Test an Ignition Coil

Test an ignition coil at home to narrow down the culprits behind its problems. Ignition coils have gone through a myriad of transitions over the years as to size, location, form and accessibility. They all have one thing in common: internal structure and function. They differ in resistance readings due to temperature and amount of coil windings relative to output voltage. For this reason, these methods will be for the determination of functionality in real time as to a failure of the unit.

For intermittent faults, a service manual will be needed for the particular engine. The service manual will give the maximum voltage output, ohm readings for internal resistance and the method and location of the proper terminals with which to access the positive and negative terminals for the coil.

There is one common factor in all coils: They all need to have power to the positive battery terminal, a negative terminal and a coil tower or output terminal. The negative terminal can be controlled by a computer and ignition control module, an ignition igniter or a hall effect sensor and control module. These also have one thing in common: They must ground the system each time the coil is to be fired, causing oscillations which can be witnessed on a voltmeter or scope.

Disconnect one of the easiest spark plug wires to reach and push an extra spark plug in the wire end. Lay the plug on a good ground on the engine. If the engine uses a coil on plug arrangement, remove the two 10mm bolts securing the coil to the valve cover and pull the coil up and off the plug. Install a spark plug and lay it on a good ground.

Start the engine and observe the spark at the plug's electrode. If the spark is weak, inconsistent or nonexistent, shut the engine off.

Check the coil for power at the positive terminal. Turn the key on with the engine off. If the coil is exposed, check the positive terminal with a voltmeter. There should be battery voltage on this terminal. If the coil has an electrical connector or sits on an ignition control module, disconnect the electrical connector and check the terminals for battery voltage. If there is no voltage, check the fuse for the ignition. If there is battery voltage, shut the ignition key off.

Check the negative terminal for fluctuations on the voltmeter indicating that the computer, ignition control module or igniter is functioning. If the voltmeter shows a rapid rise and fall in voltage, the coil is faulty. If this is not observed, there is a problem elsewhere, either in one of the above control devices or connectors.

A good way to search for intermittent coil misfires is to connect an inline spark plug light. This is a tool that consists of a light that is connected between the spark plug wire or end and the spark plug. As the engine runs the light can be observed blinking with every spark plug firing. A miss can be easily seen by a break in the frequency. This way, the miss can be contributed to the coil or ignition. If all the plugs are checked and no break in frequency is observed, the misfire is other than ignition. These ignition tester lights can be purchased at any auto parts and are very inexpensive. They are a great tool for expediency.  

To confirm the failure of the coil as opposed to a bad connector or wiring, a service manual will be needed to get the proper resistance factors for the particular coil voltage and temperature conversions.

Test the coil with an ohmmeter by connecting one lead to the battery+ and one to the negative terminal. There should be very little resistance. On standard coils, there should be about .02-.025 ohms. Remember these resistances are very close but approximate. These approximations are going to be close enough that if the readings are far distant from these, the service manual won't be needed--the coil is bad. Now take one of the leads from the ohmmeter and touch the primary terminal in the coil tower. The reading from either the positive or negative terminal to the coil tower should be from 7,500 ohms to 18,000 ohms.

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