Points
(Ignition)
Also called breaker points or a contact breaker, a type of
electrical switch found in the distributor of older vehicles equipped with a mechanical
ignition system. The points serve to interrupt
the flow of current in the primary circuit of the ignition coil. The collapsing current in the primary
circuit induces a high voltage in the secondary circuit of
the coil, which causes a sufficiently high voltage (20,000
volts or more) to appear at the coil output to enable arcing
across the electrodes of a spark plug.
The points open and close as the distributor shaft rotates.
Half of the point set is fixed, while the other half is able
to pivot. On the moveable half of the point set is a rubbing
block. The distributor shaft has lobes that contact the rubbing
block and act as cams to push the points open, thereby breaking
the electrical connection between the points. A spring clip
causes the moveable point to snap back into contact with the
stationary point mounted on the distributor plate when the
cam rotates out of contact.
The spring clip is electrically insulated from the distributor
body so that the primary circuit is grounded only when the
points are closed. When the points are touching each other
current flows from the battery, through the coil and to the
engine block, which is grounded to the negative terminal on
the battery. The current running through the windings in the
ignition coil builds up a powerful electrical field that is
unleashed when the points separate. No longer able to go to
ground through the points, the electricity, seeking the easiest
path to ground, travels through the secondary circuit to the
coil wire to the top of the distributor cap where it is transmitted
to the distributor rotor and then on to a spark plug.
Maintenance of points
Since they open and close several times every turn of the
engine, contact breaker points are subject to a lot of wear
and need to be checked regularly for optimal engine performance.
The gap between the points (the distance between them at their
widest open setting) can be checked with a feeler gauge. The
gap specification can be looked up in a repair manual for
the particular vehicle. Typical settings are in the range
0.015- to 0.020-inch. The feeler gauge should just slide between
the points without pushing them apart. If the point set is
worn, it should be replaced.
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