How To Diagnose A Failing Ignition Coil

An ignition coil is a tube-shaped part that connects to a spark plug. It’s responsible for delivering high voltage current to the spark plug. It may be a small part, but it’s a key player in your Volkswagen’s engine. In fact, it’s such an important part that your engine can’t start or run without a set of coils.

Ignition coil location

Ignition coils don’t last forever. Most of them start to fail after 100K miles. If you have a sneaking suspicion that one of your ignition coils is on the fritz, we’ll help you confirm it.

Symptoms To Look For

A failing ignition coil can prevent the engine from starting or running properly. If this goes on long enough, it will cause other problems with the engine. So it’s always best to replace a failing ignition coil ASAP. If you notice two or more of the following symptoms, one of your ignition coils could be bad:

  • Engine not starting
  • Engine stalling
  • Check engine light being on
  • Decreased fuel economy
  • Misfiring/backfiring
  • Poor catalytic converter performance

Getting A Diagnosis

If you have a voltmeter, you don’t have to pay a shop to diagnose the issue for you. It’s easy to do it at home with this tutorial.

Accessing The Ignition Coil

VW coil

Usually, there's one ignition coil for each cylinder. You want to test each one. To access each ignition coil:

  • Open the hood.
  • Remove the plastic engine cover.
  • Disconnect the electrical connector at the ignition coil.
  • Remove the bolts holding down the ignition coil.
  • Remove the coil.
  • Test the voltage of the ignition coil (see the next 2 sections).
  • Repeat with the rest of the ignition coils in your engine.

Testing The Primary Winding

Coil winding

Each ignition coil has two separate coils wrapped around each other:

  1. The primary winding: a heavy wire that receives energy from the battery
  2. The secondary winding: a thin wire that receives energy from the primary winding and then sends it to the spark plug

You want to test the primary winding first. Here's how:

  1. Check the service manual to see what the correct resistance reading should be.
  2. Connect the ohmmeter's positive and negative leads to the corresponding terminals at the electrical connector. Sometimes these are marked with a + and -. If not, refer to the service manual.
  3. Check the reading on the ohmmeter. Compare the reading to the resistance specs for the primary winding.
    1. If the reading is within normal resistance range, the primary winding is fine. You still need to test the secondary winding, though.
    2. If the reading is zero or close to zero, meaning no resistance, there's a short inside the coil. In this case, you need to replace the ignition coil.
    3. If the reading falls on the higher side outside the normal resistance range, the coil is open. In this case, you need to replace the ignition coil.

Testing The Secondary Winding

The steps for this test are similar to the steps for testing the primary winding:

  1. Connect the ohmmeter's positive lead to the positive terminal on the electrical connector. Connect the negative lead to the center pole in the coil. The center pole is what touches the top of the spark plug.
  2. Check the reading on the ohmmeter. Compare the reading to the resistance specs for the secondary winding.
    1. If the reading is within normal resistance range, the ignition coil is fine. This only applies when the primary winding's reading looks good, too.
    2. If the reading is zero or close to zero, there's a short inside the coil. In this case, you need to replace the ignition coil.
    3. If the reading falls on the higher side outside the normal resistance range, the coil is open. In this case, you need to replace the ignition coil.

If you've determined that at least one of your ignition coils is bad, it's time to replace them. It's best to replace them all at the same time. If one has failed, the others usually will fail soon. You only have to put new coils in instead of putting the bad ones back in.

Do you have any questions about diagnosing a failing ignition coil? We can help you! You can call us or fill out this contact form. We'll get back to you as soon as possible.