The ignition coil, an essential component of any vehicle ignition system, is responsible for providing electricity to the spark plugs. When a vehicle won't start, fails often, or stops frequently, it means the ignition coil needs to be replaced. Fortunately, a simple and relatively quick test can determine if the ignition coil is working properly, and therefore find out whether it will be necessary to go to the auto parts store or the mechanic shop. Read step 1 to find out how to carry out the test.
Method 1 of 2: Perform the Ignition Coil Spark Test
Step 1. Turn off the vehicle and open the hood
As with most vehicle maintenance, you should start the test with the vehicle parked and the engine off. Open the hood to locate the ignition coil. While its precise location may vary from vehicle to vehicle, it is usually located near the fender, the starter, or under the distributor cap. Note that on distributorless vehicles, the spark plugs are connected directly to the coil.
- One sure way to find the ignition coil is to locate the distributor and follow the wire that does not connect to any spark plugs.
- Before you begin, it is good to make sure you are wearing goggles or any other type of eye protection and that you have insulating tools (especially pliers) to protect yourself from an electric shock.
Step 2. Remove a spark plug wire from its plug
Pull one of the spark plug wires out of the plug. Typically these wires run from the distributor cap to each of the spark plugs individually. To avoid injury, be very careful when working with your vehicle's electrical system (wear gloves and insulating tools at all times).
- If your vehicle has been running for a long time, its internal components are likely very hot. If this is the case, let your vehicle cool down for 5-10 minutes before taking this initial step.
- To save time and avoid potentially damaging the spark plug, consider using a spark plug tester instead. Instead of sticking the spark plug to the spark plug wire, stick the tester to the spark plug wire. Ground the alligator clip. Then lean forward and ask a friend to start the engine so you can see possible sparks in the spark gap.
- Using a spark plug tester will also not expose the combustion chamber to debris.
Step 3. Remove the spark plug using a socket wrench
Once you've removed the spark plug wire, take out the spark plug. This can be easily done with a special spark plug wrench called a "socket wrench".
- From this point on, be careful not to drop anything into the empty hole where the spark plug was located. Leaving debris in the hole can damage the engine while the vehicle is running, and since removing anything from this hole can be a major headache, it is best to take proper precautions to ensure that none of this happens.
- Cover the cavity with a clean rag or towel to prevent debris from entering the combustion chamber.
Step 4. Reattach the spark plug to the spark plug wire
Now, very carefully reconnect the spark plug with its wire. You should be left with a spark plug connected to the distributor, but not inserted into its "hole." Handle the spark plug with insulating pliers to avoid possible electrical shock.
Step 5. Touch the threaded part of the spark plug to look for any exposed metal on the engine
Maneuver the spark plug (with the wire still attached) so that the threaded "head" of the plug touches some metal part of the engine. Practically this can be any sturdy metal part of the engine block (even the engine itself).
We reiterate: carefully hold the spark plug using insulating pliers (and if possible, gloves). Don't risk getting an electric shock in the following steps by ignoring this simple safety measure
Step 6. Remove the fuel pump relay or fuse
Before starting the engine to test the spark plug, you must disable the fuel pump. Doing so will prevent the engine from starting, allowing you to test for coil sparks.
- If you do not remove the fuel pump relay, the cylinder to be tested will not light because there is no spark plug. However, it could fill up with fuel, which could cause serious damage.
- Check your manual to locate the fuel pump relay.
Step 7. Have someone else "start" the engine
Have a friend or assistant turn the vehicle's ignition key. This will provide power to the car's electrical system and therefore to the spark plug you are holding (assuming the ignition coil is working).
Step 8. Look for blue sparks
If the ignition coil is working properly when your helper starts the engine, you should see a bright blue spark jump in the spark gap. This spark will be clearly visible in daylight. If you don't see a blue spark, the ignition coil is probably malfunctioning and will need to be replaced.
- Orange sparks are a bad sign. These sparks mean that the ignition coil is not supplying enough electricity to the spark plug (this can be due to a number of reasons, including cracked coil housings, "weak" current, poor connections, etc.).
- The last possibility is that you don't see any spark. This is usually an indication that the ignition coil is totally "dead," that one or more electrical connections are bad, or that you've made a mistake during testing.
Step 9. Very carefully reinstall the spark plug and reconnect its wire
When you've finished testing, make sure the vehicle is turned off before repeating the preliminary steps listed above in the opposite order. Disconnect the spark plug from its wire, reinsert it into its hole, and reconnect the wire.
Congratulations! You have finished the ignition coil test
Method 2 of 2: Perform the Ignition Coil Resistance Test ("Test Bench")
Step 1. Remove the ignition coil from the vehicle
The above test is not the only way to determine if a vehicle's ignition coil is working properly. If you have an electrical instrument called an ohmmeter, which measures electrical resistance, you can measure the effectiveness of the ignition coil in a definitive and quantifiable way, rather than in a somewhat subjective way as described above. However, to begin this test, it is necessary to remove the ignition coil from the vehicle so that you can easily access its electrical terminals.
Check your service manual for precise instructions on how to remove the ignition coil. Normally, you will have to disconnect it from the distributor cable, then unscrew it from its mount with a wrench. Make sure your vehicle is turned off before beginning this process
Step 2. Find the resistance specifications for your ignition coil
Each ignition coil in a vehicle has its own unique specifications in terms of electrical resistance within the coil. If your coil's actual resistance levels fall outside of these specifications, your coil is clearly damaged. Typically, you can find the unique strength specifications for your vehicle by consulting your service manual. However, if you can't find them in the manual, it's best to contact your dealer or look up vehicle resources online.
Generally speaking, most automotive coils give a resistance reading of about 0.7 to 1.7 ohms for the primary winding and 7,500 to 10,500 ohms for the secondary winding
Step 3. Place the ohmmeter leads on the poles of the primary coil
The distributor must have three electrical contacts (two on either side and one in the center). These can be external (protruding) or internal (sunken), it does not matter. Turn on the ohmmeter and connect one terminal to each of the external electrical contacts. Record the resistance reading. This is the primary winding resistance of the coil.
Note that some recent ignition coil models have contact configurations that differ from their traditional arrangement. Consult the vehicle manual for more information in case you are not sure which contacts correspond to the primary winding
Step 4. Place the ohmmeter leads on the secondary coil poles
Keep one terminal on one of the outer contacts and connect the other to the center inner contact of the ignition coil (where the main wire connects to the distributor). Record the resistance reading. This is the secondary winding resistance of the coil.
Step 5. Determine if the readings you have recorded fall within the specifications for your vehicle
Ignition coils are delicate components of a vehicle's electrical system. If one of the windings (either the primary or the secondary) is a little out of the specifications of your vehicle, you will have to replace the ignition coil, since it is very likely that the one you currently have is damaged or malfunctioning.