How to Test Ignition Coils
If you suspect that something is wrong with your vehicle’s ignition coil, you can test it with a multimeter. A weak or damaged ignition coil can cause engine misfires, stalling, and lower fuel economy. A broken ignition coil will prevent your vehicle from running at all.
If you’re having an ignition coil issue, you’ll want to test your coils to determine which one is having problems. Follow these steps to test an ignition coil.
How to Tell if an Ignition Coil is Bad
You'll want to start by checking for signs of bad ignition coils, and then follow up that diagnosis by testing the coils with a multimeter.
Ignition coils take energy from the battery (usually 12 volts) and convert it to a high enough voltage (50,000 volts or more) to create a spark. This energy travels through the distributor to the spark plugs and ignites the fuel. Some vehicles only have one or two ignition coils that power multiple spark plugs. Other models use one ignition coil per spark plug.
Whether your vehicle has one coil or eight, you’ll need to remove them all. Before starting, gather the following supplies:
- A basic tool kit
- A multimeter or an ohmmeter
You're going to want to do your initial diagnosis by looking for common bad ignition coil symptoms, because you need to remove the coils before testing them. That way you can save yourself some time if you find that the symptoms don't match up. These are the symptoms of a bad ignition coil:
When your vehicle backfires, it’s hard not to notice. You’ll hear what sounds like a shotgun blast, your car may lurch, and black smoke will pour out of the tailpipe. The “check engine” light on the dash usually turns on, and you may smell gasoline. This unpleasant experience happens when unused fuel enters the exhaust pipe and combusts on the way out.
Low fuel pressure could be the issue, but, sometimes, the ignition system is to blame. Bad ignition coils affect when the spark plugs ignite, which causes improper timing.
If you’ve ever driven a vehicle that stalls, you already know how frustrating it is to deal with abrupt stops and starts. It’s also extremely dangerous when it happens in traffic. One or more failing ignition coils under the hood can cause stalling by sending abnormal sparks to the spark plugs. This uneven electrical charge prevents the engine from running smoothly.
Lower Fuel Economy
When ignition coils transmit too little energy to the spark plugs, your vehicle uses extra fuel. It’s the only way it can stay powered. If you’ve noticed that each tank of gas gives you fewer miles than usual, you may have one or more faulty ignition coils.
Difficulty Starting the Engine
When there’s a problem starting an engine, most people think to check the battery first. But what if you check your battery and its fine? Try checking the ignition coils. They can cause the problem if your vehicle has a one-coil-per-plug configuration. The car can still run if only a few ignition coils fail, but it won’t start as easily.
If your vehicle is experiencing these problems, it's time to break out the multimeter and start testing coils.
How to Test Ignition Coils
Let your vehicle cool down, and put on the emergency brake. Open the hood and disconnect the negative battery cable. You may need to use a wrench or a screwdriver, depending on your vehicle. Refer to the owner's manual for specific instructions.
Remove the Ignition Coils
Find the ignition coils. They should be on or near the engine. Disconnect the mounting bolts holding the ignition coils and remove each coil.
Test the Primary Winding
Each ignition coil consists of two separate coils wrapped around each other: the primary winding and the secondary winding. The primary winding is made of heavy wire and receives energy from the battery. Test this part of the coil first.
- Check your vehicle's owner manual to find out what the correct resistance reading should be
- Connect the multimeter's positive and negative leads to the corresponding terminals on the ignition coil. Each terminal, or pin, may be marked with plus and negative signs or with numbers. Check the owner's manual to find out what your vehicle's spark plugs look like
- Check the reading on the multimeter and compare it to the resistance specifications for the primary winding
If the reading falls outside the normal resistance range, you'll need to replace the ignition coil. A zero means the coil shorted internally. An excessively high reading means the coil is open. Does it look okay? If so, move on to checking the secondary winding.
Test the Secondary Winding
The secondary winding is a thin wire that coils many times. It receives energy from the primary winding, which it then sends along to the spark plugs. The steps for this test are similar:
- Connect the multimeter's leads to the positive terminal and the center pole. This center pole is where the main wire attaches to the distributor
- Compare the multimeter's reading to the resistance specifications for the secondary winding
If the reading falls outside the normal range, you'll need to replace the ignition coil.
Solve the Problem
If your vehicle has more than one ignition coil, remove and test each one. New coils should be the same kind as the originals. Reinstall or replace each one.
While you’re under the hood, you may want to test the spark plugs as well. Ignition coils can fail due to the heat and vibration of the engine, but the most common problem is faulty spark plugs. Bad spark plugs or plug wires overload the ignition coils. If you don’t solve the underlying problem, the coils will fail again. If you find that your ignition coils fail regularly, then be sure to take your vehicle to a mechanic or replace the ignition coils yourself.
It’s easy enough to test your vehicle’s ignition coils yourself with a few tools from your local AutoZone. Even if you get a normal reading, ruling them out as the problem can help you narrow the issue with the ignition system.