How to Test a Coil Pack

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The coil pack is important in the process of starting your car and keeping the engine running smoothly. The coil pack produces the high voltage needed for the spark plug to create a spark and start the process of combustion. It receives its signal from the car's computer and transmits the voltage through the spark plug wires. When it does not work properly, your car's performance suffers. In order to get an accurate diagnosis to make car repairs, it is important to test the coil pack to see if it is indeed the problem. To test a coil pack, identify the symptoms of a bad coil pack, check for a spark, test the resistance, and test for the switching signal.

Symptoms of a Faulty Coil Pack

If the coil pack is bad, then your car might not be performing properly. Usually, an engine that is misfiring is a sign that the coil pack is bad. However, the symptoms of a faulty coil pack often mimic those of a faulty spark plug. Other symptoms include the car engine idling roughly or operating louder than usual. Additionally, a lack of power or significant drop in RPMs when accelerating can also be from a faulty coil pack. Lastly, if your fuel light comes on when you have plenty of gas or if your check engine light blinks, you should test your coil pack to ensure it is working.

Remove the Spark Plug Wires

To test the coil pack, you first need to gain access to it. Turn off the car's engine and open the hood. Because the test takes place on the car, exercise caution when working with wires and hot parts. Locate the spark plugs and follow the spark plug wires until you locate the coil pack. Depending on the make and model of your car, the coil pack might be at the side or the back of the engine. Remove the spark plug wires and label them to make it easy to install them again.

Check for a Spark

To check for the spark, you need the help of an assistant. Attach a spark tester to the spark plug and ask your assistant to crank the engine. While he is doing so, check to make sure that the spark jumps from the center electrode to the metal casing of the spark tester. Do this for all of the towers on the coil pack. If you observe a spark on all of the towers, then the coil pack is good and not likely to be the source of your problem. If you do not observe sparks on all of the towers, then the coil pack may be bad, and you need to test the coil pack further to be sure.

Test the Resistance on Each Coil

Unplug the coil pack's electrical connector and use a wrench to remove the coil pack from the engine. Set the ohmmeter to the 20,000 ohms range and turn it on. With the meter leads, touch the spark plug wire terminal on each coil. This measures the secondary resistance, which should be between 5,000 ohms and 15,000 ohms. Next, check the primary resistance for each coil. Set the ohmmeter to the 10 ohms range. Touch one of the meter leads to the center prong on the electrical connector of the coil pack. Touch the other lead to the corresponding prong on the electrical connector. The reading should be between 0.3 and 1.0 ohms. Repeat the process for each coil.

Test for the Switching Signal

Before replacing the coil pack, check the switching signal. First, use a multimeter to make sure that the coil pack is receiving 12 volts with the engine cranked. If it is, make sure that it receives the switching signal from the control module. To do this, use a LED light. Disconnect the coil pack from its connector. Insert the red lead of the LED into the power circuit connector and the black lead of the LED into one of the other terminals. Ask your assistant to crank the engine again. Repeat the process with the other terminal. If the LED flashes in both circuits, then your coil pack is bad.

How to Buy Coil Packs on eBay

Find great deals on new coil packs for your vehicle by shopping on eBay. Additionally, find a large selection of tools to help you complete any auto project. Use the search bar on any eBay page to perform a basic or advanced search. Select options from the refinement menu to narrow down the list to a more manageable number of results. Compare listings and choose the items you wish to purchase.

A faulty coil pack can greatly affect the performance of your car. Because the symptoms often mimic those of faulty spark plugs, it is important to test the coil pack in order to get an accurate diagnosis.