Why does my KOSO inductive tach need ground?

4 posts in this topic

Posted

So I bought a KOSO mini tach and fabricated a mount for it to sit in front of my speedometer. Ran the inductive wire to the spark plug wire and wrapped it 5 times and secured it with a plastic tie. Go to start the engine and nothing. By happenstance, I touched the metal mount and voila -- the tach works. Don't get me wrong, it is no big deal for me to ground the holder, but this kind of tach is supposed to work without any other connections (in fact, it even comes with a velcro mount!). So while it works like a charm, I'm scratching my head as to why it needs to be grounded. Any ideas? (see pix)

M

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Posted

Maybe it needs it for the internal battery, I've read on another forum that guys had to add an external ground, some of them it would just bounce around, not read properly.

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Posted

It needs a complete circuit. The spark induces a voltage in the sensing wire that's wrapped around the ignition wire. In order for the tach to see that voltage, it needs something to compare it to, like the frame ground. If you don't do this, the tach will "float," electrically speaking. This would mean that when the spark occurs and the sensing wire sees, say 10V, the entire tach will raise to 10V so in essence nothing changes that the tach can see.

An analogy to describe it better:

You are in a car but can't see out the window. The road is too smooth to feel if you are moving or not. How do you know what is going on?

Now we give you a window to see the trees that aren't moving (the frame ground). If the trees are moving by from your perspective, then you are moving.

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Posted

It needs a complete circuit. The spark induces a voltage in the sensing wire that's wrapped around the ignition wire. In order for the tach to see that voltage, it needs something to compare it to, like the frame ground. If you don't do this, the tach will "float," electrically speaking. This would mean that when the spark occurs and the sensing wire sees, say 10V, the entire tach will raise to 10V so in essence nothing changes that the tach can see.

Fair enough (and it makes sense). But then KOSO failed to mention anything about grounding the unit in their instructions. I might add, they show the mounting screws as optional offering a pure velcro mounting option as the alternative. As such, there is no designated grounding point. SMH!

M

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