try. 

Kel


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## picclock (Apr 20, 2011)

How exactly does that work? 

Would I need large components? No, largest is pickup coil which could be an old relay coil say 1/2" dia x1" long. other bits very small

Would the gauge be measuring voltage? Yes


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## kcmillin (Apr 20, 2011)

Thanks Picclock, your drawing is nice, I can understand most of it. What would the orientation of the magnet be to the pickup coil? 

What is a set FSD? what is a bias resistor? how many ohms?

Am I in over my head if I am asking such questions? 

As far as a voltage gauge, should I test to see how much voltage it produces before I get the gauge? I would like as much swoop as I can get.

 Kel


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## picclock (Apr 20, 2011)

>>What would the orientation of the magnet be to the pickup coil? 

Magnet pole should pass near the end of the coil. Small neodymium magnet about 0.5mm from the pole piece. Stronger the magnet and closer the coupling the more volts generated. Small 12v relay coil or similar should be OK. 

>>What is a set FSD? Full Scale Deflection - adjust it to set the correct rpm figure - so optically tach it at 1000 rpm and set the variable resister so it reads 1000 on the dial - like calibration. 

>> what is a bias resistor? how many ohms? The value depends on how much current the rpm meter needs to read maximum. When you get your meter you need to figure out how much current it needs to read max rpm. Then set the bias resister to about 1/3 of that. Likely the value will be between 1K ohms and 100K ohms. Stick a value of 10K ohms in to start off with if your not sure then if the meter won't adjust high enough reduce it.

Mount the diodes next to each other so they are at the same temperature to get the best accuracy. IN4148 or any similar diodes are good. 

Hope this helps

Best Regards

picclock


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