Doug, et Listers,
Sometimes a picture is worth a thousand words, and I think we've gotten
pretty close to a millennium of words on this subject. On pages 35, 36, &
37 of the Electrical System section of my Tiger Service Manual there are
three wiring diagrams, all of which show essentially the same circuit
layout for the tach, ballast resistor, coil and distributor. It is helpful
to refer to one of these diagrams if you have a manual. Otherwise, I can
simply state that in the original wiring configuration, the current flows,
starting from the ignition switch, through the loops (2) around the tach
pickup, thence through the ballast resistor, coil, distributor points, and
finally to ground to complete the circuit. This is a series circuit and, as
such, at any instant of time has the same current flow everywhere in the
circuit. Also, it does not matter from a functional point of view, in which
order the circuit elements are arranged. For example, you could run the
current through the points first, thence through the coil, the ballast
resistor and finally through the tachometer pickup and to ground, or any
other order you might choose. For practical reasons it is more expedient to
do the way the diagram shows, but in principle you could do it in any other
order if you really wanted to. Since distributors are made with one side of
the points at ground, this is always used as the last element in the
circuit. However, one can easily switch the order of the other pieces
without any real difficulty. Obviously, some of our Tigers have had some
changes made in this circuit. But, the tach just senses electrical current.
As long as the current flowing through the coil, points, etc. also flows
through the loops around the inductive pickup of the tach, the tach will
function equally well, regardless of the order. It matters not whether
these loops are before the ballast resistor, between the ballast resistor
and the coil, or between the coil and the distributor. From the standpoint
of minimizing the length of wire required, and in the interests of
originality, putting the tach first in the circuit would be the preferred
arrangement.
A tach that operates by sensing voltage is configured differently. It has
high electrical impedance and where it senses the voltage in the circuit is
important. The best place to attach this wire would be between the coil and
points, although it could probably be made to work between the coil and
ballast resistor.
I hope this helps clarify things just a little.
Not a rocket scientist in San Diego,
Bob
At 08:57 PM 6/9/99 -0400, Doug & Rett Leithauser wrote:
>chuck & list;
>one of us is definately missing something. I have checked the hookup on my
>own car, it is connected to the distributor side of the coil. I'm just a
>dumb mechanic, not a rocket scientist (Laifman's job) but it seems to me
>that if the tach is connected in the feed to the coil before the ballast
>resistor that there should be 12 volts at that point and nothing for the
>tach to count. If this is indeed what the service manual shows then my car
>is wrong, & I understand the world even less than I thought I did.
>Doug Leithauser
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