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The steps for testing coil on plug with multimeter

The steps for testing coil on plug with multimeter

In this article, we will present the steps for testing coil on plug with a multimeter. We will also provide a detailed account of the coil on plug ignition system, and how you can use a multimeter to test the COP.

What is a coil-on-plug

The coil on plug ignition system (COP) is a component of a car’s engine system that consists of a coil of wires covering a plug that is attached directly to the top of the spark plug. You do not need distributors and spark plug wires. That is why it is also called the direct ignition system.

The ignition that is followed by the spark plug is controlled by the Engine Control Unit (ECU) or Engine Control Module (ECM) and is based on inputs from different sensors.

The COP improves the spark timing tremendously by making it accurate. COP is a system which uses redesigned ignition coils and is capable of producing higher voltage levels. Because these plugs are firmly attached to the top of the spark plug, the ignition coils are not affected by the exhaust fumes. But this makes these coils on plug difficult to handle and expensive to repair. However, there are no moving parts, so the coils are easy to maintain, and there is no need to replace them frequently.

With this direct ignition system or COP, each spark plug is ignited separately by its own coil. Therefore, the Powertrain Control Module (PCM) or the ECU can itself control the spark plug and the ignition timing, which means each individual cylinder can be used separately.

The multimeter

Multimeters or digital multimeters are instruments that are used to measure two or more electrical standards, i.e. voltage, current, and resistance.

There used to be analog meters that had needles that moved left or right to show how low or high the values were. With digital multimeters, the reading on the instruments is now much more accurate and reliable. It can also show impedance – the resistance and reactance with the electric flow.

Testing coil on plug with multimeter

If there is a problem in your car, use the digital multimeter to check if the coil on the plug ignition system is working. And you do not have to go to a mechanic to get it done. You can do it yourself.

You will need some tools, including a multimeter, wrench, racket, socket, heat gun, test spark plug, and a test light. First of all, you have to test the primary resistance in the coils. Then test the secondary resistance of the coils. Check for any voltage leakages or other faults.

Testing the coil on the plug with a multimeter

It’s time to get to the coil. The service manual will tell you how much primary and secondary resistance the coil should have. The coil will consist of two wires winding around each other.

Each wire has a different resistance force. A repair/service manual will tell you the values. If you do not have your car’s service manual, buy one for your make and model. Don’t do any testing coil on plug with multimeter without the right service/repair manual.

How to unplug the coil from the spark plug

Disconnect the cord from the power source/battery. Unscrew all bolts that connect the coil to the engine. First, check the coil/cord for dirt, cracks or faults of any kind. You must also check the plug ends.

Now attach one wire at a time to the multimeter. Your manual will tell you if the longer wire has the primary resistance or not. And the multimeter will show you a reading in Ohms when it sends a current through the wire. Do ensure that both ends are attached to the multimeter.

Repeat the same process with the second wire. You will get the reading for secondary resistance.


A common primary resistance should range from 0.3 to 1.0 Ohms. If the resistance is less, then only the coil has to be changed. But if the reading is exceeding the standard in the manual, then you have to change the ignition coil altogether.  If there is no limit to the resistance, then the cord is badly damaged and must be changed.

You must select the right instrument size for testing coil on plug with multimeter, in accordance  with your car service manual.

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