Ignition Coil Installation

from a deep article...or everything you always wanted to forget about condensors...
"The condenser must be of proper rating. If it is
either over or underspecced for the application, the points will pit and
burn. Remember the minus-minus-minus rule: If the minus (ground,
stationary) point is minus metal (pitted), then the condenser is minus
capacity (underspecced). The obverse applies, though it becomes the
"plus-minus-plus" rule: If the plus (positive, movable) point is minus
metal (pitted), then the condenser is plus capacity (overspecced).

That said, an engine will run OK with a fairly wide range of capacitance
in the ignition primary; years ago the condenser on my '65 Valiant failed
on my way to high school one morning. The engine hiccuped, coughed and
sputtered the remaining 1/4 mile; I coasted to a stop in a parking space.
No auto parts stores within convenient distance, but there was a hardware
store across the street. Come lunch hour, I went in and bought a small
condenser for a Tecumseh lawnmower engine. Wrapped a few turns of bare
wire around the condenser body and grounded the other end of that wire.
Put the condenser's lead wire on the coil negative. Car started and ran
fine and got me home, where I could install a correct condenser.
For the secondary effect of reducing AM radio noise
A different condenser -- the radio noise suppression condenser -- is used
for the bulk of this job. It is connected across coil + and ground. What
is commonly referred to as the "distributor condenser", i.e., the one that
permits the ignition system to function, is connected across coil - and
ground."

and "The condenser(capacitor) is about .22uf give or
take for design differences. I don't know what the mH rating of a coil
is, but the coil is a electrical device known as a auto-transformer.
It's like a regular transformer, but both the primary and secondary
share some of the windings.

The capacitor is wired in parallel to the points and in series to the
coil. The capacitor tries to maintain a constant voltage while the coil
tries to maintain a constant current. The basic design of a automotive
ignition circuit is basically a series resonance circuit not unlike
those used in radio.

When the points are closed, the capacitor is shorted out and current
flows from ground, through the points, through the coil, and out onto
the positive power rail. This sets up a magnetic field around the
windings of the coil. At the moment that a sparkplug is to fire, the
points open which interrupts the current flowing through the coil, but
the coil is charged (magentic field), and in an attempt to keep the
current going, the coil discharges right into the capacitor.

At this point, when the coil is fully discharged, there is a voltage
charge on the capacitor in the range of +700-1000 volts in reference to
ground. Since this is much higher than the +12V rail that the other end
of the coil is connected to, the current flows BACKWARDS from +12V,
through the coil, and into the capacitor to even the charge. Because
there is little resistance in the coil, a high amperage pulse is
generated through the coil primary, and with a winding ratio of 40:1,
causes a voltage spike between 28KV and 40KV on the secondary, which is
more than enough to jump the gap between the rotor and distributor cap,
and the gap on the plug itself.

A leaking capacitor will disapate some of the charge before it can be
used to draw the current through the coil resulting in a lowered high
voltage output and a weak, or no spark condition.

A shorted capacitor will cause a no-spark condition because the current
will still flow through the coil.

A open capacitor will burn the points because the points will be forced
to absorb the discharge from the coil causing them to arc, and you will
get a weak or no spark.

Too little capacitence will cause the points to burn do to the same
reason that a open capacitor will. But, it may or may not cause a
weak/no spark condition depending on how much under the spec the cap is.

Too much capacitence will cause a weak or no spark condition because the
voltage across the capacitor will not be high enough because of the
larger value. The bigger the cap is, the bigger the charge on the coil
has to be to get it to the 700-1000 volt range.
--
Daniel Rudy

Dont know who Dan Rudy is but he is pretty sharp on this subject.