Basic Ignition System Theory of Operation (How it Works)
The first ignition systems used a mechanical switch run by a cam on the distributor shaft to turn current flow through the ignition coil on and off. The switch was called "breaker points" or "contact points" because we were concerned with the operation and adjustment of the switch contacts that broke the circuit. You may never run into one of these systems unless you're working on a classic car from before the mid 1970s, but we'll discuss them here for two reasons. First of all, should you be asked to work on one of these systems, you should at least have an understanding of how to diagnose and service them. Second, the operation of the breaker points is instrumental in understanding how the electronic ignition systems that came later work. The next page will get into those Electronic Ignition Systems. We have enough to keep us busy with learning how the ignition coil works without complicating it right away with a mysterious electronic control module. For a more basic explanation of how the ignition coil develops its high output voltage, and for a some background history on some early electronic ignition systems, see the "Ignition System Theory of Operation (What it Does)" page under "More Circuits".
Breaker Points
Figure 1 shows a drawing of a set of breaker points that sit on a plate in the distributor, under the cap and rotor. The cam, (shown in yellow), has eight high spots that correspond to the cylinders in a V-8 engine. Four and six-cylinder engines have a matching number of high spots on their cams too.
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The plastic "rubbing block", (orange triangle), rides on the cam. New points come with a small vial of special grease to lubricate the rubbing block and reduce wear. That grease resists being thrown off from centrifugal force. The rubbing block is shown here nearing the tip of a high point so it is pushing the spring-loaded arm up and the contacts are open.
The black lower contact is adjustable. The "point gap" between the two contacts must be adjusted very precisely to the manufacturer's specification. |
 Figure 1. Basic set of breaker points. |
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In Figure 2 the cam has turned and the rubbing block is on a low spot. The contacts are closed, and they're holding the rubbing block off the cam. |
 Figure 2. Points closed. |
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The entire ignition system circuit is shown in Figure 3. All that has been added are the ignition coil and an ignition resistor, often called the "ballast resistor". It's job is to limit current flow to prevent overheating the contact points. The two lines between the primary and secondary windings in the coil indicate they're connected by an iron core. That concentrates the magnetic field and greatly increases its efficiency. |
 Figure 3. Ignition system circuit. |
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The points are closed, there is a complete circuit, and current flows through the resistor, ignition coil, and breaker points to ground. That current path is shown in red. Even though the coil is grounded the instant the points close, current takes a little time to reach its highest value because it takes energy to make the magnetic field build up in the coil's primary winding, and that building field opposes the current flow from the battery. |
 Figure 4. Current flow through the ignition coil and points. |
The magnetic field likes to maintain a constant strength, but as it nears its maximum strength, the rate of increase slows down so there is less opposition to current flow. Even though the field reaches its maximum strength in a few milliseconds, the fact that it does take some time means the movement of the lines of force, shown in green, cuts across the secondary winding relatively slowly. We know from generator theory that it takes a magnetic field, a coil of wire, and movement between the two to induce a voltage into the coil. The secondary winding is the coil of wire. The building of the magnetic field is what creates the movement.
The difference though between the ignition coil and the generator is in the generator there are very few loops of wire in the stator to induce voltage into so the output is limited to around 14 volts. In the ignition coil, the secondary winding has thousands of loops, and a little voltage is induced into each one. Just as a generator is less efficient at low speeds and produces less voltage, the ignition coil is less efficient when the magnetic lines of force build and cut across the secondary winding slowly. A typical coil might produce about 300 volts from the secondary winding. That's not nearly enough to jump a spark plug's gap.
The difference though between the ignition coil and the generator is in the generator there are very few loops of wire in the stator to induce voltage into so the output is limited to around 14 volts. In the ignition coil, the secondary winding has thousands of loops, and a little voltage is induced into each one. Just as a generator is less efficient at low speeds and produces less voltage, the ignition coil is less efficient when the magnetic lines of force build and cut across the secondary winding slowly. A typical coil might produce about 300 volts from the secondary winding. That's not nearly enough to jump a spark plug's gap.
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Here's where the fun starts. The points have opened and current flow stops. Unlike when the magnetic field built up slowly and current flow reached maximum over a short period of time, here the open circuit forces current flow to stop instantly. The magnetic field collapses very rapidly. Since the magnetic lines of force are moving very fast, a very high voltage is induced in the secondary winding. Some coils can develop over 45,000 volts. |
 Figure 5. Magnetic field collapses instantly. |
Adjusting the Points
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Most do-it-yourselfers set the point gap with a feeler gauge. The engine is turned slowly by hand until the rubbing block is on the highest point. Loosen the screw on the lower movable contact, then slide it in the direction necessary to create a slight drag on the feeler gauge. Tighten the screw, then recheck for the proper drag on the feeler gauge. |
 Figure 6. Point gap. |
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Professionals adjusted the points more precisely with a "dwell meter". One complete revolution of the distributor shaft and cam is 360 degrees. The cam shown in Figure 7 has eight lobes and is for an eight-cylinder engine. That means 45 degrees is devoted to each cylinder. Within that 45 degrees of rotation, the points will be closed roughly half of the time and open half of the time. The time they are closed, represented by the dark yellow "pie slice", is the dwell angle. That is when current is flowing through the ignition coil primary winding and the magnetic field is building up. Since it takes time for that field to build, dwell time must be long enough to build a field strong enough to induce sufficient voltage into the secondary winding.
Dwell is adjusted by adjusting the point gap, but a dwell meter is much more precise than a feeler gauge. Most GM distributors had a small metal plate that could be raised on the side of the distributor cap to allow access for the Allen wrench used to make that adjustment while the engine was running. All other manufacturers required stopping the engine, removing the distributor cap, loosening the movable contact to make the adjustment, then reassembling everything to run the engine and remeasure dwell. That was repeated over and over until dwell was set satisfactorily. |
 Figure 7. Dwell angle. |
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Dwell Meters
There are many combination meters on the market that can measure dwell, voltage, and rpm, but we're only concerned with the dwell meter function here. Dwell meters respond to voltage but they do not measure voltage. They measure the ratio of time the points are open to the time they're closed which corresponds to the number of degrees of cam rotation the points are open to the time they're closed. For the eight cylinder engine, the number of degrees is displayed on a scale from 0 to 45 degrees, shown in red in Figure 8. Here the points are open so 12 volts will be found all the way to the upper contact point, shown in yellow. The dwell meter is connected to ground and to the negative terminal on the ignition coil. The 12 volts makes the meter read full scale. Older dwell meters often had an adjustment to set the pointer to the "set" mark near full scale to compensate for variations in the cars' system (battery) voltage. |
 Figure 8. Points are open. |
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When the points are closed, as in Figure 9, the coil's negative terminal is grounded and the dwell meter reads to the left. This is when current flows through the coil and the magnetic field builds. As the point gap increases, dwell time decreases. By setting either one to manufacturer's specifications, that will insure dwell time is sufficient for the coil to build a magnetic field of the strength required. |
 Figure 9. Points are closed. |
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Now that the engine is running in Figure 10 and the points are opening and closing rapidly, 30 degrees of dwell has been measured. That means that for the 45 degrees of rotation for each cylinder, the points are open only 15 degrees and closed 30 degrees. The amount of time they're open isn't critical because very little time is needed for the coil's magnetic field to collapse, but the amount of time they're closed is important because enough time is needed to build that field which takes much longer.
The typical dwell specification for many V-8 engines was 32 to 34 degrees. To increase that from the 30 degrees shown here, the point gap would have to be decreased very slightly. |
 Figure 10. 30 degrees of dwell. |
"D I T" (Dwell, Idle,Timing)
When performing a tune-up or just making a few adjustments, point gap, (dwell), must be adjusted first. Changing the point gap affects when the cam will hit the rubbing block and push the contacts open. When the contacts open is when the ignition coil fires a spark plug, so changing point gap changes timing. Adjusting timing is done later by turning the distributor and that has no affect on point gap. That's why it's done last.
Since changing point gap changes timing, it is going to affect the idle speed of the engine so again, dwell, (point gap), is adjusted first. Idle speed is set next, ahead of timing because it's possible for the mechanical advance system to come into play if engine speed is too fast. There's no way to defeat that advance while setting base timing. The vacuum advance should not be a factor yet if ported vacuum is used, but it can be easily defeated by disconnecting the vacuum hose and plugging the port. The hose is typically disconnected at the distributor and plugged. That is a standard part of the procedure for almost all manufacturers and is usually spelled out in the service manuals.
Idle speed and ignition timing won't affect point gap, so that's done. Small changes of a few degrees in timing won't have a noticeable affect on idle speed, but if big adjustments are needed, such as after the distributor was removed, you can expect the engine speed to change as timing is changed. It will be necessary to readjust idle speed, then recheck timing until both are set to specifications.
Problems with Breaker Points
Point Bounce
Breaker points were all we had up to the mid 1970s along with their problems. Point bounce, point pitting, and rubbing block wear were the most common. "Point bounce" is when the contact arm and movable contact bounce back up after making contact and open the circuit much too soon to fire the next cylinder. Often there is no problem noticed because the magnetic field hasn't had time to build again sufficiently to induce enough secondary voltage to fire the spark plug again, and if it did, the fuel in that cylinder is already burning. The problem comes in when the rotor has turned far enough to send the additional spark to the next cylinder. That piston is still on its way up on the compression stroke and igniting the fuel too soon can have catastrophic results. See the section on "Spark Plug Wire Routing" for a related problem. Point bounce becomes more of a problem when the movable arm's spring becomes weak, but a stronger spring leads to increased rubbing block wear. This is also more likely to occur at high speeds when the movable contact is setting down on the fixed contact very rapidly. More on the solution in the "Dual Breaker Points" section.
Pitted Contacts
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Pitting of the points was another very common problem. The arcing from electrons jumping the gap just as the contacts separate would remove metal from one contact and deposit it on the other one. Some metal was also burned away. That left uneven surfaces on each contact that had to match up to form a solid electrical connection. Figure 11 shows an expanded view of that pitting.
A part of regular maintenance included filing the contacts to remove the pitting and to insure the contact surfaces were perfectly parallel so they'd make a good electrical connection. Once filed, the gap would need to be readjusted. Performing this service every 10,000 to 15,000 miles was considered normal. If ignored long enough, some of the deposited metal could slide alongside a hole in the other contact and continue to make an electrical connection long after the contacts had separated. The spark would occur much too late. The rotor tip would have moved away from the terminal in the distributor cap too, so the increased gap there had to be jumped by the spark along with the spark plug's gap. If the coil didn't have the capacity to develop enough voltage to jump both gaps, a misfire would result. |
 Figure 11. Pitted contacts. |
Rubbing Block Wear
Rubbing block wear wasn't so much a problem as just a fact of life that had to be dealt with. The instant a new set of points was put into service, the rubbing block began to wear down. That gradually reduced the point gap and increased the dwell angle. The cam had to rotate further before the contacts would open so ignition timing changed. Timing was late which reduced power but it also had an adverse effect on emissions. The problem of gradually-increasing emissions is what spurred the development of electronic ignition systems with no moving parts to wear. More on that in "Advances in Ignition Systems". To keep the rubbing block wear to a minimum, always apply the little packet of grease supplied with new points to the cam when they are installed.
Point Float
"Float" is more commonly associated with intake and exhaust valves not closing fast enough at high engine speeds. Stronger valve springs are used in applications where that is a problem, such as with racing engines. Point float is similar in that it occurs at high speeds when the momentum of flinging the movable contact arm open prevents it from closing in time. The rubbing block pounds down onto the cam rather than riding smoothly on it. All kinds of misfires will occur which prevents engine speed from increasing any further. We normally suspect valve float first, but the clue is hydraulic lifters will tend to pump up when the valves are floating. When engine speed comes back down, misfires will continue for perhaps as long as 15 seconds until the lifters gradually bleed back down. Only the one or two cylinders with the weakest valve springs are affected, then it's nearly impossible for the engine to pick up even more speed.
When point float is the problem, all cylinders are affected, you'll likely hear popping and backfiring, and the problem will clear up as soon as engine speed comes down.
Timing Advance
Mechanical and vacuum timing advance systems are necessary and not exactly a problem, but as discussed in "Basic Ignition System Theory of Operation (What it Does)", only two operating conditions are addressed. Those are engine speed and engine load. The first electronic ignition systems used the same two timing advance systems. It wasn't until "Computer-Controlled Ignition Systems" came along in the mid 1970s that ignition timing could be fine tuned for many more operating parameters.
Dwell Time
We've discussed dwell angle and it can be seen that it stays constant with changes in engine speed, but that is not true about dwell time. For simplicity, the two waveforms in Figure 12 show a 50 percent "duty cycle" meaning 12 volts is only present half of the time. This correlates to the breaker points being open half the time. On a V-8 engine with 45 degrees of cam rotation per cylinder, this would equate to half of that, or 22.5 degrees of dwell angle. That is true at 800 rpm, 2000 rpm, and any other speed. What is different though is dwell time. 0 volts is found at the ignition coil negative terminal when the points are closed, (circled in red). That is the time when the magnetic field is building in the coil's primary winding.
Rubbing block wear wasn't so much a problem as just a fact of life that had to be dealt with. The instant a new set of points was put into service, the rubbing block began to wear down. That gradually reduced the point gap and increased the dwell angle. The cam had to rotate further before the contacts would open so ignition timing changed. Timing was late which reduced power but it also had an adverse effect on emissions. The problem of gradually-increasing emissions is what spurred the development of electronic ignition systems with no moving parts to wear. More on that in "Advances in Ignition Systems". To keep the rubbing block wear to a minimum, always apply the little packet of grease supplied with new points to the cam when they are installed.
Point Float
"Float" is more commonly associated with intake and exhaust valves not closing fast enough at high engine speeds. Stronger valve springs are used in applications where that is a problem, such as with racing engines. Point float is similar in that it occurs at high speeds when the momentum of flinging the movable contact arm open prevents it from closing in time. The rubbing block pounds down onto the cam rather than riding smoothly on it. All kinds of misfires will occur which prevents engine speed from increasing any further. We normally suspect valve float first, but the clue is hydraulic lifters will tend to pump up when the valves are floating. When engine speed comes back down, misfires will continue for perhaps as long as 15 seconds until the lifters gradually bleed back down. Only the one or two cylinders with the weakest valve springs are affected, then it's nearly impossible for the engine to pick up even more speed.
When point float is the problem, all cylinders are affected, you'll likely hear popping and backfiring, and the problem will clear up as soon as engine speed comes down.
Timing Advance
Mechanical and vacuum timing advance systems are necessary and not exactly a problem, but as discussed in "Basic Ignition System Theory of Operation (What it Does)", only two operating conditions are addressed. Those are engine speed and engine load. The first electronic ignition systems used the same two timing advance systems. It wasn't until "Computer-Controlled Ignition Systems" came along in the mid 1970s that ignition timing could be fine tuned for many more operating parameters.
Dwell Time
We've discussed dwell angle and it can be seen that it stays constant with changes in engine speed, but that is not true about dwell time. For simplicity, the two waveforms in Figure 12 show a 50 percent "duty cycle" meaning 12 volts is only present half of the time. This correlates to the breaker points being open half the time. On a V-8 engine with 45 degrees of cam rotation per cylinder, this would equate to half of that, or 22.5 degrees of dwell angle. That is true at 800 rpm, 2000 rpm, and any other speed. What is different though is dwell time. 0 volts is found at the ignition coil negative terminal when the points are closed, (circled in red). That is the time when the magnetic field is building in the coil's primary winding.
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At 2000 rpm the ratio of on-time to off-time is the same as at 800 rpm but the amount of on-time, (dwell time), is significantly shorter. That gives the magnetic field less time to build up. As long as it has reached its maximum strength before the points open, no adverse change in secondary output will occur. But, . . . as engine speed continues to increase, the dwell time gets shorter and shorter until the point is reached when the points are opening before the magnetic field has had time to build sufficiently. Weak spark and misfires will occur. The effects of increased engine speed can be reduced by decreasing the point gap to increase dwell angle. That will increase dwell time at every engine speed. There's a limit though to how much the gap can be reduced because wear on the rubbing block could suddenly prevent the contacts from opening at all. There's a better way to address the reduced dwell time. |
 Figure 12. Dwell time. |
 Figure 13. Dual Points. |
Dual Point Distributors
The answer to some of these problems was the dual point distributor. The two sets of breaker points were wired together and either one of them could run the engine independently, but combining them had some advantages. The point gap specification remained approximately the same as with a single point system, but the second set was rotated slightly to make it open and close a little later than the first set. By overlapping their on-times that way, the overall angle increased to perhaps as much as 40 degrees for a V-8 engine, and the dwell time increased at all speeds. That increased dwell time allowed more time for the coil to build its magnetic field where it had been a problem at very high speeds. The effects of point bounce were reduced because when the circuit was broken too early due to that, the other set of contacts was still grounding the coil. Rubbing block wear, point float, and pitting were still problems that needed to be solved. That was done with electronic ignition systems. Adjusting dual point distributors was a little more involved than before. The gap could be set just like before, but to set the dwell angle one set of contacts needed to be blocked open while the angle was measured and adjusted for the other set. Both sets of points had to be adjusted separately, then the ignition timing could be adjusted. |
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All of these problems will be handled on the next page, "Advances in Ignition Systems", (coming soon).
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