The tach/dwell meters don't use the high voltage coil wire, just the one
that goes to the points. These meters just look for any momentary short to
ground (as in "The points are closed! Hooray, the points are closed!")
followed by any positive spike in voltage (as in "The points have opened!
Hooray, the points have opened"). The dwell reading is simply a statement
of the % of time they are closed divided by the time between cycles (opening
to opening).
The tachometer reading is simply a measure of the number of times this
happens per minute.
The size of the positive voltage spike is not a critical value, merely the
timing of it, in other words. It just has to be large enough to trigger the
electronic time measuring circuitry inside the meter. In mine, and perhaps
many others of these, the 6 volt ignition system works the meter just fine,
just watch that polarity!
Dick Benjamin (who built one of these deals in 1952 as a hobby project)
----- Original Message -----
Subject: Re: IML: tach&dwell meter
> Can/t you get a 12 V battery and run the hot wire of the meter/tach from
it
> and just use the coil lead for the trigger? Might have to hook the -
wires
> of both batteries together. Some of you electrical types jump in on this.
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