How to Test an Ignition Coil

Co-authored by Rocco Lovetere

The ignition coil, a vital component of any vehicle's ignition system, is responsible for providing electricity to the spark plugs. When a vehicle will not start, misses often or stalls frequently, its ignition coil may need replacement. Luckily, a relatively quick, simple test can determine whether the ignition coil is functioning properly and thus whether a trip to the auto parts store or mechanic's garage is warranted. See Step 1 below to get started!

Method 1

Method 1 of 2:

Performing an Ignition Coil Spark Test

1
Turn the vehicle off and open the hood. As with most types of vehicular maintenance, you'll want to begin the test with the vehicle in park and with the engine off. Open the hood to locate the ignition coil. Though its precise location may vary from vehicle to vehicle, generally, it is located near the fender or bolted to a bracket fairly close to the distributor. Note that in vehicles without a distributor, the spark plugs will be connected directly to the coil.
- One sure-fire way to find the ignition coil is to locate the distributor and follow the wire that does not connect to any spark plug.
- Before beginning, it's very wise to ensure you're wearing safety goggles or other eye protection and that you have access to insulated tools (especially pliers) to protect from electric shock.
2
Remove one spark plug wire from its plug. Next, remove one of the spark plugs' wires from the plug itself. Usually, these wires run from the distributor cap to each of the spark plugs individually. To prevent injury, be very careful when working with your vehicle's electrical system - use gloves and insulated tools at all times.
- If your vehicle has been running for a while, its internal components are likely to be very hot. A car that has been driven for as little as 15 minutes can heat the engine to around 200 degrees. Allow the car to sit and cool for an hour to prevent significant injury.
- To save time and avoid potentially damaging your spark plug, consider using a spark plug tester instead. Instead of attaching the spark plug back to the wire, attach the spark plug tester to the wire. Ground the alligator clip. Then skip ahead and have your friend crank the engine, watching for sparks in the tester's gap.
- Using a spark plug tester also means you won't expose your combustion chamber to debris.
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3
Remove the spark plug using a spark plug socket. Once you've removed the spark plug wire, remove the spark plug itself. This is easiest with a specialized socket wrench called a spark plug socket.
- From this point forward, be careful not to let anything drop into the empty hole left where your spark plug was. Leaving debris in this hole can cause damage to the engine as the vehicle runs and, since removing anything from this hole can be a big pain, it's best to take preventative care to ensure nothing of the sort happens.
- Cover the cavity with a clean rag or towel to prevent debris from entering the combustion chamber.
4
Attach the spark plug back to the spark plug wire. Now, carefully reattach the spark plug to its wire. You should be left with a spark plug that's connected to the distributor but not seated in its "hole." Handle the spark plug with insulated pliers to avoid the possibility of electric shock.
5
Touch the threaded portion of the spark plug to any exposed metal in the engine. Next, maneuver your spark plug (wire still attached) so that the threaded "head" of the plug is touching some metal part of the engine. This can be virtually any sturdy metal part of the engine block - even the engine itself.
- Again, hold the spark plug carefully with insulated pliers (and, if possible, gloves). Don't risk electric shock in the next few steps by neglecting this simple safety measure.
6
Remove the fuel pump relay or fuse. Before you crank the engine to test the spark plug, you must disable the fuel pump. When this is done, the engine will not start, allowing you to test the coil for spark.
- Failing to remove the fuel pump relay means cylinder being tested will not fire because there is no spark plug. It will, however, still be flooded with fuel, which may cause serious damage.
- Check your manual to locate the fuel pump relay.
7
Have a friend "crank" the engine. Get a friend or assistant to turn the key in the vehicle's ignition. This will provide power to the car's electrical system and, thus, to the spark plug you're holding (assuming your ignition coil is working)
8
Look for blue sparks. If your ignition coil is working properly, when your friend cranks the engine, you should see a bright blue spark jump across the spark plug gap. This spark will be clearly visible in the daylight. If you don't see a blue spark, your ignition coil is probably malfunctioning and needs replacement.
- Orange sparks are a bad sign. These mean that the ignition coil is supplying insufficient electricity to the spark plug (this can be for any number of reasons, including cracked coil casings, "weak" current, faulty connections, etc.).
- The final possibility you may observe is that no spark occurs. This is usually a sign that either the ignition coil is completely "dead," that one or more electrical connections are faulty, or that you've done something wrong in your test.
9
Carefully re-install the spark plug and re-connect its wire. When you've concluded your test, ensure the vehicle is turned off before essentially repeating the preparatory steps above in reverse order. Disconnect the spark plug from its wire, re-insert it into its hole, and re-connect the wire.
- Congratulations! You've completed your ignition coil test!

Method 2

Method 2 of 2:

Performing an Ignition Coil Resistance Test ("Bench Test")

1
Remove the ignition coil from the vehicle. The test above isn't the only way to determine whether the ignition coil in your vehicle is functioning as it should. If you have access to a piece of electrical equipment called an ohmmeter, which measures electrical resistance, you can measure the effectiveness of your ignition coil in a definitive, quantifiable way, rather than in the somewhat subjective way described above. However, to begin this test, you'll need to remove the vehicle's ignition coil so that you can easily access its electrical terminals.
- Refer to your service manual for precise instructions on removing your ignition coil. Usually, you'll need to disconnect it from the distributor wire, then unscrew it from its mounting with a wrench. Ensure your vehicle is turned off and has had a chance to cool before beginning this process.
2
Find the resistance specifications for your ignition coil. Every vehicle's ignition coil has its own unique specifications in terms of the electrical resistance within the coil. If your coil's actual resistance levels fall outside of these specifications, you'll know that your coil is damaged. Usually, you'll be able to find the resistance specifications unique to your vehicle by consulting your service manual. However, if you can't find it here, you may find success by contacting your dealership or by searching online vehicular resources.
- Generally speaking, most automotive coils will have a resistance reading of about .7 - 1.7 ohms for the primary winding and 7,500 - 10,500 ohms for the secondary winding.^{[1]
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3
Position the leads of the ohmmeter on the poles of the primary coil. The distributor will have three electrical contacts - two on either side and one in the middle. These may be either external (jutting out) or internal (sunken in) - it makes no difference. Turn on your ohmmeter and touch one lead to each of the outer electrical contacts. Record the resistance reading - this is the resistance of the coil's primary winding.
- Note that some newer models of ignition coil have contact configurations that differ from this traditional arrangement. Consult your vehicle's manual for information if you are unsure which contacts correspond to the primary winding.
4
Position the leads of the ohmmeter on the poles of the secondary coil. Next, keep one lead on one of the outer contacts and touch the other to the central, inner contact of the ignition coil (where the main wire to the distributor connects). Record the resistance reading - this is the resistance of the coil's secondary winding.
5
Determine whether the readings you recorded fall within your vehicle's specifications. Ignition coils are delicate components of a vehicle's electrical system. If either the primary or secondary windings are even a little outside of your vehicle's specifications, you'll want to replace your ignition coil, as your current one is likely damaged or malfunctioning.