By Staff
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A recent topic on the SmokStak bulletin board at brings up an item that is responsible for
spark plug ignition failure -the capacitor.

I would like to offer my help to those folks who may not
understand capacitors, also commonly called condensers.

First off, what is a capacitor? A capacitor consists of two
metal plates separated by an insulating material, such as paper.
Engine capacitors are usually two long strips of aluminum foil
about an inch wide sandwiched between layers of waxed paper, then
rolled up and placed in a metal case. One plate is connected to the
case and the other to a lead out wire. Fine, but what does it

When a capacitor is connected to a voltage source, such as a
battery, it acts momentarily as a closed circuit as current flows
to charge the plates; negative electrons on one plate and positive
on the other. When the voltage across the plates builds up to the
battery voltage, current flow stops and the capacitor acts as an
open circuit. The plates remain in a charged condition even when
disconnected from the battery. A capacitor, therefore, stores
electric power just as a battery does.

But, if a circuit path is connected to the plates the capacitor
will discharge its stored energy through that path. And, if the
voltage polarity is reversed it will recharge to the opposite

So, how do you test a capacitor to see if it’s good? If you
have an ohmmeter, connect the test leads together and zero the
meter. If the meter won’t zero replace its battery. Select the
highest R times range you have; R times 10,000 works well. Connect
the test leads to the capacitor while watching the meter. Movement
on the scale means the meter is taking the charge, and prompt
movement back to full left on the scale means there are no leaks in
the capacitor. A leak is a small amount of continuous current
flowing through a high resistance bridge across the plates.
Don’t touch the conductors, because you will read leakage
through your hands. Repeat the test while reversing the test leads,
and it’s a good idea to test several known good capacitors to
establish a reference. – Ralph

Good post. Magneto condensers (capacitors) have several failure
modes that a simple ohm-meter test will not identify. The first is
the intermittent failure of the leads that connect to the foil
capacitor plates within the capacitor.

If you have ever taken an older automotive or magneto capacitor
apart you will find that the connections are made by pressure on
the two ends of the capacitor by a flat spring of some type at the
bottom of the capacitor can. Over time, minute amounts of moisture
migrate into the can (mostly due to temperature cycling) and
corrode the connections. The result is an intermittent connection
that breaks down under the charge and discharge cycling of the

What seems to happen is when the capacitor is operating in an
ignition system the current surge causes a temporary weld of the
connection, and when the capacitor sits unused the connection
breaks down again. This is a very difficult symptom to identify
with any simple tests.

The second mode is leakage. I have found that leakage of less
than a megohm indicates there is moisture in the capacitor and
sometime soon the intermittent connection symptom described above
is about to cause capacitor failure. Assuming the capacitor shows
capacity, leakage is the most telling indication of its condition.
Any leakage at all tells you to replace the capacitor.

The old ignition capacitor checkers tested the leakage at 500 to
600 volts and would tend to break down the capacitor if there was
any potential for failure. They also checked the capacitance by
applying 50 to 100 volts AC to the capacitor and measuring the AC
current flow through the capacitor. These two tests stress the
capacitor in much the same way actual operation does.

What this all means is if your points are clean and gapped
correctly replace the capacitor next. If the magneto still
doesn’t work then suspect the coil. Keep in mind you will
occasionally find a defective (new) capacitor. I have a known good
capacitor set up with two clip leads. I hook it up to the points
after removing the connection of the original capacitor, one lead
to ground and the other to the contact point. This substitution
method is the best way to identify a faulty capacitor. -Sherm

I read someplace that scientists originally thought the
electrons sort of ‘condensed,’ much the way steam does,
inside the Leyden jar that was originally used as a capacitor. A
Leyden jar was a glass jar with a layer of foil on both the inside
and the outside surfaces of the jar. The glass jar served as the
insulating medium between the two metallic surfaces. The connection
to the inside surface was usually made by a small length of chain
that contacted the surface of the inside conductor. The chain was
attached to a rod, usually with a small ball on the top of it that
stuck out of the top of the glass jar. You could charge the Leyden
jar from some source of electricity, usually a static electricity
generator of some sort. The Leyden jar could be discharged across a
spark gap to show that a charge of electrons was built up. In any
case, they called these things and their later developments of the
same nature ‘condensers.’

It wasn’t until long after the real operation of the device
became clear that the name was changed sometime in the 1930’s
or 1940’s to ‘capacitor.’ This was because the device
had the ability to store electrons and thus it had a
‘capacity’ of one value or another.

Most common capacitors are of less value than a ‘farad’
and are thus rated in microfarads, one millionth, or
micro-microfarads, one millionth of one million. The term
‘micro-micro’ in the metric system is known as pico, as in
picofarad. The term farad came from the name of the scientist
Michael Farady (1791-1867) who discovered the capacity effect along
with a host of other early developments in the new field of
electricity and magnetism.

The value of a farad is equal to the amount of one coulomb of
charge for each volt of applied potential. A car battery, while it
is not a capacitor, may, for comparison purposes, have an effective
capacitance of approximately one farad.

In any case, a condenser (old term) and a capacitor (present
term) mean exactly the same thing and can be used interchangeably,
with the term ‘capacitor’ being preferred.

Incidentally, a capacitor in good condition can self charge from
the static electricity in the air. A large capacitor, such as found
in commercial xenon flash lamp power supplies, can, under the right
conditions, self store a charge sufficient to kill someone. Always
store capacitors, particularly large ones, with the terminals
shorted together to prevent self charging. Always discharge a
capacitor before working with it. -Russ

If the condenser in a Wico EK magneto no longer works, I can
replace it with a capacitor. What should the rating of the
capacitor be to match the old condenser? The electronics supplier
has hundreds of capacitors. Can anyone provide a list of what

capacitors would be correct replacements for the condensers in
the most common magnetos? -Eugene

My experience is the capacity of both magneto and spark coil
capacitors range from 0.2 microfarads to 0.33 microfarads. Almost
all automotive distributor coils use a 0.25-0.29 microfarad
capacitor. The operating voltage rating should be no less than 600
volts DC. There are now capacitors available in the 0.25 microfarad
range rated at 630 volts AC. Since the AC peak voltage is 1.4 times
the nominal value, these capacitors are actually 880 volt DC
capacitors making them ideal

for ignition use. I have used these capacitors inside of Bosch
magnetos to replace the mica capacitors inside the armature. They
can be soldered in and anchored with RTV sealant. – Sherm

Thinking in terms of a low-tension igniter system, when the
igniter opens and spark occurs, there has to be quite a bit of
voltage to continue jumping the gap. The collapse of the magnetic
field in the low-tension coil is similar to a high-tension
coil’s primary. The capacitor across the points is used to
suppress the arc that would ordinarily occur, which would
eventually burn out the points. – Russ

I would like to expand on Russ’ mention of high-tension
ignition. When the points open the capacitor is connected in series
with the coil. The voltage/current generated by the collapsing
magnetic field charges the capacitor. This is the voltage/current
source that is induced into the stepped up secondary winding. A
coil opposes current flow while a capacitor enhances current flow.
The enhancement of the capacitor balances out or cancels the
opposition of the coil. The result is a faster collapse of the
magnetic field and the highest possible voltage generated in the
primary circuit. It’s true that the capacitor reduces or
prevents arcing at the points, but its main function is to provide
a circuit path for the coil after the points open and to speed up
the collapse of the magnetic field. – Ralph

For additional internet reference on battery and coil, buzz coil
and magneto ignitions, see
SmokStak is an engine conversation bulletin board with over 15,000
messages on file and is part of the Old Engine series of web sites
that started in 1995 as ‘Harry’s Old Engine.’ Harry
Matthews is a retired electronic engineer and gas engine collector
from Oswego, N.Y., now residing in Sarasota, Fla.

‘A coil opposes current flow while a capacitor
enhances current flow. The enhancement of the capacitor balances
out or cancels the opposition of the coil. The result is a faster
collapse of the magnetic field and the highest possible voltage
generated in the primary circuit.’

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