DIY Auto Service: Ignition Systems Operation Diagnosis and Repair
Ignition Systems Intro
Gasoline, compressed natural gas (CNG) and propane gas powered engines require a spark to ignite the air/ fuel mixture. Diesel fuel powered engines take advantage of the compression ignition quality of the diesel fuel. In order to supply the spark of high voltage and occurring at the proper time, several ignition systems were developed over the years.
The ignition system coil is a step-up transformer the takes about 12 volts and steps it up from about 6,000 Volts (6kV) to as much as 100,000Volts (100kV) depending on the system. The high voltage that leaves the coil is connected to a spark plug by either a spark plug wire or a coil connecter. When the spark reaches the spark plug it is forced to jump across the open gap. This spark ignites the air/fuel mixture in the cylinder to provide the heat (fire) for expansion of the gases. The expanding gasses push down on the piston to rotate the crankshaft. This sends power to the drivetrain.
Ignition System Diagram
Ignition Coil Construction and Operation
The ignition coil, as mentioned earlier, is a step-up transformer. Two coils of wire are electrical circuits inside the coil. The primary coil circuit is connected to 12 volts. The secondary circuit captures the voltage and amplifies it. The voltage to the primary circuit was originally controlled by a set of mechanical ignition points that were opened by camshaft lobes in the distributor. The ignition points controlled the ground side of the primary circuit. When the points were closed, the current flow through the primary coil of wire built up a magnetic field. When the cam lobe forced the points open, the magnetic field collapsed across the secondary coil windings. Since the 1970’s, manufacturers have replaced the points with an electronic solid state ignition system. Points needed adjustment and typically annual replacement. Solid state ignition modules with no moving parts theoretically could last indefinitely. The ignition module didn’t have a way to reference engine position and speed (RPM) like the cam lobes for the points. Sensors, pick-up coils and expensive control modules were needed to give the module the information. The module then controlled the ground side of the primary circuit like the points did.
The number of coil windings or turns controls the amount of step-up or induced voltage produced. The amount of voltage produced is controlled by the ratio of primary wiring coils to secondary wiring coils. Today’s coils are typically over a 100:1 (secondary/primary) Ratio. This means if there are 200 primary coil windings (turns), the secondary has over 20,000 turns. To help amplify and shape the magnetic field, an iron core is used in the middle of the windings.
Up until the middle 1980’s, a single ignition coil was used on all engines whether they were 4, 6 or 8 cylinders. The time that the current is flowing through the primary circuit (on time) allows the magnetic field to build up. This is called the saturation time or dwell. The longer the saturation time (within limits) the stronger the magnetic field produced and the stronger spark output possible. With a four stroke engine, all the cylinders fire within two turns of the crankshaft. On an eight cylinder engine, the coil discharges every 90 degrees or 1/4 turn of the crankshaft. A four cylinder engine would fire every 180 degrees or 1/2 turn. This limits the amount of time the coil can build up especially at higher RPM’s. Most ignition systems today use multiple coils either one coil for two cylinders or one coil for each cylinder.
Round Ignition Coil
Ignition Coil Tests
As mentioned earlier the ignition coil is two circuits; primary and secondary. Each circuit can be tested for resistance. The primary circuit is a low resistance circuit because of the larger wire size while the secondary circuit is high resistance due to the small and long length of the wire.
To test the ignition coil resistance:
Primary Circuit test: With the Ohmmeter on the lowest scale, test across the B+ and negative terminal of the coil. The resistance specification is generally .5 to2.0Ω but is typically between .8 and 1.0Ω for a good reading on a late model coil.
Secondary Circuit test: Secondary resistance (Ω) is typically between 10,000 (10k) and 20,000Ω (20k). Set the Ohmmeter on the proper scale to measure the expected amount of resistance for the system you are working on according to the specifications. For a single tower coil, place one lead in the tower and the other on the coil negative terminal. For a waste spark coil, test between the two coil towers that are paired together.
GM High Energy Ignition Distributor
Electronic Distributor Ignition Systems
In the early 1970’s many manufacturers came out with a variety of distributor electronic ignition systems referred to as "DI". An electronic module replaced the mechanical point style ignition contacts. A magnetic pickup (pickup coil) inside the distributor sensed the engine position, sent a signal to the module controlling the primary circuit and the rotor distributed the spark to the right cylinder. Some of the systems were:
GM High energy Ignition System (HEI) – The HEI system used a coil built into the top of the distributor cap. A copper spring and carbon button made the contact from the coil secondary to the rotor. The entire ignition system was inside the distributor. The distributor needed 12V to operate. Inside the distributor was the ignition module and pickup coil. Later versions separated the coil to a remote version and added a knock sensor with a timing module.
Motorcraft (Ford) Duraspark – Ford and Lincoln Mercury products used a remote coil with the module on the side of the distributor. The control box mounted on the fenderwell controlled the timing.
Chrysler Breakerless Ignition – Chrysler, Dodge and Plymouth products used this type of ignition with an adjustable pickup. If you look at a feeler gauge set and see a .009” brass feeler gauge, it was for adjusting this gap.
Service Tip: All of these systems had one thing in common; they all needed to have the timing set with a timing light. In order to set the timing, the distributor had to be put into base timing by disconnecting a plug or vacuum line. Look up the procedure for the vehicle you are working on. On some vehicles the procedure was on the emission label under the hood. If the procedure wasn’t followed the timing was incorrectly set.
HEI Spark Plug Wires
Electronic Ignition (EI) System Operation
EI systems do not use a distributor and are referred to as distributorless ignition systems. All ignition systems today are EI. The advantage is the versatility and the precise control of timing. Timing with a distributor was only as good as the timing adjustment. Plus the distributor over time wore out and affected the timing control. Other factors like; timing chain and distributor gear wear also affects the accuracy of the timing. EI has no distributor to wear out. It uses sensors on the crankshaft (CKP) and camshaft (CMP) to feed engine position and speed to the engine computer (ECM or PCM) or ignition module. The timing accuracy should be maintained throughout its life. This speed and position information (timing) is also used by the fuel injection system. Timing is especially critical to exhaust emissions. EI systems are typically in two varieties:
- Waste Spark – uses one coil for two companion cylinders.
- Individual Coil (per cylinder) – Each cylinder has its own ignition coil. Coil on Plug (COP) or a coil and short plug wire.
Waste Spark Ignition Coils
Spark Plug Wires and Connections
Spark Plug Wires
Spark plug wires make the connection between the distributor cap or coil depending on the system. The wires have a thick silicone outer layer for insulation. To reduce radio interference, the wires are designed using a resistance type conductor of carbon and fiberglass. This added resistance reduces the magnetic radio interference. Damage to the outer insulation could cause the spark to ground where it is not wanted and cause a misfire due to weak or no spark to the spark plug. The outer cover is oil resistant but, can be damaged by hot oil leaking. Wire routing is also critical because if the wire is routed too close to a metal bracket, another wire or exhaust, the spark may leak through the insulation and cause a misfire. The wires are usually routed with wire clips and it is important to put them back. Wires that are too short or too long are especially prone to shorting out. The spark plug boot end of the wire is usually very close to the exhaust manifold and may have a heat shield. Always reinstall the heat shield or the exhaust manifold heat could melt the boot. The spark plug wire connections depend on the type of ignition system.
With a distributor, the spark plug wires are connected to the top of the distributor cap in a circle. As the rotor inside the cap turns, it will point the contact to the different connections around the circle. The wires are connected to the cap in the engine firing order. A typical V8 GM firing order is “1-8-4-3-6-5-7-2”. This means that this order is used to connect the wires to the cap in the direction the rotor turns. If the wires are connected improperly, the cylinders will fire at the wrong time and cause a misfire.
Coil wires are used if the distributor has a remote coil. This means a spark plug wire (coil wire) connects the coil tower to the center (usually) of the distributor cap. The coil wire delivers the spark from the coil to the rotor inside the cap. Like the spark plug wires, the routing of this coil wire is critical so it will not short out on a bracket or other metal part.
Waste spark systems are a distributor-less ignition system that uses one ignition coil for every two cylinders. The spark plug wires connect directly to the coils. The cylinders that are paired on each coil are “companion cylinders”. Companion cylinder pistons are moving up and down at the same position but are on opposite strokes of the four stroke cycle (Intake – Compression – Power - Exhaust). If the pistons are moving up, one will be on the Compression Stroke and the other will be on the Exhaust Stroke. Companions on a Buick V6 engine are 1-4, 2-5 and 3-6. On a 4 cylinder they are typically 1-4 and 2-3. Many of the factory installed coils are numbered for easier connections. Like the distributor cap, the wires must be connected to the right coil position or a misfire will occur. It is referred to as the “Waste Spark Ignition System” because when one cylinder fires at the top of the compression stroke, the other cylinder is on the top of the exhaust stroke. The spark plug that is on the exhaust stroke does not use the spark so it is wasted and has no effect on that cylinder. Both spark plugs are fired as the spark completes the circuit through both spark plugs in a loop. The spark also uses the heads and block to complete the circuit to the other spark plug. One spark plug fires from the center electrode to the ground strap while the other fires from the ground strap to the center electrode.
One coil for each cylinder is very popular today. This is used in two variations; Coil with a spark plug wire and coil over plug (COP). Both types use one individually controlled coil for each cylinder. Longer coil saturation and cool down times should provide a hotter spark and longer coil life. Although these two types look quite different, they are controlled the same. The engine computer (ECM or PCM) typically controls the ground of the primary coils. A combination of crankshaft (CKP) and camshaft (CMP) sensors report the engine position and speed to the computer. The spark can be tailored to the needs of the engine at a variety of engine conditions. The two types are:
Coil with a short spark plug wire; uses a coil that is typically mounted on the valve cover and has a short spark plug wire connected from the coil to the spark plug. The two wires connected to each coil are the positive and negative connections for the primary circuit. It is important that the right connecter is plugged into the correct coil or the computer will fire the wrong spark plug. The plugs are typically numbered or arranged to make it difficult to connect them improperly. This arrangement also makes it easy to switch the coils to see if an intermittent misfire follows a coil.
Coil over plug or COP; use a coil that has the spark plug connector either built into or attached directly to the coil. The coil is bolted in above the spark plug usually through the valve cover. Another option has the coils mounted in a holder that bolts in above the spark plugs and is removed as a unit. Either way, this is used on engines that have the spark plugs in the center of the cylinder in “Hemi” cylinder fashion. Depending on the manufacturer, on some of these systems the plug connector and coil can be replaced separately or as one piece together.
Coil Over Plug or COP Ignition Coils
One Coil per Cylinder with a Short Spark Plug Wire
Electronic Distributor Ignition Diagnosis and Testing
Inside the distributor is a permanent magnet generator referred to as a pickup coil. The pickup coil consists of a magnet and coil of wire. As the points of the magnet line up (4 points for 4 cylinder, 6 points for 6 cylinder and 8 points for 8 cylinder), a magnetic field is generated. As the center (timer core) continues to rotate away from the points, the magnetic field collapses across the coil and an AC voltage is produced. This pulse is sent to the ignition module to identify the engine position and signal the module to open the ground circuit to the ignition coil. This causes the ignition coil magnetic field to collapse across the coil secondary windings and produce the high voltage spark.
Two tests are available for the pickup coil.
Pickup Coil Resistance test: With the coil leads disconnected, test the resistance of the coil. The resistance is typically between 200 and 1,500Ω. The GM HEI spec is 500 and 1,500Ω with the reading typically between 700 and 900Ω for a good coil. Also check the pickup coil to ground for a shorted coil.
AC Voltage test: With a good DVOM, test the AC voltage output across the two terminals of the coil while cranking or rotating the shaft. At cranking speed the output should be at least .25 to .50 AC volts. The faster it is spun the higher the AC voltage.
Service Tip: Some worn distributors allowed engine oil up the shaft to the pickup coil. This caused the insulation to break down and cause intermittent rough running or crank but no starts. The pickup coil, distributor shaft and bushings need replacement to correct the problem.
GM V6 Waste Spark Coil Test
EI System Diagnosis and Testing
The EI system uses components that are scattered around the engine. The ignition coils can be tested as earlier described by testing the primary and secondary circuits in the coil. With the key “on”, there should be 12V to the B+ side of the coil. The ignition module or engine computer controls the ground. For engine position and speed crankshaft and camshaft sensors are used. Two types of sensors are available and used depending on the engine model and year.
Permanent Magnet Generators are similar in construction to a pickup coil and can be tested for resistance and AC voltage output.
Hall Effect Sensors use a 5 or 8 volt source voltage and create a DC digital voltage signal. They can be tested for source voltage across power and ground. Test the signal wire for source voltage (5 or 8 volts) and low (0 to 1 volt) while the engine is being cranked or barred over.
More on Hall Effect and PM generators in my Hub: DIY Auto Service: Pemanent Magnet and Hall Effect Sensor Diagnosis and Testing