Ignition System Basics
The four stroke gasoline engine relies upon an ignition system that can deliver an electric current across the spark plug gap. Air and fuel is drawn into a cylinder during the intake stroke. This mixture is then compressed to raise the temperature of the air fuel mixture. As the temperature increases, the fuel is vaporized. When the spark occurs, the fuel begins to burn. This drastically raises the pressure inside the cylinder and forces the piston down into the power stroke.
The purpose of an electric current across the spark plug gap is to generate enough heat to ensure all the fuel burns within the cylinder. To make a good spark the ignition coil must develop enough power, and the insulation of the secondary ignition system must keep the electric current flowing across the spark plug gap. The ignition system must also deliver that spark to the correct cylinder at the correct time.
A high electrical pressure or voltage is needed to push the electrons across an air/fuel gap at the spark plug. Most automobiles use a 12 volt battery. The job of the ignition coil is to raise battery voltage high enough to push electrons across the spark plug gap. The low voltage found in the battery is called primary voltage, and the high voltage available to the spark plug is called secondary voltage. Once electrons have jumped the spark plug gap, the coil must produce sufficient power to keep the spark going for about 1ms to 1.5mS. (1mS is one thousandth of a second)
How to Make Sparks
All ignition systems use the same basic steps to raise battery (primary) voltage up to the several thousand (secondary) volts required to jump the spark plug gap. A good understanding of both primary and secondary ignition theory will make it easier to diagnose the variety of ignition systems found on gasoline engines. Remember that a weak or missing spark can be caused by a failure in either the primary or the secondary ignition system.
Ignition coils consisting of one primary winding, and one secondary coil windings. An ignition coil transforms the electric power of the primary winding (low volts and high amps) into the electric power of the secondary winding (high volts and low amps). The more power we flow through the primary windings, the more power we can get out of the secondary windings.
How Primary Ignition Works
The primary coil is made by winding wire into several hundred loops. As electric current flows through a wire it creates a magnetic field. Changing the amount of current flowing through a wire will change the strength of the magnetic field around that wire. As current begins to flow through one loop of an electric coil, the expansion of the magnetic field in that loop will create a Counter ElectroMotive Force (CEMF) in the adjacent loops of the coil. The action of CEMF on a coiled wire is called Inductive Reactance This reactance will Impede or slow down any increase in current flow through any coil. As a result it takes between 2 to 6 milliseconds for the primary coil winding to reach it's maximum current flow and magnetic field strength.
How long the primary coil is turned ON is called Dwell. Once the maximum current is reached, the coil will be at it's maximum power potential (strongest magnetic field). This is called coil saturation. Short Dwell times will not give enough time for the coil to become saturated and a weak spark will result. Dwell times that are too long will not increase the magnetic field, but it will raise the temperature in the coil windings. An ignition system must turn on the primary coil long enough to reach maximum power, but not so long as to overheat the coil.
The primary ignition system must turn ON for the proper dwell, and then turn OFF current flow at the proper time for spark to occur in each cylinder. The instant primary current flow stops is called ignition timing. Ignition timing must advance and retard to match engine speed and load. There are many ways to control dwell and timing. Older systems use ignition points that close and open to turn on and off current to the primary coil windings. Dwell (ON time) was set by the size of the point gap and timing (signal to turn OFF) was controlled by mechanical and vacuum advance units.
Modern systems use a power transistor to turn ON and OFF the primary coil windings. This transistor is part of the Ignition Control Module. It might be a separate unit (often called an Igniter or ICM) or part of the Powertrain Control Module (PCM). Proper dwell and timing require sensors that monitor engine RPM, crankshaft and/or camshaft position, engine load, engine temperature, and a knock sensor. Most engines use a Crankshaft Position Sensor (CKP) to monitor RPM and indicate which pistons are going up or down. Camshaft Position sensors (CMP) will indicate if the piston is on compression or exhaust. It can also be used to monitor RPM. Manifold Absolute Pressure (MAP) sensors in combination with Throttle Position sensors (TPS) will monitor engine load however most vehicles use Mass Airflow sensors (MAF) to monitor engine load. The Engine Coolant Temperature (ECT) and Intake Air Temperature (IAT) will also be used when calculating spark timing. Finally most systems use a Knock Sensor (KS). The knock sensor is a feedback sensor. Ignition Timing will be calculated based on the RPM, Engine Load and Temperature. If the timing is too advanced (happens too early) the cylinder will "spark knock" or "Ping". When a ping or knock occurs the knock sensor signal will retard the timing until the knock goes away.
How Secondary Ignition Works
All ignition coils are transformers that step up the 12 - 15 volts at the battery to several thousand volts required for the spark plug. The primary winding of the coil is in series with the battery and the secondary winding is in series with the spark plug(s). The primary winding gets it's electrical power from the battery while the secondary winding gets electrical power from the collapsing magnetic field generated by the primary windings.
Electrical power is generated any time you move a magnetic field past a coil of wire. When the primary circuit is turned ON a magnetic field slowly builds in the primary windings of the coil. The secondary windings are coiled close to the primary windings and are made up of several thousand turns of wire. When current is turned OFF to the primary windings, the magnetic field collapsed very quickly. As this magnetic field moves quickly past the several thousand turns of the secondary coil windings, a very high voltage is generated. This high voltage (electrical pressure) forces electrical current (amps) out of one end of the coil, through the plug wires, across the spark plug gap, and back to the other end of the secondary coil windings. Current will continue to flow through the coil, wires, and spark plugs, until all the electrical energy created by the ignition coil is used up.
A modern ignition coil must create enough power to maintain current flow across the spark plug gap for between 1 and 1.5 milliseconds. You can easily measure the milliseconds of current flow with an oscilloscope. Any defect in the primary or secondary ignition circuits will lower the time that current flows across the spark plug gap. If secondary voltage is tool low or current flow is too short (low power) the cylinder will misfire.
What are the secondary ignition system components? This depends upon the type of ignition system. Older engines use DI or Distributor Ignition systems. When you have a distributor Ignition (DI) there will be one coil, a coil wire, distributor cap and rotor, spark plug wires and spark plugs. Newer engines use EI or Electronic Ignition. As there are multiple coils in EI systems you have more dwell time and can make a more powerful spark. In addition the lack of moving parts (the rotor inside the distributor cap) creates a more reliable system. Two basic styles of Electronic Ignition (EI) systems are used. The Waste Spark system uses one coil for every two cylinders. A four cylinder would have 2 coils, 2 plug wires and 2 spark plugs. The most powerful and efficient Electronic Ignition (EI) systems use one coil for each spark plug. These are often called Coil On Plug. For this style a four cylinder would have four coils and four spark plugs. Some of these systems use no plug wires. Other COP systems use a short plug wire for each cylinder.
If the ignition system does not work (no spark) a very basic test is to ensure that electric current to the primary coil is getting turned ON and OFF. By hooking a 12 volt test light between the positive battery terminal and the negative side of the ignition coil we can easily see if primary current is tuning ON and OFF. Your test light will turn ON when current is flowing and turn OFF when the current has stopped. This is easy to see at cranking speeds (200 - 300 RPM). If the 12 volt test light does not turn ON, or it never turns OFF, we know that the primary ignition system is broke. We don't know exactly what is wrong but we have narrowed our search to the components of that vehicles primary ignition system. If the primary current is turning on and off, but there is no spark, the problem is likely in the ignition coil, or in the secondary ignition system.
An ohmmeter might find defects in the ignition coil. Check and compare the resistance for both the primary windings, and the secondary windings of the coil. The proper specification for any coil will depend upon the year, make, model and engine size of the vehicle. An open circuit is easy to spot with the ohm meter. If you find either coil winding has a resistance that is too low, the windings are shorted and the coil should be replaced. Because an ohm meter will not measure insulation that breaks down under high voltage, an ignition coil that passes the ohmeter test may still be defective. The best way to test an ignition coil is with a labscope. Before replacing an ignition coil be sure to continue checking the secondary ignition system.
A common defect in the secondary wires is an open circuit and this can be easily found with an ohmmeter. Coil and plug wires have several thousand ohms per foot of length so do not confuse high resistance with an open circuit.
A short circuit in the secondary is also common, rarely shows up on an ohm meter, and can occur any place in the secondary current path. A short circuit is an alternate path to ground (return current path). When a short occurs, current will take this alternate path instead of jumping the spark plug gap. These can be found by careful inspection. You might see or hear the spark jumping to ground. Places where high voltage current has been flowing leave very thin charcoal or burn lines called carbon tracks. Visual inspection, with a good light, can find these carbon tracks. Distributor caps and rotors are famous for carbon tracks allowing the secondary current an easier path to ground. If you find a carbon track, replace the defective component. If you find either a bad distributor cap or rotor, be sure to replace both of them. An easy test for short circuits in the coil and plug wires is to use a spray bottle filled with soapy water. Spraying or misting the secondary system with water make it easier for the shorts to happen and will cause the engine to misfire and stumble. If you find one bad plug wire replace all the plug wires.
Most often a problem in the secondary ignition system will show up as a rough running engine, or one that lacks power, long before it gets bad enough to cause an engine to not start. Secondary ignition problems are much easier to diagnose with the labscope. Reading the secondary ignition labscope pattern on a running engine can point out problems in both the primary and secondary ignition systems. It will also identify misfiring cylinders, problems with fuel injectors, and cylinders with low compression.
Diagnosing driveability problems using an ignition scope pattern takes practice and careful study. By looking at normal scope patterns on many different vehicles you will learn to quickly spot abnormal patterns when they occur. This will reduce your diagnostic time and improve your ability to restore an engine to normal performance.
Through research such as watching films, reading, talking to other technicians, asking questions and attending seminars you will learn to make the ignition scope patterns one of your most valuable diagnostic tools.
Graphical version of Matt Dubanoski's Website: cf.linnbenton.edu/eit/auto/dubanom/