I sent this to the list because I read a couple of the responses and feel
the need to suggest that while working on the ignition system, one should
avoid putting your hands near the ignition wires, either primary OR
secondary, while the car is running, unless you need a "kick-start"
excitment in your life.
8 vdc is likely right when the points are closed, iginition on. Of course,
when the points open, the voltage goes down to nothing, in practical terms.
But this is not completely true. When the current flow stops, as when the
points open, the voltage actually shoots up to 250-300v suddenly. This is
because of the collapsing magnetic field, which induces voltage in the
primary in the same way as the secondary, only at a much smaller value
because of the fewer amount of wraps of wire in the primary. This is the
reason for the "condensor"; or more accurately, the Capacitor. The
condensor "absorbs" the high voltage peak, and reduces points arcing, as
well as allowing a rapid collapsing of the magnetic field, which gives a
better induction in the secondary of the coil.
That said, it should be noted that when you use a common voltmeter while the
car is running, you'll not read that sudden peak voltage in the primary,
simply because neither a digital nor analog voltmeter will respond quickly
enough. That is why we mechanics of yesteryear used "scopes" to read
primary and secondary voltages.
To get to your question, why do you say "weak coil"? are you getting poor
spark? Poor performance? Misfiring? any number of maladies could cause
these problems. But to check the coil, the best indicator is an ohmmeter.
Very few coils of our year cars used internal resistors in the coils, and a
rough idea of the resistance should be around 250-450 ohms (across the
primary terminals, while at least one of the wires is disconnected, and the
ignition OFF). Seldom was a secondary resistance check used, although there
are specifications for that, too.
B/T/W..... If you have an early type of Electronic system, many of them will
"pulse" when the ignition is on, and the engine stopped.
If you can find a repairman in your area who still has an ignition
oscilliscope, (drat, it's been years since I spelled that word) it's the
best tool for checking the ignition systems of our era cars. By reading the
voltages of each the primary and secondary while the engine is running, and
performing open circuit tests of the available voltage, the mechanic can
easily isolate the condition of your coil.
Good luck
Mike Higgins
1955 Belvedere Sport Coupe
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