[TR] '3 ignition revisited..

Jeff Scarbrough fishplate at gmail.com
Tue Jul 17 17:56:41 MDT 2018

On Sun, Jul 15, 2018 at 12:10 AM, Carl Sereda <carlsereda at aol.com> wrote:
> it might help you see what's going on with your testing - seems to me you
> should be getting 12v whether points are open or not..

> https://www.google.com/search?q=ignition+coil+wiring+diagram&client=firefox-b-1&tbm=isch&source=iu&ictx=1&fir=_KDMhwaFko-lLM%253A%252CTfSYecqNPNEi0M%252C_&usg=__rjFhOK9M5qVKracklvyd8TG4EQo%3D&sa=X&ved=2ahUKEwj3z4yRnKDcAhXjqFkKHQvBD5MQ9QEwCXoECAUQFg#imgrc=yhdSXogp_qHvwM:

OK, I'm going to jump on Carl's post and give a physics lesson...but
don't be afraid!

Your ignition works in the very same way that your electric power
utility distributes power to your house - specifically, think of that
transformer out there on the pole or pad.  It's just a little

Looking at the referenced diagram, you have to envision the coil a
little differently:  There's a picture of it on this page, as well as
a fun little interactive graphic

Just as the transformer takes high voltage and, through the magic of
inductance, makes low voltage for your house, your automotive coil
takes low voltage from the battery and makes high voltage for a spark.
The transformer uses the constantly varying voltage of Alternating
Current to produce the transformation, but the car is Direct Current,
so it takes another tool: a switch, in the form of points.

When the points are closed, the current flows through the coil and
through the points to ground.  (It may actually be the holes that flow
that way, and the current flows the other way, but let''s not get off
track here).  When the current flows through the coil's Primary coil,
it creates a magnetic field that induces a current in the Secondary
coil (that's the one that's connected to the spark plug).

When the points open, that magnetic field collapses.  This causes the
Secondary coil to make a huge current in the High Tension wire, which
rushes out and across the spark plug gap on its way to ground.

The condenser is just a capacitor: ahandy device to keep the points
from burning up.  If you've ever opened a high-current switch, you can
see an arc across the switch contacts - anything from a little blip to
a four-foot arc, depending on the size of the switch and the current.
The condenser, being connected in parallel to the points with one end
grounded, provides an alternate path for that energy that has a lower
resistance, so instead of an arc burning the points, it charges the
capacitor.  The capacitor gets fully charged eventually, and won't
accept any more current, but then the points close and return both
sides of the capacitor to the same voltage, so it discharges and is
ready the next time the points open.

So, how do we test the ignition?

First, I'd check the power to the coil disconnect the power wire from
the coil, and switch on the ignition.  Then check the voltage between
the power wire and ground.  You should have 12 Volts (or 6 Volts,
depending on the ignition system).

If you have voltage there, switch off, then connect eh power wire back
to the coil, and disconnect the wire to the points.  Switch on, and
check the voltage on the coil terminal that had the points wire.
Again, you should have same voltage as in the first step.  Switch off.

Now, reconnect the points and disconnect the condenser.  Rotate the
engine until the points are closed.  Then switch the ignition on.
Read the voltage at the terminal connected to the points.  It should
read zero Volts, because the terminal is connected to ground through
the points.With an insulated tool (like a bamboo skewer), open the
points - you should see the voltage jump up to the previous reading,
and the system should produce a spark - make sure you know where it's
going to go, or it might jump on you!

If opening and closing the points doesn't affect the voltage, then
that's where your problem likely lies.  If it works fine, then I'd
replace the condenser with a new one and see what happens.

If none of that works, come back here and we'll dig a little deeper...

Jeff Scarbrough
Corrosion Acres, Ga.

More information about the Triumphs mailing list