In a vehicle, an ignition coil converts the 12 volts of battery power to the thousands of volts needed to ignite the spark plugs. This part is essentially an induction coil and simply a high-voltage transformer. Although the ignition coils are generally very strong and reliable, over time, due to heat, vibrations and faulty insulation, they can be damaged.
The ignition coil of your vehicle can be tested in two ways: the spark plug test and the bench test. While the spark plug test is effective, the bench test is more complete. Why? In the first, rely on the spark to
determine the condition of your coil; in fact, only in a non-spark condition would you be sure that your coil is defective. In the second, it relies on resistance readings and data to test the status of the ignition coil. Here, even if there is only slight damage to your coil, your readings will indicate it.

Read on for proper instructions on how to perform these tests so you can be sure your coil needs to be replaced.

1.
Take precautions

When testing vehicle parts, particularly the engine, you must be extremely careful. Before starting, be sure to put on safety glasses, that you are not wearing loose clothing and that your hair, if it is long, is properly pulled back. In addition, the motor of an automobile produces electricity, so you will have to take all precautions to avoid unfortunate incidents.

2.
Detach the spark plug

For the spark plug test, start by removing the spark plug wire. It is advisable to consult the service manual of your vehicle to make sure you are removing the correct cables. Then, with the help of a spark plug socket, detach the spark plug.

If you are performing the bench test, refer to the service manual and remove the two windings (primary and secondary) that are inside your ignition coil.

3.
Inspect for sparks (spark plug test)

After removing the spark plug, restore the spark plug wire. Now, hold the spark plug wire with insulated pliers and let the naked threaded end of the spark plug touch a grounded surface (any exposed metal area, such as a bolt). Ask for help to start the ignition in the meantime. With the key twisted, you should be able to see a bright blue spark at the end of the spark plug. If you do, your coil is fine. If it does not, its coil needs to be replaced. However, even a coil that malfunctions can give a small spark; Therefore, to be careful, you need the bench test.

4.
Check the primary and secondary windings (bench test)

Consult the service manual to obtain the correct resistance readings applicable to your vehicle and model. In general, for most automotive coils, a reading of 0.75 to 0.81 ohms for the primary winding and 10,000 to 11,000 ohms for the secondary winding is correct. To check the resistance, connect the multimeter / ohmmeter cables to the two outer poles in the primary winding. In the secondary winding, connect one cable to either of the side poles and the other to the central high-voltage terminal. If the readings are even slightly out of the resistance indicated in your service manual, replace your ignition coil.

NOTABLE POINTS
Please follow the useful tips above and get effective result, if this is done periodically then you can be sure of a long lasting vehicle.
[img]https://2.bp..com/-Ty5pAPCgB6A/XLpUMS7ZxfI/AAAAAAAACOI/54dfjo5APL08OR7hDZDU2EWOusD4ddbJgCLcBGAs/s320/Ignition%2Bcoil.jpg[/img]
Source:
http://www.eliancars.com/2019/04/how-to-test-ignition-coil-in-vehicle.html#more


cc: Lalasticlala, Marpol

Inside every ignition coil are two sets of windings around a laminated or segmented iron core. The "primary" windings, which number a few hundred, are connected to the two external low voltage terminals on the coil. The positive (+) primary terminal connects to the ignition switch and battery while the negative (-) primary terminal connects to the ignition module which provides ground. The "secondary" windings, which have thousands of turns, are connected at one end to the primary positive terminal and the high voltage secondary output terminal in the center of the coil at the other end.
Ignition coils are very rugged and reliable, but can fail for a variety of reasons. Heat and vibration can damage the coil's windings and insulation causing shorts or opens in the primary or secondary windings. But the number one killer of ignition coils is voltage overload caused by bad spark plugs or plug wires.

If a spark plug or plug wire is open or has excessive resistance, the ignition coil's output voltage can rise to the point where it burns through the coil's internal insulation causing a short. The insulation in many coils can be damaged if output exceeds 35,000 volts. Once this happens, the coil's output voltage may drop causing ignition misfire when the engine is under load, or the coil may cease to put out any voltage preventing the engine from starting or running.

If a coil has battery voltage at its positive terminal and is being grounded on and off by the ignition module or circuit but is not producing a spark, the coil is defective and needs to be replaced.