Ignition Coil – Function – Coil Types – Failure Symptoms – Testing

Ignition Coil - Function - Coil Types - Failure Symptoms - Testing
Ignition Coil - Function - Coil Types - Failure Symptoms - Testing

The function of the ignition coil, is to produce a high voltage from a low voltage.

The high voltage from your ignition coil, is needed to ignite the fuel and start the engine.

So, for an ignition coil to work it has to have battery voltage at its positive terminal. Also, it needs to be grounded on and off by the ignition module or circuit. But, if it is not producing a spark the coil is defective and needs to be replaced.

So, the ignition coil is very rugged and reliable. But, it can fail for a variety of reasons. Heat and vibration can damage the coil windings and insulation; causing shorts or opens in the primary or secondary windings. But, the number one killer of ignition coils is, voltage overload. Consequently, caused by, bad spark plugs or plug wires.

Ignition Coil Types
Ignition Coil Types

So, inside the ignition coil are two sets of windings. The primary coil windings, containing hundreds of turns of heavy wire. While the secondary side, contains thousands of turns of fine wire. In older vehicles, a single coil would service all the spark plugs and use a distributor. In modern systems, the distributor is omitted. Because, the ignition coil is now electronically controlled. But, with all this new technology come new issue to solve.

Ignition Coil Failure Symptoms

The severity of the symptoms vary, according to which ignition coil is failing:
  • Backfiring
  • Fuel Economy
  • Vehicle Stalling
  • Engine Jerking, Rough idling, Poor Power
  • Engine does not start
  • Vehicle misfires
  • Poor acceleration or loss of power
  • Engine control unit switches to limp-home mode
  • Engine fault codes

If the coil malfunction is causing misfiring, this will allow raw fuel to enter; permanently damaging the catalytic converter.

Catalytic Converter
Catalytic Converter

Take steps to check for oil leaks, moisture and spark plug issues; to prevent serious engine damage.

So, the most likely cause, is a oil leak from the valve cover gasket. On many (COP) style engines, the spark plug and ignition coil are mounted; inside a spark plug tube. This tube is sealed in place around part of the valve cover.

Oil Leaking Into Spark Plug Tube
Oil Leaking Into Spark Plug Tube

Overtime the seal between the valve cover and that spark plug tube can fail. As a result, causing oil to leak in and fill up around the spark plug and ignition coil. This in turn, can lead to spark plug and ignition coil failure.

Ignition Coil Types

There are basically four main types of ignition coils;


Conventional Ignition Coil
Conventional Ignition Coil

So, with the conventional breaker point-type ignition system; the primary circuit receives power from the battery through a resistor. Current flows through the windings of the primary coil, creating a magnetic field. When the points are opened, the current’s electrical circuit is broken. As a result, collapsing the magnetic field.

The force from the collapse, crosses the windings of the secondary coil and creates an electrical current within them.

The current flows into the distributor cap and eventually into the spark plugs; all in a split second. But, these early fully mechanical distributor systems had their shortcomings. The ignition points would break down and change spark timing, messing up engine efficiency. As a result, requiring replacement, as often as every 12,000 miles.


Electronic Ignition Coil
Electronic Ignition Coil

So, this type of ignition is very similar to the conventional system. But, instead of a distributor cam and points; the electronic system uses a pickup coil to signal the control module.

Doing so increased reliability. But, the solid-state switches, still took their marching orders from the distributor shaft.

Distributor shafts would tend to develop a certain amount of “lash” or slop after 120,000 or so miles. Since gear wear would always be an impediment to proper spark timing, mechanical ignition systems had to evolve.

Distributor-less (DIS)

Distributor-less Ignition Coil (DIS)
Distributor-less Ignition Coil (DIS)

So, in a distributorless ignition system (DIS) its design allowed, more energy to be available from multiple coils. Also, there are three or more mounted together in a coil pack.

This system uses a magnetic triggering device to determine engine speed and crankshaft position. This system determines spark timing based on, two shaft position sensors and a computer.

The Crankshaft Position Sensor (CKP) and the Camshaft Position Sensor (CMP). These sensors continually monitor both shafts’ positions and feed that information into a computer.

Coil-on-Plug (COP)

Coil-on-Plug Ignition Coil (COP)
Coil-on-Plug Ignition Coil (COP)

So, the coil-on-plug (COP) ignition system incorporates; all the electronic controls found in a (DIS) ignition system. But, instead of two cylinders sharing a single coil, each (COP) coil services just one cylinder.

As a result, some (COP) ignition systems generate, as much as 50,000 volts and much hotter, stronger sparks.

The coil mounts directly on top of the spark plug. As a result, the need for spark plug wires is no longer an issue.

Finally, The Top Failure Reasons:

  • Bad spark plugs
  • Bad plug wires
  • Voltage overload

Testing For Spark

So, never pull off a plug wire or the coil’s high voltage output wire to test for a spark. Besides risking a severe shock an open plug wire or coil wire; will increase the voltage demands on the coil to the point where it may damage the coil. The only safe way to test for spark is to use, a spark plug tester tool.


So, heat and vibration can damage the coil windings and insulation. As a result, causing shorts or open circuits in the primary or secondary windings. Finally, new spark plugs will help ignition coils last longer.

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