How To – How To Tell If Your Engines Coil Is Bad

Be Sociable, Share!

    Your car’s electrical system is complex but by following simple diagnostic steps you can tell what parts are bad and need replacing.

    In order to ignite the fuel air mixture in your engine’s combustion cylinders you need a high voltage spark. Your car’s battery and charging system can only output from 10 to 15 volts of DC Current but by using a coil you can take the high amperage but low voltage of your electrical system and convert it into a lower amperage but high voltage spark.

    The same technique is used to send electricity from a power station along high voltage lines and then drop that electric down to a usable voltage at your local street for your home appliances.


    When your car has trouble starting it is most often either your fuel system or electrical system.

    When coils die they usually do it during operation of the vehicle. This can happen soon after you start the car but more often it will happen while you are driving.

    Single Coil Engines – The symptom may feel like you are out of gas and your engine will sputter to a stop. At that point you should try to coast your way to the side of the road in a safe location.

    Multi Coil Engines – If you have a new vehicle with a coil above each spark plug then a single coil dieing will not give you the same experience as found in single coil engines. Most likely your coil will die and the engine will begin to run very rough or lose much of its power.

    Some multi coil systems can sense if spark is getting to the cylinder and shut off the flow of fuel to that cylinder however many can not so continuing to run a car with a single coil out can cause fuel buildup in both the engine and exhaust system. This extra fuel can cause your Catalytic converter to run extremely hot and even begin to glow red. For this reason you should not run your vehicle if you suspect you have a single dead coil more then necessary to reach help or get to a safe location for repair or towing.


    We will first cover a single coil system then add in the steps to diagnose a multi-coil system where appropriate.

    First you should make sure that your car has a fully charged battery. Your engine should crank easily as if to start but will not. Do not continue to try starting the engine as it will quickly drain the battery.

    Make sure you have fuel getting to your engine. To do this you can access your fuel line from its connector to your injector throttle body, fuel rail or at a connection past your fuel filter. Remove the cap on your gas tank to release the pressure then remove the fuel line connection.

    Simply turning the ignition key to the on position should prime the fuel line and cause gasoline to come out the line. DO NOT DO THIS IF YOUR ENGINE IS HOT! you should also have a catch can under the fuel line to catch the few ounces of fuel that will escape.

    Now that you know your fuel lines are not blocked either inline or at the filter and you know you have sufficient pressure to deliver fuel to your injectors or carburetor on an older vehicle you can begin looking at your ignition system.

    Testing the spark

    On multi-coil ignitions your car should still be running so the easiest method for testing if a single coil is bad is to start the engine and listen to and feel smoothness of how it is running.

    Now turn the engine off and remove a single spark plug wire. Position the end so it will not come in contact with any part of the vehicle including the body you need at least 3-4 inches distance. Start the engine and try to notice any difference in how the engine is running.

    If the plug wire you pulled has a bad coil the engine will remain the same. If you pulled a good plug and coil wire you should notice the difference. A 4 cyl will react much more then a 6 or 8 but you should still notice.

    Once you have found the bad coil you know which one to replace.

    On a Single Coil Ignition System your engine will not be running the following tests can be taken with both a single coil and multi-coil ignition system.

    Test the spark by removing one spark plug wire and inserting a spare spark plug. Now with the plug end touching ground on your vehicle or engine BUT NOT TO THE BATTERY have a helper try to start the engine for 5 seconds or less.

    Make sure you are holding the wire well away from the plug at least 6 inches and use insulated rubber gloves or a pair of insulated pliers to reduce your chance of shock.

    If the coil is working and you have spark to the plugs you should see a bright blue or white spark between the electrodes on the plug. If you do not see a spark then the problem is prior to the plug meaning the wires, cap, rotor or coil.

    You could also use this spark plug test for multi-coil systems to check for spark if you have doubts which coil is bad.

    Testing The signal to the Coil

    Computer controlled ignition systems will send a signal to the coil to fire at the exact time the piston is at the top of its stroke or when it has the air / gasoline mixture compressed as much as possible.

    To regulate that timing means a cam or crank sensor needs to send a signal to your computer which will either send voltage directly to the coils on a multi-coil system or send it to an ignitor on a single coil system.

    To test either system you will need specific information about your vehicle and how it sends that signal.

    Once you have found the input wire to the coil then you can use a test light to see if the voltage causes the test light to blink while the engine is cranking.

    If the test light does blink while you crank the starter then you “most likely” have a bad coil. If not then the problem may be between the coil and your computer. This can include many parts including the sensors, wires and the computer its self.

    Understand once you have got to the point of  the coil test you should have already eliminated the cap, rotor and wires on a single coil system.

    Testing the coil alone

    Many manuals will provide specs on your coil and this will allow you to use a volt meter with an OHM setting to test if the wires are open or if they are shorted.

    The basic test is performed by testing from the positive to negative connector wires on the coil and you should have a full resistance connection showing those wires are connected.

    The second test is from the battery positive connector and the output of the coil and this circuit should show some resistance because coils are long spools of wire. The exact resistance will need to be given in your manual but may be around 8,000 to 11,000 ohms.

    Older Vehicles

    On older vehicles that do not have computer controlled ignition systems the coil is often independent of the distributor and can be tested by using the spark plug test explained above.

    If you do not get spark from the coil and you have positive battery to the input you simply replace the coil.

    The YouRepair Store sells a full line of test equipment, manuals and replacement parts for your vehicle at great discount prices.


    Replacement Parts

    Ignition Parts


    Batteries & Accessories

    Performance Parts & Accessories

    Be Sociable, Share!