Understanding Breaker Point Ignition Systems
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.
The rotor is fixed to the top of the distributor shaft and revolves around the inside of the distributor, its contact giving each post on the circumference of the distributor cap a shot of electricity as it passes by. The posts have wires attached to them and these lead to the spark plugs that ignite the fuel/air mixture in the cylinder.
The spark has to be timed so that it sets off the gas at the right part of the piston cycle, usually when the piston is near the top of the cylinder. On most engines ignition timing is set by loosening a distributor hold down bolt and rotating the distributor to advance or retard the spark timing. Old-time garage jockeys used to set timing by ear, turning the distributor until the engine sounded “right.” Most mechanics use a timing light that takes a signal from a spark plug wire and releases a pulse of light every time the spark plug wire has electricity running through it. The light is aimed at one of the pulleys on the front of the engine and the distributor is rotated until a notch on the pulley lines up with a mark on the pulley guard.
Knowing how a breaker point system works puts you in a much better position to repair it when it breaks down. If your engine isn’t running and you suspect the ignition system, the first thing to do is look the whole thing over for anything that is obviously wrong, like loose or broken wires.
Push down hard on the boots at the end of the spark plug wires to make sure that they are on securely. Take a glance at the points; if they look corroded replace them. Check the gap between the points (the space formed when the points are at their widest open setting) with a feeler gauge, getting the proper specification for the gap from a repair manual. A typical setting is 0.015- to 0.020-inch. Use a socket and a breaker bar to rotate the engine so that the points are at their widest gap. The gauge, either a wire type or feeler type, should just slide between the points without pushing them apart.
If this doesn’t solve the problem, try tracing the entire circuit, starting with the battery. Test the battery with a voltmeter and a hydrometer. You want the battery to read at least 12.6 volts, assuming you have a 12-volt system. If the battery needs to be recharged make sure to use a trickle charger, a charger that runs at no more than 2 amps. High amp chargers can ruin a battery if used often,, something I had to learn the hard way.
Double-check the battery with a hydrometer.
Be sure to wear splash-proof goggles. Each cell should read almost the same as the others. If you get a drastically different reading in one cell you may have a bad battery.
Use the voltmeter to take a reading at the ends of the battery cables. The voltage should be the same as at the battery itself. If not, clean the ends of the cables and try again. If you still get a voltage drop at the end of the cables, junk them and get new cables. While you are at it, try wiggling the cables around with the voltmeter probes firmly attached. If you see a low or nonexistent reading you have a cable that is corroded on the inside.
Assuming you have a good battery that is fully charged, good battery cables and clean, tight connections you can start testing other parts of the circuit. Place the positive probe of the test meter on the positive battery terminal and the negative probe on a clean part of the engine block. This tests the ground connection between the negative battery terminal and the block. If the voltmeter reads lower than the battery you need to clean and/or tighten the ground connection.
You can go through the whole circuit testing the voltage at each wire and component. If you find a significant drop in voltage, stop to check for a bad connection or wire. Some engines have an external resistor near the ignition coil. This will affect the voltage reading you get according to the strength of the resistor.
You can check the resistor with an ohmmeter. Get the resistance of the resistor from the manual for your engine (some resistors may have their ohm rating marked on them). The coil can be checked the same way.
Use your voltmeter to check for a short to ground between the battery and the points. Block the points open with a small piece of wood and put one probe on the appropriate battery terminal and the other probe on the point itself. Just make sure that you have your polarities straight. With the points blocked open one will be positive, the other negative. If the meter doesn’t show any voltage when the probe is on the “spring clip,” you may have a bad insulating washer at the distributor, which would let electricity go to ground through the block before going to the points. Test for continuity between the block and the negative coil terminal to confirm this theory. Test for continuity between the block and the stationary point attached to the distributor plate.
Rotate the engine until the points are closed. Use the multi-meter to test for a good connection between the points. A slight gap when the points are supposed to be closed will keep your machine from running.
If you do not have a test meter you can use a self-powered test light to do essentially the same thing. Always use the test light with the battery disconnected. When the circuit is closed the light will shine. If you have a fault in the circuit, such as a broken wire, the light will not come on.
Starting at the battery cables go around the circuit testing each wire and connection. Block the points open and place each probe on one of the points. If the light goes on you have found the problem. Look carefully to find the bare patch of insulation or missing rubber washer on the distributor wire.
With the points touching and the probes on each point the light should shine for you. If the light doesn’t go on they aren’t really touching or they are so corroded that they won’t conduct electricity. You can file them down, or better yet replace them. It is a good idea to replace the point condenser at the same time. The condenser is usually located inside the distributor, but sometimes is attached to the outside housing. It has a single lead that attaches to the points where the negative wire from the ignition coil attaches.
If you’re still not having any luck, try checking the resistance of the spark plug wires. I know a lot of us hate manuals, but it’s good to have the specs for your piece of iron so you can test these things out. Any spark plugs wires with cracked insulation should be replaced.
Use your multi-meter to test the spark plugs. There should be continuity between the top of the plug and the electrode. There shouldn’t be any continuity between the screw threads and the electrode. Put the end of the plug in the boot on the end of the ignition wire and test for continuity between the electrode and the end of the plug wire. This will rule out a bad plug wire or a bad connection between plug and wire.
If you have gone through all of this and still can’t get any spark take a look at the cap and rotor. These are usually the first parts that get replaced when someone is going over the ignition system. If they look old or damaged, I would replace them.
After going through all of this you should have a good feel for how your breaker point ignition system works, and how to work your way through when you’re having ignition-related problems. Understanding how the system works is key, and if you take your time and trace the system down, you’ll always find your way to making it work.
Contact engine enthusiast Gary Grinnell at: 9 Laurel Park, Northampton, MA 01060-1196.
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