The ignition circuit supplies high voltage surges (some as high as 100,000 volts in electronic ignition circuits) to the spark plugs in the engine cylinders. These surges produce electric sparks across the spark plug gaps. The heat from the spark ignites the compressed air-fuel mixture in the combustion chambers. When the engine is idling, the spark appears at the spark plug gap just as the piston nears top dead center (TDC) on the compression stroke. When the engine is operating at higher speeds, the spark is advanced. It is moved ahead and occurs earlier in the compression stroke. This design gives the compressed mixture more time to burn and deliver its energy to the pistons.
The functions of an ignition circuit are as follows:
The ignition circuit is actually made of two separate circuits which work together to cause the electric spark at the spark plugs: the primary and secondary.
The primary circuit of the ignition circuit includes all of the components and wiring operating on low voltage (battery or alternator voltage). Wiring in the primary circuit uses conventional wire, similar to the wire used in other electrical circuits on the vehicle.
The secondary circuit of the ignition circuit is the high voltage section. It consists of the wire and components between the coil output and the spark plug ground. Wiring in the secondary circuit must have a thicker insulation than that of the primary circuit to prevent leaking (arcing) of the high voltage.
Various ignition circuit components are designed to achieve the functions of the ignition circuit. Basic ignition circuit components are as follows:
The ignition switch enables the operator to turn the ignition on for starting and running the engine and to turn it off to stop the engine (Figure 7-24). Most automotive ignition switches incorporate four positions: off, accessory, ignition on, and start:
Figure 7-24 — Ignition switch.
An ignition distributor can be a contact point (Figure 7-25, View A), or pickup coil type (Figure 7-25, View B). A contact point distributor is commonly found in older vehicles, whereas the pickup coil type distributor is used on many modern vehicles. The ignition distributor has several functions:
Figure 7-25 — Ignition distributors.
The distributor cap is an insulating plastic component that covers the top of the distributor housing. Its center terminal transfers voltage from the coil wire to the rotor. The distributor cap also has outer terminals that send electric arcs to the spark plugs. Metal terminals are molded into the plastic cap to provide electrical connections.
The distributor rotor transfers voltage from the coil wire to the spark plug wires. The rotor is mounted on top of the distributor shaft. It is an electrical switch that feeds voltage to each spark plug wire in turn.
A metal terminal on the rotor touches the distributor cap center terminal. The outer end of the rotor almost touches the outer cap terminals. Voltage is high enough that it can jump the air space between the rotor and cap. Approximately 4,000 volts are required for the spark to jump this rotor-to-cap gap.
An electronic ignition, also called solid state ignition, uses an electronic control circuit and distributor pickup coil to operate the ignition coil.
An electronic ignition is more dependable than a system of contact points because there are no mechanical breakers to burn out or wear down. This avoids trouble with ignition timing.
An electronic ignition is capable of producing a significantly higher secondary voltage over a points system. This allows for a wider spark plug gap and higher voltage to burn lean air-fuel mixtures. Leaner mixtures are now used to reduce emissions and improve fuel economy.
A distributorless ignition uses multiple ignition coils, a coil control unit, engine sensors, and a computer to operate the spark plugs (Figure 7-26).
Figure 7-26 — Distributorless ignition.
The electronic coil module consists of more than one coil and a coil control unit that operates the coils. The module’s control unit performs about the same function as the Ignition Control Module (ICM) in an electronic ignition. It will analyze data from different engine sensors and the system computer.
The coils are wired so they fire two spark plugs at the same time. One plug will fire on the power stroke and the other will fire on the exhaust stroke (there is no effect on engine operation). This system reduces the number of ignition coils required to operate the engine. For instance, a four cylinder would have only two coils, a six cylinder would have only three coils and so on.
A camshaft position sensor is installed in place of the ignition distributor. It sends an electrical pulse to the coil control unit providing data on camshaft and valve position.
A coil over plug ignition system has coils mounted on top of each spark plug (Figure 7-27). This type of system operates very similar to the distributorless ignition except for the lack of spark plug wires and the increase of coils. Sensor inputs allow the electronic control module to alter ignition timing with changes in operating conditions.
Figure 7-27 — Coil over plug ignition.
The spark plug consists of a porcelain insulator in which there is an insulated electrode supported by a metal shell with a grounded electrode. Spark plugs have the simple purpose of supplying a fixed gap in the cylinder across which the high voltage surges from the coil must jump after passing through the distributor.
The spark plugs use ignition coil high voltage to ignite the fuel mixture. Somewhere between 4,000 and 10,000 volts are required to make current jump the gap at the plug electrodes. This is much lower than the output potential of the coil.
Spark plug gap is the distance between the center and side electrodes. Normal gap specifications range between .030 to .060 inch. Smaller spark plug gaps are used on older vehicles equipped with contact point ignition systems.
Spark plugs are either resistor or non- resistor types (Figure 7-28). A resistor spark plug has internal resistance (approximately 10,000 ohms) designed to reduce the static in radios. Most new vehicles require resistor type plugs. Non-resistor spark plugs have a solid metal rod forming the center electrode. This type of spark plug is NOT commonly used except for racing and off-road vehicles.
Figure 7-28 — Spark plugs.
The heat range of the spark plug determines how hot the plug will get. The length and diameter of the insulator tip and the ability of the spark plug to transfer heat into the cooling system determine spark plug heat range.
A hot spark plug has a long insulator tip that prevents heat transfer into the water jackets. It will also bum off any oil deposits. This provides a self-cleaning action.
A cold spark plug has a shorter insulator tip and operates at a cooler temperature. The cooler tip helps prevent overheating and preignition. A cold spark plug is used in engines operated at high speeds.
Vehicle manufacturers recommend a specific spark plug heat range for their engines. The heat range is coded and given as a number on the spark plug insulator. The larger the number on the plug, the hotter the spark plug tip will operate. For example, a 54 plug would be hotter than a 44 or 34 plug.
The only time you should change from spark plug heat-range specifications is when abnormal engine or operating conditions are encountered. For instance, if the plug runs too cool, sooty carbon will deposit on the insulator around the center electrode. This deposit could soon build up enough to short out the plug. Then high voltage surges would leak across the carbon instead of producing a spark across the spark plug gap. Using a hotter plug will bum this carbon deposit away or prevent it from forming.
Spark plug reach is the distance between the end of the spark plug threads and the seat or sealing surface of the plug. Plug reach determines how far the plug reaches through the cylinder head. If spark plug reach is too long, the spark plug will protrude too far into the combustion chamber, and the piston at TDC may strike the electrode. However, if the reach is too short, the plug electrode may not extend far enough into the cylinder head, and combustion efficiency will be reduced. A spark plug must reach into the combustion chamber far enough so that the spark gap will be properly positioned in the combustion chamber without interfering with the turbulence of the air-fuel mixture or reducing combustion action.
The spark plug wires carry the high voltage electric current from the distributor cap side terminals to the spark plugs. In vehicles with distributorless ignition, the spark plug wires carry coil voltage directly to the spark plugs. The two types of spark plug wires are solid wire and resistance wire.
Solid wire spark plug wires are used on older vehicles. The wire conductor is simply a strand of metal wire. Solid wires can cause radio interference and are no longer used.
Resistance spark plug wires consist of carbon-impregnated strands of rayon braid. They are used on modern vehicle because they contain internal resistance that prevents radio interference. Also known as radio interference wires, they have approximately 10,000 ohms per foot. This prevents high voltage-induced popping or cracking of the radio speakers.
On the outer ends of the spark plug wires, boots protect the metal connectors from corrosion, oil, and moisture that would permit high voltage to leak across the terminal to the shell of the spark plug.
The basic difference between the contact point and the electronic ignition system is in the primary circuit. The primary circuit in a contact point ignition system is open and closed by contact points. In the electronic system, the primary circuit is open and closed by the electronic control unit (ECU).
The secondary circuits are practically the same for the two systems. The difference is that the distributor, ignition coil, and wiring are altered to handle the high voltage produced by the electronic ignition system. One advantage of this higher voltage (up to 60,000 volts) is that spark plugs with wider gaps can be used. This results in a longer spark, which can ignite leaner air-fuel mixtures. As a result, engines can run on leaner mixtures for better fuel economy and lower emissions.
The components of an electronic ignition system regardless of the manufacturer all perform the same functions. Each manufacturer has its own preferred terminology and location of the components. The basic components of an electronic ignition system are as follows:
With the engine running, the trigger wheel rotates inside the distributor. As a tooth of the trigger wheel passes the pickup coil, the magnetic field strengthens around the pickup coil. This action changes the output voltage or current flow through the coil. As a result, an electrical surge is sent to the electronic control unit as the trigger wheel teeth pass the pickup coil.
The electronic control unit increases the electrical surges into on/off cycles for the ignition coil. When the ECU is on, current passes through the primary windings of the ignition coil and develops a magnetic field. Then, when the trigger wheel and pickup coil turn off the ECU, the magnetic field inside the ignition coil collapses and fires a sparkplug.
Ignition timing refers to how early or late the spark plugs fire in relation to the position of the engine pistons. Ignition timing must vary with engine speed, load, and temperature.
Timing advance happens when the spark plugs fire sooner than the compression strokes of the engine. The timing is set several degrees before top dead center (TDC). More time advance is required at higher speeds to give combustion enough time to develop pressure on the power stroke.
Timing retard happens when the spark plugs fire later on the compression strokes. This is the opposite of timing advance. Spark retard is required at lower speeds and under high load conditions. Timing retard prevents the fuel from burning too much on the compression stroke, which would cause spark knock or ping.
The basic methods to control ignition system timing are as follows:
Centrifugal advance makes the ignition coil and spark plugs fire sooner as engine speed increases, using spring-loaded weights, centrifugal force, and lever action to rotate the distributor cam or trigger wheel. Spark timing is advanced by rotating the distributor cam or trigger wheel against distributor shaft rotation. This action helps correct ignition timing for maximum engine power. Basically the centrifugal advance consists of two advance weights, two springs, and an advance lever.
During periods of low engine speed, the springs hold the advance weights inward towards the distributor cam or trigger wheel. At this time there is not enough centrifugal force to push the weights outward. Timing stays at its normal initial setting.
As speed increases, centrifugal force on the weights moves them outwards against spring tension. This movement causes the distributor cam or trigger wheel to move ahead. With this design, the higher the engine speed, the faster the distributor shaft turns, the farther out the advance weights move, and the farther ahead the cam or trigger wheel is moved forward or advanced. At a preset engine speed, the lever strikes a stop and centrifugal advance reaches maximum.
The action of the centrifugal advance causes the contact points to open sooner, or the trigger wheel and pickup coil turn off the ECU sooner. This causes the ignition coil to fire with the engine pistons not as far up in the cylinders.
The vacuum advance provides additional spark advance when engine load is low at part throttle position. It is a method of matching ignition timing with engine load. The vacuum advance increases fuel economy because it helps maintain idle fuel spark advance at all times. A vacuum advance consists of a vacuum diaphragm, link, movable distributor plate, and a vacuum supply hose.
At idle, the vacuum port from the carburetor or throttle body to the distributor advance is covered, thereby no vacuum is applied to the vacuum diaphragm, and spark timing is not advanced. At part throttle, the throttle valve uncovers the vacuum port and the port is exposed to engine vacuum.
The vacuum pulls the diaphragm outward against spring force. The diaphragm is linked to a movable distributor plate, which is rotated against distributor shaft rotation and spark timing is advanced. The vacuum advance does not produce any advance at full throttle. When the throttle valve is wide open, vacuum is almost zero. Thus vacuum is not applied to the distributor diaphragm and the vacuum advance does not operate.
The computerized advance, also known as an electronic spark advance system, uses various engine sensors and a computer to control ignition timing. The engine sensors check various operating conditions and sends electrical data to the computer. The computer can change ignition timing for maximum engine efficiency.
Ignition system engine sensors include the following:
The computer receives different current or voltage levels (input signals) from these sensors. It is programmed to adjust ignition timing based on engine conditions. The computer may be mounted on the air cleaner, under the dash, on a fender panel, or under a seat.
The following is an example of the operation of a computerized advance. A vehicle is traveling down the road at 50 mph; the speed sensor detects moderate engine speed. The throttle position sensor detects part throttle, and the air inlet and coolant temperature sensors report normal operating temperatures. The intake vacuum sensor sends high vacuum signals to the computer.
The computer receives all the data and calculates that the engine requires maximum spark advance. The timing would occur several degrees before TDC on the compression stroke. This action assures that high fuel economy is attained on the road.
If the operator begins to pass another vehicle, intake vacuum sensor detects a vacuum drop to near zero and a signal is sent to the computer. The throttle position sensor detects a wide open throttle and other sensor outputs say the same. The computer receives and calculates the data, then, if required, retards ignition timing to prevent spark knock or ping.
Ignition troubles can result from a myriad of problems, from faulty components to loose or damaged wiring. Unless the vehicle stops on the job, the operator will report trouble indications, and the equipment is turned in to the shop for repairs.
Unless the trouble is known, a systematic procedure should be followed to locate the cause. Remember, electric current will follow the path of least resistance. Trace ignition wiring while checking for grounds, shorts, and open circuits. Bare wires, loose connections, and corrosion are found through visual inspection.
After checking the system, you must evaluate the symptoms and narrow down the possible causes. Use your knowledge of system operation, a service manual troubleshooting chart, basic testing methods, and common sense to locate the trouble. Many shops have specialized equipment that provides the mechanic a quick and easy means of diagnosing ignition system malfunctions.
Bad spark plugs cause a wide range of problems such as misfiring, lack of power, poor fuel economy, and hard starting. After prolonged use, the spark plug tip can become coated with ash, oil, and other residue. The spark plug electrodes can also burn and widen the gap. This makes it more difficult for the ignition system to produce an electric arc between the electrodes.
To read spark plugs closely, inspect and analyze the condition of each spark plug tip and insulator. This will give you information on the condition of the engine, the fuel system, and the ignition system. The conditions commonly encountered with spark plugs are as follows:
Figure 7-29 — Spark plug conditions.
When a spark plug is removed for cleaning or inspection, it should be regapped to the engine manufacturer’s specifications. New spark plugs must also be regapped before installation, as they may have been dropped or mishandled and may not be within specifications.
Use a wire type feeler gauge to measure spark plug gap. Slide the feeler gauge between the electrodes. If needed, bend the side electrode until the feeler gauge fits snugly. The gauge should drag slightly as it is pulled in and out of the gap. Spark plug gaps vary from 0.030 inch on contact point ignitions to over 0.060 inch on electronic ignition systems.
When you are reinstalling spark plugs, tighten them to the manufacturer’s recommendation. Some manufacturers give spark plug torque, while others recommend bottoming the plugs on the seat and then turning an additional one-quarter to one-half turn. Refer to the manufacturer’s service manual for exact procedures.
A faulty spark wire can either have a burned or broken conductor, or it could have deteriorated insulation. Most spark plugs wires have a resistance conductor that can be easily separated. If the conductor is broken, voltage and current cannot reach the spark plug. If the insulation is faulty, sparks may leak through to ground or to another wire instead of reaching the spark plugs. To test the wires for proper operation, you can perform the following:
Installing new spark plug wire is a simply task, especially when you replace one wire at a time. Wire replacement is more complicated if all of the wires have been removed. Then you must use engine firing order and cylinder numbers to route each wire correctly. You can use service manuals to trace the wires from each distributor cap tower to the correct spark plug.
The distributor is critical to the proper operation of the ignition system. The distributor senses engine speed, alters ignition timing, and distributes high voltage to the spark plugs. If any part of the distributor is faulty, engine performance suffers.
When problems point to possible distributor cap or rotor troubles, remove and inspect them. The distributor cap should be carefully checked to see that sparks have not been arcing from point to point. Both interior and exterior must be clean. The firing points should not be eroded, and the interior of the towers must be clean.
The rotor tip, from which the high-tension spark jumps to each distributor cap terminal, should not be worn. It also should be checked for excessive burning, carbon trace, looseness, or other damage. Any wear or irregularity will result in excessive resistance to the high-tension spark. Make sure that the rotor fits snugly on the distributor shaft.
A common problem arises when a carbon trace forms on the inside of the distributor cap or outer edge of the rotor. The carbon trace will short coil voltage to ground or to a wrong terminal lug in the distributor cap. A carbon trace will cause the spark plugs to either fire poorly or not at all.
Using a droplight, check the inside of the distributor cap for cracks and carbon trace. Carbon trace is black, which makes it hard to see on a black colored distributor cap. If you find carbon trace or a crack, replace the distributor cap or rotor.
In a contact point distributor, there are two areas of concern: the contact points and the condenser.
Bad contact points cause a variety of engine performance problems. These problems include high speed missing, no-start problems, and many other ignition troubles. Visually inspect the surfaces of the contact points to determine their condition. Points with burned and pitted contacts or with a worn rubbing block must be replaced.
However, if the points look good, point resistance should be measured. Turn the engine over until the points are closed and then use an ohmmeter to connect the meter to the primary point lead and to ground. If resistance reading is too high, the points are burned and must be replaced.
A faulty condenser may leak (allow some DC current to flow to ground), be shorted (direct electrical connection to ground), or be opened (broken lead wire to the condenser foils). If the condenser is leaking or open, it will cause point arcing and burning. If the condenser is shorted, primary current will flow to ground and the engine will not start. To test a condenser using an ohmmeter, connect the meter to the condenser and to ground. The meter should register slightly and then return to infinity (maximum resistance). Any continuous reading other than infinity indicates that the condenser is leaking and must be replaced.
Installing contact points is a relatively simple procedure but must be done with precision and care in order to achieve good engine performance and economy. Make sure the points are clean and free of any foreign material.
Proper alignment of the contact points is extremely important (Figure 7-30). If the faces of the contact points do not touch each other fully, heat generated by the primary current cannot be dissipated and rapid burning takes place. The contacts are aligned by bending the stationary contact bracket only. Never bend the movable contact arm. Ensure the contact arm-rubbing block rests flush against the distributor cam. Place a small amount of an approved lubricant on the distributor cam to reduce friction between the cam and rubbing block. Once you have installed the points, you can adjust them using either a feeler gauge or dwell meter.
Figure 7-30 — Contact point alignment.
To use a feeler gauge to set the contact points, turn the engine over until the points are fully open. The rubbing block should be on top of a distributor cam lobe. With the points open, slide the specified thickness feeler gauge between them. Adjust the points so that there is a slight drag on the blade of the feeler gauge. Depending upon point design, use a screwdriver or Allen wrench to open and close the points. Tighten the hold-down screws and recheck the point gap. Typically point gap settings average around .015 inch for eight-cylinder engines and .025 inch for six- and four-cylinder engines. For the gap set of the engine you are working on, consult the manufacturer’s service manual.
Ensure the feeler gauge is clean before inserting it between the points. Oil and grease will reduce the service life of the points.
To use a dwell meter for adjusting contact points, connect the red lead of the dwell meter to the distributor side of the ignition coil (wire going to the contact points). Connect the black lead to ground.
If the distributor cap has an adjustment window, the points should be set with the engine running. With the meter controls set properly, adjust the points through the window of the distributor cap using an Allen wrench or a special screwdriver. Turn the point adjustment screw until the dwell meter reads within manufacturer’s specification. However, if the distributor cap does not have an adjustment window, remove the distributor cap and ground the ignition coil wire. Then crank the engine; this action will simulate engine operation and allow point adjustment with the dwell meter.
Dwell specifications vary with the number of cylinders. An eight-cylinder engine requires 30 degrees of dwell. An engine with few cylinders requires more dwell time. Always consult the manufacturer’s service manual for exact dwell values.
Dwell should remain constant as engine speed increases or decreases. However, if the distributor is worn, you can have a change in the dwell meter reading. This is known as dwell variation. If dwell varies more than 3 degrees, the distributor should either be replaced or rebuilt. Also, a change in the point gap or dwell will change ignition timing. For this reason, the points should always be adjusted before ignition timing.
Most electronic ignition distributors use a pickup coil to sense trigger wheel rotation and speed. The pickup coil sends small electrical impulses to the ECU. If the distributor fails to produce these electrical impulses properly, the ignition system can quit functioning.
A faulty pickup coil will produce a wide range of engine troubles, such as stalling, loss of power, or failure to start at all. If the small windings in the pickup coil break, they will cause problems only under certain conditions. It is important to know how to test a pickup coil for proper operation.
The pickup coil ohmmeter test compares actual pickup resistance with the manufacturer’s specifications. If the resistance is too high or low, the pickup coil is faulty. To perform this test, connect the ohmmeter across the output leads of the pickup coil. Wiggle the wire to the pickup coil and observe the meter reading. This will assist in locating any breaks in the wires to the pickup. Also, using a screwdriver, lightly tap the coil. This action will uncover any break in the coil windings.
Pickup coil resistance varies between 250 and 1,500 ohms, and you should refer to the service manual for exact specifications. Any change in the readings during the pickup coil resistance test indicates the coil should be replaced. Refer to the manufacturer’s service manual for instructions for the removal and replacement of the pickup coil.
Once you have replaced the pickup coil, you need to set the pickup coil air gap. The air gap is the space between the pickup coil and the trigger wheel tooth. To obtain an accurate reading, use a nonmagnetic feeler gauge (plastic or brass).
With one tooth of the trigger wheel pointing at the pickup coil, slide the correct thickness non-magnetic feeler gauge between the trigger wheel and the pickup coil. Move the pickup coil in or out until the correct air gap is set. Tighten the pickup coil screws and double check the air gap setting.
The ignition system must be timed so the sparks jump across the spark plug gaps at exactly the right time. Adjusting the distributor on the engine so that the spark occurs at this correct time is called setting the ignition timing. The ignition timing is normally set at idle or a speed specified by the engine manufacturer. Before measuring engine timing, disconnect and plug the vacuum advance hose going to the distributor. This action prevents the vacuum advance from functioning and upsetting the readings. Make the adjustment by loosening the distributor hold-down screw and turning the distributor in its mounting.
Turning the distributor housing against the distributor shaft rotation advances the timing. Turning the distributor housing with shaft rotation retards the timing (Figure 7-31).
Figure 7-31 — Determining direction of rotor rotation.
When the ignition timing is too advanced, the engine may suffer from spark knock or ping. When ignition timing is too retarded, the engine will have poor fuel economy and power and will be very sluggish during acceleration. If extremely retarded, combustion flames blowing out of the open exhaust valve can overheat the engine and crack the exhaust manifolds.
A timing light is used to measure ignition timing. It normally has three leads—two small leads that connect to the battery, and one larger lead that connects to the number one spark plug wire. Depending on the type of timing light, the large lead may clip around the plug wire (inductive type), or it may need to be connected directly to the metal terminal of the plug wire (conventional type).
Draw a chalk line over the correct timing mark. This will make it easier to see. The timing marks may be either on the front cover in harmonic balance of the engine, or they may be on the engine flywheel.
With the engine running, aim the flashing timing light at the timing mark and reference pointer. The flashing timing light will make the mark appear to stand still. If the timing mark and the pointer do not line up, turn the distributor in its mounting until the timing mark and pointer are aligned. Tighten the distributor hold-down screw.
Keep your hands and the timing light leads from the engine fan and belts. The spinning fan and belts can damage the light or cause serious personal injury.
After the initial ignition timing, you should check to see if the automatic advance mechanism is working. This can be done by keeping the timing light flashes aimed at the timing mark and gradually increasing speed. If the advance mechanism is operating, the timing mark should move away from the pointer. If the timing mark fails to move as the speed increases or it hesitates and then suddenly jumps, the advance mechanism is faulty and should either be repaired or replaced.
Replace the distributor vacuum line and see if timing still conforms to the manufacturer’s specifications. If the timing is NOT advanced when the vacuum line is connected and the throttle is opened slightly, the vacuum advance unit or tubing is defective.
Most computer-controlled ignition systems have no provision for timing adjustment. A few, however, have a tiny screw or lever on the computer for small ignition timing changes.
A computer-controlled ignition system has what is known as base timing. Base timing is the ignition timing without computer-controlled advance. Base timing is checked by disconnecting a wire connector in the computer wiring harness. This wire connector may be found on or near the engine or sometimes next to the distributor. When in the base timing mode, a conventional timing light can be used to measure ignition timing. If ignition timing is not correct, you can rotate the distributor, in some cases, or move the mounting for the engine speed or crank position sensor. If base timing cannot be adjusted, the electronic control unit or other components will have to be replaced. Always refer to the manufacturer’s service manual when timing a computer-controlled ignition system.
9. Of the two circuits within the ignition circuit, which one uses conventional wiring?
10. What are the two types of sparkplugs?