Wiring in a tach

[Scurvy Knave]

I want to wire a tach into my old Fleetwin. Just for diagnosis and tuning.

Can I simply tap a wire between the primary on the coil and the condenser, and then ground back to the crankcase?

I don't want to spend money on an inductive tach (like tinytach) but have access to old/surplus analog tachs for free.

any ideas?

 

[Chris1956]

Re: Wiring in a tach

If that fleetwin is a 2 cylinder, you will get 1/2 the RPM reading, if you get any reading at all. Those tachs were likely made for a battery/points/coil ign. The OB should be a magneto ign.

 

[Papa gone fishing]

Re: Wiring in a tach

Never mind...:embarassed:

Papa :fish2:

 

[Scurvy Knave]

Re: Wiring in a tach

Chris, how do you figure 1/2 RPM. On a four stroke engine I can understand, since any given cylinder fires every other revolution of the crankshaft, but on 2 stroke each cylinder fires every revolution of the crankshaft. I know the signal might not be strong enough, I might have to to rig something up. But would tapping in at that point work?

 

[Chris1956]

Re: Wiring in a tach

Well, If you had a 2 cylinder 4 cycle setting on the tach, it may work, although I do not know if the magneto signal will drive an auto tach. Magnetos are a volt or two, whereas auto battery/coil.points igns are 12VDC. Try it. it may even work.

If you could get the signal off both sets of points, and if it does indeed drive an auto tach, you could set the tach to 4 cylinder, and you may get an accurate signal. Of course you would need some way (diodes?) to keep the points from shorting each other out.

I never saw an automotive tach with a 2 cylinder setting.

 

[Scurvy Knave]

Re: Wiring in a tach

Well, it might not work, but its worth a try. They say necessity is the mother of invention, I say it's cheapness and laziness

 

[Scurvy Knave]

Re: Wiring in a tach

alternatively, a multimeter with a frequency counter function could also be used. no?

 

[gm280]

Re: Wiring in a tach

That may work if the signal can trigger enough input to register on a multimeter per the frequency setting. I too have an old tach/dwell/volt meter and it doesn’t work even on signal cylinder 4 cycle engines correctly (obviously). So I ordered a Tiny Tach and will calibrate my old tach/dwell/volt meter to the correct RPMs I read from the Tiny Tach. I may even design a new circuit for that old meter to allow me to switch from 2 cycle to 4 cycle and select the number of cylinders as well. Shouldn’t be too hard to do… Then I’ll have a large analog meter to work with and give me subtle readings for carb settings at low speeds. I also bought a Faria 7K tach for the actual boat instead of any speedometer. I think the engine RPMs tells me more about the operation of engine and what is going on than any speedometer could ever…

 

[Scurvy Knave]

Re: Wiring in a tach

well, if you design the circuit, I would love to see it. Would a simple op-amp amplifying circuit suffice to boost the signal?

 

[gm280]

Re: Wiring in a tach

well, if you design the circuit, I would love to see it. Would a simple op-amp amplifying circuit suffice to boost the signal?

An op amp could help, BUT I'd wait until you actually try the frequency option on the meter first. It may work without any problems. I don't know the update rate but try it and see... A simple linear amp circuit may work for the analog meter conversion circuit too...