A bad ignition coil is one of many things that can cause your car to run poorly. It helps to know what an ignition coil does, what the signs are that it is going bad, how to test the coil and how to replace it if it's needed.
What an Ignition Coil Does
Your car has spark plugs that ignite the fuel in the cylinders and propel your car forward. These simple spark plugs rely on a more complex ignition system to fire them at the right time and with the proper intensity. The ignition coil takes power from the battery and increases the voltage to a level high enough to create a spark in the plugs.
How an Ignition Coil Works
Ignition coils are made up of two parts. The first part is called the primary coil, and the second is called the secondary coil. The primary coil has fewer windings than the secondary coil. Current running from the battery into the primary coil creates a magnetic field. The magnetic field causes a current to flow in the secondary coil. Because the secondary coil has more windings, the voltage in the secondary coil is much higher than that of the primary coil.
Symptoms Of a Bad Coil
An ignition coil that is completely dead will prevent the engine from starting or will stop the engine if the car is running at the time of failure. This is because no current will reach the spark plugs. An ignition coil that is failing will still provide current to the plugs, but it might not be enough for a full spark and the car will run sluggishly. If the current being provided by the coil isn't consistent, the car will run rough.
Testing a Coil
Disconnect the battery and remove the coil before testing it. The repair manual for your make and model will give you the resistance specifications for your coil. To test the resistance of the primary coil, touch the leads of a multimeter to the smaller poles on the outside of the coil. If the reading matches up with the specs in your repair manual, the primary coil is functioning properly. To check the secondary coil, keep one of the leads attached to the 12-volt pole and attach the other to the big pole in the middle. Compare the results with the specifications in your manual.
Replacing a Coil
When placing a new coil in the car, make sure the battery is disconnected. Bolt the new coil into the place of the dead coil. Replace the primary ignition wires. Add a little grease to the coil wire boot before plugging it into the coil. You are now ready to reconnect the battery and start the car.