- Do It Yourself Auto Repair
How to Test an Ignition Coil
A bad or weak ignition coil can start by causing a misfire, rough idle, power loss, shutting off of the engine after warming up, or some other performance problem, until your engine refuses to start altogether.
A typical ignition coil is a pulse-type transformer with a soft iron core and two coil windings on top of each other, known as primary and secondary. Using battery voltage, the coil supplies the spark plugs with a low current, high-voltage electric arc of between 4000 to 60,000 volts or more — depending on the vehicle model. The spark plugs use this electric arc to burn the air-fuel mixture inside the combustion chamber.
Although not a common issue, ignition coils do run into problems sometimes. Lack of engine maintenance, dirty electrical connections, engine compartment heat, and vibration during engine operation can all eventually interfere with or damage a coil. However, problems with other components in the ignition system can cause some of these symptoms as well.
Thus, you need to confirm that you're dealing with a malfunctioning ignition coil.
The simple troubleshooting procedure described here uses a digital multimeter to check the ignition coil — or the ones in your ignition coil pack, depending on your particular vehicle model. So you can apply this test at home.
Tools and Items You'll Need:
Yout coil windings' resistance specifications
Wrench or ratchet and socket set
Hair dryer or heat gun, if necessary
Locating Your Ignition Coil
On some vehicles, you can find a single (cylindrical or rectangular assembly) ignition coil, usually mounted on the fender (driver side) of the engine compartment, away from direct engine heat. This single coil supplies each cylinder with a high-voltage spark. To locate it, follow the high-tension, center wire on the distributor cap to the other end where it connects to the coil. Still, some distributors have the ignition coil mounted on the cap.
Other car models use a coil pack (usually two or more coils) mounted on one side of the engine. To locate the pack, follow the spark plug wires going from the engine towards the other end, where they connect to their corresponding coils — each two cylinder pair will share one coil. If you have a 6 cylinder engine, for example, you'll see three coils.
This guide deals with conventional ignition coils. Some newer vehicle models use a coil-on-plug (COP), a coil that mounts directly over the spark plug. Since this type of coil comes under different configurations requiring different troubleshooting procedures, it's not covered here.
Before you begin these tests, check the resistance specifications for the primary and secondary windings of your ignition coil or coil pack. You'll find this information in the service manual for you particular vehicle make and model. If you don't have the manual, you can buy a copy from most auto parts stores or online (see the Amazon ad below). You'll need these resistance specifications to compare them to your own test results.
Vehicle Repair Manuals
HAYNES ENGINE SERVICE MANUAL -- Using a haynes manual is like having a mechanic in every book, each manual is written and photographed from the hands-on experience gained by a complete teardown and rebuild of the engine, includes procedures for everything from routine maintenance to complete engine overhaul.
Testing the Coil's Primary Resistance
If you have good access to the coil pack, you may test it without removing it from the engine. Otherwise, use a wrench or ratchet and socket to remove the coil or coil pack. Just make sure to note how the wires connect to it.
1. Depending on the type of coil your car uses, disconnect the wires or unplug the coil's electrical connector, and high-tension distributor or spark plug wire(s).
2. Visually inspect the wire connectors or electrical connector for corrosion, dirt and physical damage.
3. Then, check the coil itself for potential physical damage. Any of these conditions will prevent your coil from producing the necessary voltage for the spark plugs.
4. Set your digital voltmeter to a low range on the Ohms scale.
5. Then, measure the coil's primary resistance by hooking up the digital meter leads to the two electrical terminals (usually marked [-] and [+]).
* On a coil with a rectangular assembly or a coil pack, the primary terminals are located on the electrical connectors. On a coil pack, one of the three or four terminal contacts (on a 6 or 8 cylinders engine, depending on your model) is positive, and the other two or three are negative (check your service manual to locate the positive terminal, if necessary). Hook up your meter to the positive and each negative terminal in turn.
6. For the primary winding, your reading may fall between 0.3 and 1.0 Ohms of resistance.
7. Compare your reading to the specifications in your service manual.
8. If you get more or less resistance than the range specified by your car manufacturer, replace the ignition coil.
Testing the Coil's Secondary Resistance
1. To measure the secondary resistance, set your voltmeter to the 20K range on the Ohms scale.
2. On a single coil model, hook up one meter lead to the negative primary terminal (look for the [-] sign) and the other lead to the secondary terminal (where the high-tension distributor wire connects on the coil). (NOTE: On some models, you hook up to the positive, primary terminal for this test and the high-tension wire connector. Check your service manual, if necessary).
* On a coil pack, you connect your meter leads across each of the coil's corresponding cylinder pairs terminals. On a V6 engine, these terminals have numbers like 1-5, 4-3, and 2-6; On a V8 engine, 1-6, 3-5, 4-7, and 2-8, on the coil pack itself. So you measure secondary resistance across each of the indicated terminal pair. Check your vehicle service manual, if necessary.
3. The secondary resistance should have between 6,000 to 12,000 Ohms or more, depending on your manufacturer specifications. If not, replace the corresponding coil. On some vehicle models, you have to replace the coil pack as a single unit.
Testing an Ignition Coil for a Broken Winding
A winding resistance reading within specifications doesn't necessarily mean you have good coil or coils. A coil with a broken winding on the primary or secondary circuit may still work at a low temperature. But, as soon as the heat in the engine compartment goes up, the windings expand, causing a broken winding to separate and cut electrical flow to the spark plugs.
Thus, if you've noticed that after a few minutes of engine operation a cylinder misfires or your engine suddenly dies but resumes normal operation after cooling, you may have a damaged coil or ignition control module (if used). On some vehicle models the computer performs the function of the control module as well, though.
This next test will help you check for a possible broken winding.
1. Repeat the two previous resistance tests, but this time heat up the coil with a hair dryer or heat gun. Just don't expose the coil to too much heat or for two long or you'll damage its internal components.
2. If your results still indicate a good ignition coil, take the ignition control module to your local auto parts store for a quick check. Most auto parts stores will check it free of charge.
If you don't know the location of the ignition control module on your vehicle, consult your service manual.
Test the ignition coil whenever you suspect a problem in the ignition system. Just like any other component, the coil wears out or eventually break, causing all kinds of engine performance problems. Although you can easily test the coil using a digital multimeter, remember that the coil windings can also come apart and fail when the engine warms up. Keep this guide handy and use it whenever you suspect trouble with the ignition system in your car.