How to Test Coil Wire

by Darla Ferrara
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The ignition coil wire sits in the ignition distributor of your car and transforms the battery’s voltage to the voltage needed to stimulate your spark plugs. The voltage from a battery is only 12 volts. A spark plug needs thousands of volts to have enough energy to turn your engine over. A faulty coil wire will keep your car from starting when you need it. A few tests will tell you if the coil is doing its job. However, you should have some experience working with car engines before attempting to deal with a coil wire. Testing the coil wire is an excellent way to avoid problems down the road.

Shadetree Test (Coil Wire is Still Installed)

Step 1

Remove one spark plug using a spark plug socket.

Step 2

Hold the spark plug with insulated pliers and insert it into the coil wire.

Step 3

Lay the solid end of the plug against a piece of exposed metal. This will serve as a ground. An exposed screw or even the negative post on your battery will work fine.

Have a second person start the car. Look for a blue spark from the plug. If you receive a nice, bright spark, your coil is fine. No spark indicates a bad coil.

Bench Test (Coil Wire is Uninstalled)

Step 1

Connect your ohmmeter or multimeter to the primary studs of the coil. When you look at the coil, the primary studs will be sticking out the top and look something like two bolts or poles. Enclosed units will have a diagram pointing out the primary studs.

Step 2

Make note of the reading on the ohmmeter. It should be within the range indicated in your service manual. Anything below that number proves a deficient coil.

Step 3

Connect the ohmmeter to the secondary winding. Attach the probes to the outer 12V pole and the center pole.

Check to see if the reading is within the specs listed in your service manual for the secondary winding. If not, the coil is bad.

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