Understanding Breaker Point Ignition Systems
Breaker point ignition system basics – knowing how the system works is key to keeping it working right
Figure 1: Ignition points must lineup correctly when they are closed. If they don't shut (left), or aren't aligned (center), the system won't work.
Breaker point ignition systems were, until the advent of electronic ignition systems, used on millions of engines. From the engines powering rum runners of the 1930s to all those Jeeps in World War II, all of them had breaker point ignition systems. Simple to troubleshoot and repair, they are, like anything else, infinitely complex if you don't understand the basics of how they work.
Breaker Point Basics
The breaker point ignition system circuit starts and ends with the battery. When the engine is running the battery is continuously being recharged by an alternator or, on older systems, a generator. Current flows from the positive terminal of the battery to the ignition switch and an ignition coil. The ignition coil is really a transformer that steps up the 12-volt current of the battery to somewhere in the neighborhood of 25,000 volts. In engines of medium to high compression this kind of voltage is necessary to reliably arc across the gap on a spark plug and make enough fire to ignite the fuel/air mixture in a cylinder.
The coil has two circuits in it; the primary, which runs from the positive coil terminal to the negative coil terminal; and the secondary circuit, which goes from the positive terminal on the coil to the ignition wire in the center of the distributor cap. The negative wire on the primary circuit runs from the coil to the base of the distributor and the breaker points inside. This may sound a little confusing, but it makes sense when you understand that the points act to open and close the ground circuit.
The breaker points open and close as the distributor shaft rotates. One half of the point set is fixed, the other half pivots and there is a rubbing block on the moveable half of the point set. The distributor shaft has lobes that contact the rubbing block. These lobes act as cams to push the points open, thereby breaking the electrical connection between the points. The points have a spring clip that acts to hold the points shut, and this spring causes the moveable point to snap back into contact with the stationary point mounted to the distributor plate when the cam rotates out of contact. If this is unclear, take the distributor cap off of a breaker point-equipped engine and rotate the engine manually, watching the parts move. The interplay will become obvious.
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 electricity runs 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, which is seeking the easiest path to ground, rushes through the secondary circuit to the coil wire to the top of the distributor cap where it is transmitted to the distributor rotor.
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