The Ignition System

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The sole function of the ignition system is to generate a spark at the correct time to ignite the air/fuel mixture in the cylinder during the compression cycle, so the expanding gases generated by the exploding fuel apply maximum force (power) against the piston. This article provides an overview of the ignition system, its components, theory of operation, and steps to diagnose ignition system malfunctions.

To ensure maximum power is applied to the piston, combustion must take place when the air/fuel mixture is fully compressed, and pressure within the cylinder is at its peak. Since there is a slight delay between the time of ignition (when the spark plug fires) and combustion (when the air/fuel mixture ignites and burns), the spark must occur slightly before the piston reaches top dead center (the top of the compression stroke). If the spark occurs exactly at the top of the compression stroke, the piston will already be moving down in the cylinder before the air/fuel mixture fully ignites, reducing the amount of power applied against the piston during combustion.

Ignition System Components

The ignition system is made up of a “primary” (low voltage) circuit and a “secondary” (high voltage) circuit. On a "conventional ignition system, the primary circuit includes the keywsitch, battery/alternator, starter motor, primary ignition coil, breaker points and low voltage wiring; the secondary circuit includes the secondary ignition coil, coil-to-distributor cable, distributor cap and rotor, spark plug cables and spark plugs.

Electronic, or "distributorless" ignition systems use an ignition module and ignition "coil pack" that replace the coil and distributor, rotor and breaker points in "conventional" ignition systems.

The purpose and function of the ignition system components are described below.

• Ignition Keyswitch: Supplies power from the battery to the ignition system.
• Battery: Supplies low voltage current to the primary ignition coil during startup. Once the engine is running, low voltage current is provided by the alternator.
• Alternator: Once the engine is running, low voltage current is provided by the alternator.
• Starter Motor: Engages the engine’s flywheel to rotate the engine during startup.
• Ignition Coil ("Conventional" ignition systems): A high-voltage transformer made up of two coils of wire, the primary coil and the secondary coil; provides high voltage current to the distributor.
• Ignition Coil Pack ("Distributorless" ignition systems): Essentially a "group" of ignition coils, with each coil providing high voltage current to a single spark plug, under control of the ignition module.
• Breaker Points ("Conventional" ignition systems): “Interrupt” current flow to the primary ignition coil.
• Distributor ("Conventional" ignition systems): Distributes the high voltage output of the ignition coil to the cylinders/spark plugs at the correct time to ensure proper ignition and combustion.
• Spark Plug: Uses the high-voltage electricity generated by the ignition coil to generate a spark of sufficient strength to ignite the air/fuel mixture.
• Ignition Module ("Distributorless" ignition systems): Receives signals from the crankshaft sensor and camshaft sensor, as well as the spark signal from the vehicle's ECM, to control the ignition coils.
• Crankshaft Timing Sensor ("Distributorless" ignition systems): Sends a signal that supplies timing and RPM information to the ignition module and the vehicle's Engine Control Module (ECM).
• Camshaft Sensor ("Distributorless" ignition systems): Supplies information related to cylinder position for the ignition coils and fuel system.

Theory of Operation

When the keyswitch is turned on, current flows from the vehicle’s battery through the primary winding of the coil. Current to the primary coil is then “interrupted”, causing its magnetic field to collapse, which induces a high-voltage current in the secondary coil. The output of the secondary ignition coil is supplied to the rotor, which rotates over a series of contacts, one for each cylinder/spark plug. When the tip of the rotor passes a contact, the ignition coil generates a high voltage pulse. The pulse arcs across the gap between the rotor and the contact, and continues through the spark plug wire to the spark plug on the associated cylinder. The strength of the high-voltage input is sufficient to cause electricity to arc across the gap between the electrodes, resulting in a spark, which ignites the air/fuel mixture in the combustion

"Conventional" vs. "Distributorless" Ignition Systems

On “conventional” ignition systems, breaker points perform the function of breaking the current to the coil in order to generate the high-voltage pulse. The ground side of the coil is connected to the breaker points. A cam in the center of the distributor applies pressure to a lever connected to one side of the points. When the lever is actuated, the points open, interrupting the current to the primary ignition coil, resulting in a high-voltage pulse.

Newer vehicles employing "distributorless" ignition systems use sensors (primarily the crankshaft timing sensor and camshaft sensor) to detect the exact position of the piston in the cylinder. This information is supplied to the vehicle’s Engine Control Module (ECM), which uses a transistorized circuit (ignition module) to interrupt current to the primary ignition coil at the correct time.

About Spark Timing

Spark timing is critical to proper combustion. Depending on engine operating requirements and engine load, it is sometimes necessary to “advance” or “retard” spark timing.

The time required for combustion of the air/fuel mixture is fairly constant under all engine operating conditions. However, the rate of travel of the pistons within the cylinders increases as engine speed increases. To compensate for increases in engine speed, spark must occur earlier in the compression stroke. This is referred to as spark advance.

When maximum power is not required, spark timing is “retarded”, reducing cylinder pressures and temperatures. This helps minimize certain vehicle emissions. Retarding the timing may also helps to eliminate engine knocking.

On “conventional” ignition systems, spark advance and spark retard are accomplished using engine vacuum and/or centrifugal advance mechanisms. On “distributorless” ignition systems, this is accomplished by the vehicles computer in response to crankshaft timing and camshaft sensor inputs that provide engine speed and piston position information.

Inspection, Troubleshooting and Diagnosis

The specific procedures for diagnosing ignition related problems on vehicles equipped with “distributorless” ignition systems vary depending on vehicle manufacturer, make and model. The following are general guidelines for diagnosing and troubleshooting ignition-related problems.

Diagnosing ignition system problems involves a thorough inspection of the ignition system. These checks include visual and functional inspections to verify proper operation of the ignition system and to determine the serviceability of components. Perform physical and functional inspections of the following components, as applicable:

Inspection

• Battery: Be sure the battery is fully charged. Inspect and clean the battery terminals as needed.
• Ignition Coil: Verify the ignition coil electrical characteristics are within manufacturer’s specifications.
• Ignition Coil and Spark Plug Cables: Be sure all cables are in good condition. Check for cuts, tears or worn insulation. Be sure the cables are firmly attached to the ignition coil, distributor or ignition module, and spark plugs.
• Distributor: Replace the distributor cap, rotor and breaker points periodically, as recommended by the vehicle manufacturer.
• Spark Plugs: Be sure the electrodes show no signs of fouling, corrosion, or electrical deterioration. Be sure the gap between electrodes is in accordance with the manufacturers specifications. Replace the spark plugs periodically, as recommended by the vehicle manufacturer.
• Ignition Module: Testing an ignition module requires a dedicated tester capable of exercising the full functionality of the module.

Ignition Coil Test ("Conventional" Ignition Systems)

The following test is appropriate to determine serviceability for most “externally mounted” ignition coils. You will need an approved spark tester and an ohmmeter to perform this test.

• Disconnect the coil wire at the distributor, and connect to one lead of the spark tester. Connect the remaining lead to a known good ground. Crank the engine and observe the spark tester. You should see a crisp, blue spark, indicating the ignition coil is serviceable. If the spark is pale yellow, proceed to step two.
• Disconnect the negative battery cable.
• Tag and disconnect each wire from the coil.
• Check the coil for physical damage. If any damage is noted, the coil should be replaced.
• Refer to the vehicle’s service manual for coil specifications.
• Connect an ohmmeter to the primary studs of the coil. Set the ohmmeter to the 0-2M ohm scale, and verify the primary winding readings are within the manufacturer’s specifications.
• Disconnect one of the ohmmeter leads from the primary stud or the coil, and connect to the coil wire lead. Set the ohmmeter to the 2M-20M ohm scale, and verify the secondary winding readings are within the manufacturer’s specifications.
• If either reading is our of limits, the coil should be replaced.

Ignition Module Test ("Distributorless" Ignition Systems)

Verifying the serviceability of the ignition module in “distributorless” ignition systems is the first step in complete system diagnosis. A malfunctioning ignition module CANNOT be diagnosed simply through visual inspection. Ignition modules look the same whether they are good or bad!

Proper evaluation of the ignition module is CRITICAL to diagnosing ignition system problems. Unnecessary replacement of ignition modules create high, non-defective return rate, typically ranging between 9% to 15%. Accurate testing of ignition modules requires a dedicated tester, such as the Innova DIS Tester (for more information, visit www.distester.com).

This countertop tester is designed to test nearly 100% of OEM and aftermarket automotive DIS and electronic ignition modules (EIM) and determine whether they are good or bad within seconds. It is intended for use by auto parts stores that sell ignition modules and related ignition parts, auto repair shops that replace modules, or any business that processes returned modules.

The Tester performs a detailed set of diagnostics specific to the module under test, provides “pass/fail” indications for each test as well as the full set of parameters for each test performed, and provides recommendation for additional tests or diagnostics for related systems based on the ignition module test results.

Once ignition module test results have been obtained, perform the following additional tests/inspections, as appropriate to the condition of the ignition module:

• If the ignition module is SERVICEABLE: The ignition module is a sealed unit. If you determine it is defective, it must be replaced. No repair or adjustment is possible. When replacing the ignition module, BE SURE to perform the recommended "Related System Checks" (see below) to ensure proper repair of the ignition system.
• If the ignition module is DEFECTIVE: The ignition module is a sealed unit. If you determine it is defective, it must be replaced. No repair or adjustment is possible. When replacing the ignition module, BE SURE to perform the recommended "Related System Checks" (see below) to ensure proper repair of the ignition system.

Ignition Module System Checks

If the ignition module is determined to be serviceable, perform the following inspections/checks to ensure all components of the ignition system are operating properly. BE SURE to refer to the manufacturer's specifications when performing these tests. You will need the service manual for the vehicle under test, as well as a digital volt-ohmmeter (DVOM) to perform these tests.

The "distributorless" ignition system has five major circuits that must be checked: battery voltage, ground (low reference), crankshaft signal (high reference), EST/SPOUT (electronic spark timing/spark output timing) and tachometer signal (reference/PIP). In order for the ignition module to supply voltage to the ignition coil(s), the module must also be supplied with “fused” battery voltage. In turn, to complete the circuit to the battery, the ground to the module must be intact.

Preliminary Checks

Use an approved spark tester to confirm spark is present at all spark plugs. If spark is not present, check for misrouted wiring near the high-voltage plug wires. An improperly routed wiring harness (one that has been routed too close to the high voltage plug wires) can disrupt the signal, causing intermittent problems. Re-route any misrouted harnesses before proceeding.

Battery Voltage and Ground Circuit Check

• Unplug the harness connector from the ignition module.
• Place the DVOM positive (+) lead into the power supply pin in the connector; place the negative (-) lead into the ground pin.
• Set the DVOM to measure DC voltage. Turn the ignition key on and verify that "battery" voltage (roughly 12VDC) is indicated on the DVOM.
• If no voltage is indicated, remove the DVOM negative (-) lead from the connector and place it on the negative (-) battery terminal.
  
  ● If no voltage is indicated, check for a "blown" fuse or damaged/defective wiring in the ignition module power supply circuit.
  
  ● If voltage is indicated, inspect the ignition module's ground wire circuit.
  

Some ignition modules utilize the module case for the ground circuit. To check the ground on these types of modules:

• Place the DVOM positive (+) lead on the module case; place the negative (-) lead on the engine block.
• Set the DVOM to measure DC voltage.
• Turn the ignition key on, crank the engine, and verify that the DVOM indicates 200 millivolts maximum.
  
  ● If more than 200 millivolts is indicated, check the ignition module ground connection (verity that the attaching hardware used to secure the module to its mounting bracket are tightened properly).

Crankshaft Signal Check

Determine the type of crankshaft sensor (Hall Effect or reluctance) used on the vehicle under test, then perform the appropriate test as follows:

NOTE: Always double-check the wiring and connectors for opens or shorts, and verify the voltage and ground supply to the sensor before replacing the component (refer to the service manual for the vehicle under test for procedures).

Hall Effect Sensors:

• Place the DVOM positive (+) lead on the SIGNAL (CKP+) pin in the sensor harness connector; place the negative (-) lead on the engine block.
• Set the DVOM to measure frequency (Hz).
• Turn the ignition key on, crank the engine, and verify that the DVOM indicates approximately 10 to 1,000 Hz, depending on the specific vehicle under test.
  
  ● If the frequency indication is out of limits, or if there is no frequency indication, the sensor is defective and must be replaced.

When installing certain crankshaft sensor models, the clearance between the interrupter rings and sensor is CRITICAL. BE SURE none of the interrupter rings are bent BEFORE replacing the sensor.

Reluctance Sensors:

• Place the DVOM positive (+) lead on the SIGNAL (CKP+) pin in the sensor harness connector; place the negative (-) lead on the engine block.
• Set the DVOM to measure DC voltage.
• Turn the ignition key on, crank the engine, and verify that the DVOM indicates 2 to 4 volts.
  
  ● If the voltage indication is out of limits, or if there is no voltage indication, the sensor is defective and must be replaced.

Ignition Coil Check.

The procedure for checking ignition coils varies between vehicle manufacturers. Refer to the service manual for the vehicle under test for specific procedures. The following general procedures are appropriate for most Ford and General Motors vehicles equipped with "distributorless" ignition systems, and may apply to other similar vehicles.

NOTE: When replacing a defective ignition coil, most manufacturers recommend that the spark plugs and plug wires be replaced at the same time.

Ford Ignition Coils:

• Disconnect the ignition coil connector.
• Connect an approved test light between the battery positive (+) terminal and the coil trigger pins, one at a time.
• Verify the test light illuminated at each trigger pin.
  
  ● If the test light fails to illuminate, the coil is defective and must be replaced.

General Motors Ignition Coils:

• Remove the ignition coil(s) from the vehicle.
• Connect the test light across the ignition module terminals.
• Turn the ignition key on, crank the engine, and observe the test light.
  
  ● If the test light flashes, the ignition module is serviceable, and the coil(s) and must be replaced.

Additional Checks.

• If the vehicle has spark, it may be necessary to check the reference high circuit for opens or shorts, as some systems require this signal for the computer to synchronize fuel delivery. Connect an approved “noid” test light to an injector harness and crank the engine. If there is a no flash condition, the fuel injectors are not being pulsed.
• If the fuel injector is being pulsed, check for a fuel pressure or other fuel related problem.

Related System Checks

Ignition module failure can result in damage to other ignition-related components. Conversely, failure of an ignition-related component can result in damage to the ignition module.

If the ignition module is determined to be defective, make note of the specific area in which the module failed to aid in identifying related components or systems that should also be inspected. Use the following guidelines to assist in identifying other potentially affected areas. BE SURE to inspect these systems/components and make any necessary repairs BEFORE installing the new ignition module:

Parts Replacement

Most manufacturers recommend the replacement of the ignition coil(s), plug wires and spark plugs whenever an ignition coil has failed. If you are reusing any component, BE SURE to thoroughly inspect and test the component to ensure it conforms to the manufacturers specifications for the year, make and model of vehicle being repaired.

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