How To Diagnose A Failing Ignition Coil

A malfunctioning ignition coil causes engine performance issues and a reduction in gas mileage. It can be a pretty serious problem.

The ignition coil’s primary job is to help the spark plugs fire up the engine and keep the engine running. So when one of your ignition coils stops working, your engine will no longer run well. You’ll experience some (or all) of the following issues:

  • Misfires
  • Rough idle
  • Loss in power
  • Sluggish acceleration
  • Lower gas mileage

The best way to restore your engine’s performance is to replace the malfunctioning ignition coil(s). First, you need to confirm that your Ford has at least one bad ignition coil. You also want to pinpoint which ignition coil(s) is bad. We'll show you how to do this.

Why Does An Ignition Coil Fail?

Ford ignition coil

Image Credit: 2CarPros

Under normal circumstances, ignition coils last about 100,000 miles. Sometimes an ignition coil can fail early. Here are some of the most common factors that can lead to this happening:

  • Excessive heat and moisture can damage the copper wire inside the ignition coil
  • Leaking oil or other fluids can short out an ignition coil
  • Excessive heat and vibration can break down the insulating material
  • Worn secondary ignition components (like spark plugs) can cause an ignition coil to wear out faster

How To Test Your Ignition Coils

test coils

Image Credit: 2CarPros

If you’re wondering how to test an ignition coil, read on to find out how.

You need to test the resistance across the terminals on each ignition coil. The only tools you’ll need are:

  • Digital ohmmeter (or digital multimeter). You can get one for about $10-$20 from a big box store like Walmart or an online retailer.
  • Wrench set.

Here are the steps to take:

1. Find Out The Resistance Range Of The Primary Windings And Secondary Windings

Resistance ranges vary between ignition coil types and manufacturers. So it’s best to find the specific ranges for your ignition coils. If you’re using OEM ignition coils, you can check the service manual. That, or you can call your nearest Ford dealer service department.

2. Disconnect The Negative Battery Cable From The Battery

Be sure to isolate the battery cable after it’s disconnected. This ensures that the cable doesn’t come in contact with the battery.

3. Remove The Ignition Coil

Find one of the ignition coils in your engine. Next, remove the retaining nuts. Disconnect all the wires attached to the side terminals on the coil.

4. Check The Primary Resistance

Here’s how to find out if it’s a bad ignition coil: check the primary winding resistance and secondary winding resistance. To check the primary resistance:

  1. Place the test leads from the ohmmeter on the side terminals (one on each side)
  2. Note the reading on the meter

Does the reading fall within the normal range? If so, the primary resistance is still good. It doesn’t mean that the ignition coil is good, though. There’s a chance that the secondary resistance is bad.

5. Check The Secondary Resistance

To do this:

  1. Remove the test lead from the negative post on the ignition coil
  2. Connect the test lead to the center terminal
  3. Be sure to leave the other lead on the positive post of the coil
  4. Note the reading on the meter

Does the reading fall within the normal range? If it’s outside the range, the coil is defective. In that case, you need to replace it.

6. Repeat The Above Steps With The Rest Of The Ignition Coils In Your Engine

If an ignition coil looks good, reinstall it in the reverse order of removal. If an ignition coil is defective, get it replaced ASAP.

7. Put Everything Back Together

When you’re done testing the ignition coils, reinstall the negative battery cable.

Do You Need To Replace Your Ignition Coils?

OEM ford coil

The good news is that it’s quite easy to replace the ignition coils in your Ford engine. You only need some basic tools and 10-20 minutes of your time. Of course, you also need replacement ignition coils. In that case, we can help you save money on OEM ignition coils!

We offer wholesale pricing for genuine OEM Ford parts, including ignition coils. Visit our catalog of ignition coils and look up your Ford model. You’ll get the right part number. You’ll also get to see how much money you can save by ordering from us!