This ignition system uses an ignition coil wound as an autotransformer to step up the voltage from the 12 volts supplied by the battery. When the points close, current flows through the primary side of the coil, which has a few hundred turns of wire, creating a magnetic field. When the points open, interrupting the current, this field collapses and as it does the lines of flux cut across the secondary coil which has several thousand turns of very fine wire, inducing an open-circuit voltage in the range of 15 to 30 thousand volts at the coil outputterminal. This high voltage pulse is carried by the coil wire to the center terminal of the distributor cap.
Capacitor
There is a capacitor connected across the points. If the capacitor were missing or defective, the primary current would arc across the opening points allowing the current and therefore the magnetic field to collapse slowly causing a weak or no spark. When the points begin to open the primary current is diverted to charge the capacitor, checking the rate at which the primary voltage rises and limiting the arcing.
Distributor
The distributor rotor connects the high voltage to one of the outer terminals around the cap where a spark plug wire carries it to a spark plug. When the voltage is high enough it jumps the gap of the spark plug and ignites the fuel air mixture in the cylinder's combustion chamber.
Problems
One problem with this design is that, even with a properly sized capacitor, there will be some arcing at the points. Arcing causes the points to "burn." This in turn introduces resistance at the point contacts that reduces primary current and resulting spark intensity. A second problem involves the mechanical cam-follower block that rides on the distributor cam and opens the points. The block wears over time, reducing how much the points open and causing a corresponding change in both the ignition timing and the fraction of time during which the points are closed. Tune-ups for older vehicles usually involve replacing the points and condenser and setting the gap to factory specifications. A third problem involves the distributor cap and rotor. These components can develop conductive "sneak paths" on their surfaces across which the coil's secondary voltage produces a current, often in the form of an arc, that bypasses the spark plug. When sneak paths develop, the only remedy is replacement of the cap and/or rotor. A fourth problem can arise when one or more of an engine's spark plugs becomes "fouled." Fouling, caused by combustion-byproducts that form deposits on a spark plug's internal insulator, creates an electrically conductive path that dissipates the coil's energy before its secondary voltage can rise high enough to produce a spark. So-called capacitive discharge ignition systems create coil voltages with much shorter rise times and can produce a spark across spark plugs with some fouling. Electronic ignition systems replace some or all of the components the Delco ignition system with solid state and/or optical devices and provide both higher voltages and more reliable ignition.
