Knowledgebase: General
Ignition Basics
Posted by Alex (Im) E. on 01 February 2013 01:13 AM


The major components in the ignition system include the spark plugs, plug wires, distributor, distributor cap and rotor, ignition coil(s), ignition module and crankshaft position sensor.
There are several basic types of electronic ignition systems.

Older vehicles use a distributor to route high voltage from a single ignition coil to the individual spark plugs.

This type of system may require replacing the distributor cap and rotor periodically to maintain ignition performance.

Many late model vehicles have "distributorless" ignition systems (DIS) that do not use a distributor.

On these applications, multiple ignition coils are used. One coil may be shared by two cylinders, or there may be a separate coil for each cylinder.

The coils are connected directly to the spark plugs either by plug wires. On some engines, the coils are mounted directly atop the spark plugs and are called "coil-on-plug" (COP) ignition systems.



Coil On Plug systems are used on a growing number of late model engines for a number of packaging, performance, emissions and maintenance reasons.

Placing individual ignition coils directly over each spark plug eliminates the need for long, bulky (and expensive) high voltage spark plug cables.

This reduces radio frequency interference, eliminates potential misfire problems caused by burned, chaffed or loose cables, and reduces resistance along the path between the coil and plug. Consequently, each coil can be smaller, lighter and use less energy to fire its spark plug.

From a performance standpoint, having a separate coil for each cylinder gives each coil more time to recharge between cylinder firings. With single coil distributor systems, the coil must fire twice every revolution of the crankshaft in a four cylinder engine, and four times in a V8.

With a multi-coil system, each coil only has to fire once every ever revolution of the crankshaft. This provides more saturation time for a hotter spark, especially at higher rpm when firing times are greatly reduced. The result is fewer misfires, cleaner combustion and better fuel economy.

According to the original equipment supplies who make multi-coil ignition systems, having a separate coil for each cylinder also improves the engine's ability to handle more exhaust gas recirculation to reduce oxides of nitrogen emissions (important with today's low emission vehicle standards).

A hotter spark also makes spark plugs more resistant to fouling and helps 100,000 mile plugs go the distance. A multi-coil ignition system also improves idle stability and idle emissions, too.

The typical multiple coil ignition system may have one of several different configurations. On Chrysler, Toyota and many other imports, the coils are mounted directly over the spark plugs. Many of these are the thin "pencil" style coils that extend down into recessed wells in the engine's valve covers.

On other applications, such as GM's Quad 2.2L Four, the individual coils are mounted in a cassette or carrier that positions the coils over the spark plugs.

On late model Corvette, Camaro and other V8s, a Coil-Near-Plug (CNP) setup is used because the spark plugs protrude from the side of the cylinder head and there isn't room to mount a coil on the end of each plug. Here, the individual coils are mounted on the valve cover and attached to the plugs by short plug wires.



When a coil failure occurs on a distributor ignition system, it affects all the cylinders. The engine may not start or it may misfire badly when under load. But with multi-coil ignition systems, a single coil failure will only affect one cylinder (or paired cylinders in the case of waste spark DIS systems).

On 1996 and newer vehicles, the OBD II system should detect coil problems as well as misfires and generate a fault codes that identify the problem coil or cylinder. A misfire code P0301, for example, would indicate a misfire problem in cylinder #1. Of course, misfires can be caused by a lot of things.

It could be a worn or fouled spark plug, a weak coil, a bad plug wire or connection in the case of a DIS or CNP system, a dirty or dead fuel injector, or a loss of compression (burned exhaust valve or leaky head gasket).

Further diagnosis is always needed to isolate and identify the cause -- which creates a problem on multi-coil systems that do not have spark plug wires because you can't observe the secondary ignition pattern unless you use some type of adapters or inductive pickups that fit on the coils themselves.



Snap-On offers a number of inductive pickup adapters that can be attached directly to the coils on various COP systems to gather secondary ignition information.

Most of these adapters cost less than $50 each and allow you to use a Snap-On kV Module to observe secondary ignition data for each coil.

In most applications, the coils do not have to be removed to connect the adapter (it fits over the top of the coil and uses induction to pick up coil voltage).

COP adapters are available for various BMW models, Chrysler 2.7L, 3.2L and 3.5L engines (Dodge Intrepid, Chrysler Concorde LHS and 300M), Ford 3.4L Taurus SHO, 4.6L Town Car and Mark VIII, Mustang, Crown Vic and Grand Marquis, and F-Series and E-Series trucks with 5.4L and 6.8L engines, Acura SLX, Honda Passport, Isuzu Amigo, Rodeo and Trooper,
Mercedes with M112 and M113 engines, Toyota and Lexus with with 1UZ-FE and 2UZ-FE engines, Audi A4 1.8L turbo and A8 4.2L, Volkswagen Passat 1.8L turbo, Volvo 960 and 9000.

Another handy tool that can be used to quickly find a dead or misbehaving coil is Waekon's Coil On Plug Ignition Quick Probe (WAE76560).

This hand-held tool sells for less than $100 and is simple to use. It has an inductive paddle that is placed over the coil to detect coil activity.

A super bright LED strobe flashes every time the coil fires and produces sufficient kV. A green indicator LED also flashes when the presence of adequate spark duration is detected. This tool eliminates the need to back-probe connectors and to disassemble and test each coil at its connectors.

Another tool worth considering is Ferret's FER72 Primary Ignition Probe Inductive Power.

This tool, which costs less than $150, has a clamp-style inductive pickup that can be used on the spark plug wires of coil near plug, DIS and distributor ignition systems.

The pickup allows coils to be tested without piercing wires or backprobing connectors.

The tool has a 20 segment bar scale that displays peak amps (amps used to drive the coil or module), build time (time it takes the amperage to reach its maximum) or drive time (on time of the module) with the press of a button. It uses "current ramping" technology to detect problems in ignition coils and modules.

The ignition signal from the inductive pickup can also be outed through a BNC connector to a lab scope or graphing multimeter to display amperage waveforms.



Located in the cylinder head, there is one spark plug for each cylinder (though a few engines use two per cylinder).

Spark plugs provide a spark to ignite the air/fuel mixture in each combustion chamber. Each spark plug has a ceramic insulator around a conductive core.

At the firing end of the plug is an electrode gap across which voltage jumps to create a spark.

Plugs have different diameters, thread pitches and lengths depending on the engine application. The "heat range" (operating temperature) of the plugs will also vary depending on the application, as will the electrode gap.

Many spark plugs have a "copper core" to extend the heat range for improved fouling resistance, and many have platinum or iridium electrodes to reduce wear and extend service life. Every time a spark plug fires, the hot spark blasts a few molecules of metal off the electrodes.

As the miles add up, the electrode gap widens and the center electrode becomes rounded and dull. This increases the firing voltage needed to jump the gap.

Eventually the point it reached where the ignition system can't generate enough juice to jump the gap causing the plug to misfire.

When platinum or iridium is used for the electrode(s), wear is greatly reduced. Most platinum plugs can go up to 100,000 miles before they have to be replaced.

The same is true for plugs that use other exotic metals such as iridium for their center electrode. Plugs with platinum on both electrodes ("double" platinum plugs) experience even less wear than plugs with only a single platinum or platinum-tipped electrode.

Long life spark plugs drastically reduce the need for maintenance while helping the engine maintain like-new performance and emissions.

The recommended replacement interval for standard spark plugs is usually 45,000 miles, but for platinum plugs it can be up to 100,000 miles, and up to 120,000 miles for some iridium plugs.

Extended life plugs and performance plugs with special electrode configurations (surface gap, U-gap, fluted center electrode or split outer electrode) are designed to reduce misfiring and may be a good upgrade option when the spark plugs are replaced.



Plug wires carry high voltage from the ignition coil(s) to the spark plugs. Ignition cables come in various types (suppression and solid core, silicone or EPDM insulated, etc.) as well as different lengths and diameters (7 and 8 mm).

Replacement cables must be the same size and length as the original. Plug wires may be replaced individually or in complete sets (wires should be changed one at a time to avoid mixing up the firing order).

Replacement is needed if internal resistance in the wires exceeds specifications, the wiring is damaged (cracked or burned insulation, or visible arcing or misfiring when the engine is running), or the plug boots or terminals fit poorly or are loose.



The distributor, cap & rotor route high voltage from the coil to the individual spark plugs.

On most applications, the ignition module is mounted inside or on the distributor. Replacement of the cap and rotor is needed if cracks, burns, carbon tracks or pitting are present.

The distributor itself may also have to be replaced if the shaft bushings are worn.

Many older distributors also contain a mechanism for advancing ignition timing as engine speed changes (improves fuel economy and performance).

If the distributor needs to be replaced, the replacement unit must be calibrated the same as the original.



A module is used in electronic ignitions to switch the ignition coil on and off. The module contains a switching transistor and circuitry that picks up ignition timing pulses from a sensor inside the distributor.

The module is usually located inside the distributor (GM, Chrysler & most imports) or on the outside of the distributor housing (Ford) of older engines.

On distributorless ignitions, the module is usually part of the coil pack and receives its ignition timing pulses from a crankshaft position sensor.

On coil-on-plug ignition systems, the module may be a separate component or integrated into the PCM itself.



The crankshaft position sensor generates a timing signal for the ignition system by reading notches in the crankshaft balancer, flywheel or crankshaft.

Two types are used: magnetic and Hall effect. Magnetic sensors produce an alternating current (AC) signal that changes in frequency and amplitude with rpm. Hall effect switches produce an on-off digital signal. The powertrain control module (PCM) may modify the timing signal from the sensor to advance or retard ignition timing.

Loss of the sensor signal will prevent the engine from starting (no spark).

A "cranks but won't start" condition may be due to a bad crankshaft (or camshaft) position sensor. If the sensor is shorted, open or out of range, the PCM can't get a good crank position signal to trigger the ignition coils.

NOTE: A magnetic crankshaft position sensor can be checked with an ohmmeter. Replace the sensor if the resistance reading is out of range (typically around 700 ohms).

ALSO NOTE: Some crank position sensors may test okay at room temperature, but go open or short when they get hot. An intermittent cranks but won't start condition may be caused by this kind of problem.



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