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Points,
Condensors and Coils
By
John Grady
I
came across a car where the coil was wired backwards; the wire from
the battery should be on the + coil post but was on the - post. I had
heard this was a bad thing to do so I asked John Grady. He said:
It will
work with the coil wired wrong, but there will be less volts
produced. The wiring of the two windings inside the coil is set up so
the primary voltage adds to secondary. The 12 Volt winding can put
out almost 1000 Volts. Coils typically have a 60:1 ratio so with a
primary of 1,000 the secondary output is 60,000 Volts. If a coil is
wired backwards the 1000 subtracts instead of adds and you get a
little weaker spark but it will run.
Let's talk
about how the coil works. Spark plugs arc over about 18-20,000 volts
(like starting a weld) then arc steady at about 4,000 volts until
the coil energy is used up. A few words to keep your coil healthy:
I said
earlier that the secondary output can be 60,000 Volts, but notice
plugs only need 18-20,000 volts. If you make the coil put out 60,000
Volts, it is working too hard and isn't going to last long. Let's say
you are holding a plug wire and trying check for spark by arcing to
the block. Most likely, at first you are holding the wire too far off
the block and nothing happens. Inside the coil you have 60,000 Volts
and it has to go somewhere. Usually it arcs inside the coil within
the coil insulation, causing a carbon track. Now your coil is hurt
but it may not show you that it is. Someday you might start to have
intermittent spark or it might just fail someday -- who knows when.
For that reason, never try to the hold wire to check spark. Always
use a spark tester. You can get them anywhere (Amazon, ebay, parts
stores) or make your own tester from old plug at .040" gap with
ground clip.
A few
words about ignition points:
Points
never wear out (*) and are not bad even if they have a rough face. I
don't recommend points files. Filing can remove hard coatings and
leave high points that burn. Just be sure the point faces are square,
concentrically aligned and adjust the point gap until you get the
dwell meter reading specified in the manual. Dwell is all that
matters, not a pretty shine on point faces.
A few
words about condensors (capacitors):
New
capacitors are likely the cheap Chinese version. Avoid the ones that
have a black rubber cone at one end where wire is. They may work for
a few years before internal corrosion causes problems. You want the
older Mopar capacitors which are hermetically sealed. They have a
copper strap instead of a wire. The best of them may even be oil
filled. Use these when you can.
You don't
have to use capacitors that fit inside the distributor. I have used a
Panasonic radio film capacitor (2025 tech, bulletproof plastic film)
mounted outside the distributor. I used a .27 uF 1000 v or slightly
higher. See the Mouser catalog.
What the
capacitor does are two very important things;
1) It
holds volts near zero as points first open, for the first 10
millionth of a second. Without it, points would strike an arc right
then and waste all your spark energy melting the points.
2 ) Once
the points open, the capacitor helps the magnetic field inside the
coil to collapse FAST by pulling current out of the 12v winding. Coil
output (E) is change of current over time {E = di/dt}. In other
words, the rate of change of current is volts. If you had no
capacitor, you get 10,000 Volts instead of 60,000. With a failed or
failing capacitor, the car won't start or run well. Add in the
frustrating complication of a capacitor that works now and then, and
you chase your tail trying to figure why the car runs then doesn't
then does. Although you can test capacitors, I have found that is not
reliable. Those cheap Chinese caps can test OK one day and not the
next.
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