The alternator does not fire the spark plugs directly.
The alternator keeps voltage producing to the battery as well as the ignition system and the vehicles electrical system.
That electricity is then used to supply the vehicle with power and also send voltage to the ignition coil which then steps up the voltage from the 12 volts to the thousands of volts needed to fire the spark plugs and ignite the fuel.
The ignition coil along with the spark plugs, ignition control module and sometimes a distributor if your car has one is responsible for firing the spark plugs and igniting the fuel.
12 volt ignition coils work by increasing the voltage from the cars battery and alternator and the coil sends the high voltage to the spark plugs to ignite the fuel.
In order to produce the required high voltages the 12 volt ignition coils make use of the relationships that exist between electricity and magnetism.
When an electric current flows through an electrical conductor such as a coil of wire, it creates a magnetic field around the coil.
To test a coil or ignition coil with a screwdriver remove the coil from the spark plug and place a screwdriver into the plug of the coil that plugs into the spark plug.
Then rest the screwdriver against the engine so that it grounds out.
Then crank the engine and look for spark and if you have weak spark or no spark the ignition coil is likely bad.
If you have spark plug wires and one coil then take the screwdriver and place it into one of the spark plug wires and place the screwdriver against the engine and crank the engine again and look for spark.
When a spark plug coil is bad it will cause the engine to misfire and can even sometimes cause hard engine starting.
Although most commonly you'll get a misfire on a certain cylinder that the ignition coil is on.
Some ignition coils have oil in them to dissipate the heat and if your ignition coil has oil on it then it likely is going bad and the oil has made a way through a crack in the ignition coil.
Another reason for the oil on the ignition coil is because of oil leaking from your engine or you spilled oil on the ignition coil without knowing it.
A bad ignition coil will cause a vehicle to not start if it's a vehicle that has one ignition coil.
If the vehicle has more than one ignition coil then the car should start on the other cylinders but will misfire on the bad ignition coil.
Other causes of car not starting include, bad fuel pump, bad spark plugs, bad fuel injectors, low fuel pressure, clogged fuel filter etc.
When ignition coils fail your car or other vehicle may be hard to start, not start at all, have loss of engine power, engine shaking, use more fuel, misfire.
Common signs of ignition coil failure include.
Loss of engine Power. This is one of the first symptoms of ignition coil failure.
Check Engine Light On.
Poor Fuel Economy.
Backfiring.
Misfiring Engine.
Hard Starts and Stalling.
Spluttering and Coughing Sounds.
Jerking and Vibrating.
Ignition coils are basically transformers that increase the voltage from your cars battery to create the high voltage needed for the spark plugs to ignite the fuel.
The voltage from your battery is not enough to ignite the fuel with the spark plugs so it needs an ignition coil which is a transformer to step up that voltage.
Some vehicles that have a distributor have 1 ignition coil while the other vehicles without a distributor use a coil for each spark plug.
Some ignition coils plug directly into the spark plugs while others sit on top of the engine and then use a wire to plug into the spark plug.
It's a good idea to replace the ignition coil or ignition coils when you replace spark plugs as well as spark plug wires.
If an ignition coil is not working properly, your engine will likely misfire.
A misfiring engine results in a jerking or sputtering feel when you are moving at normal speeds.
When you stop, a misfire from a bad ignition coil may cause your vehicle to run roughly, vibrate or shake.
Common signs and symptoms of bad or failing ignition coil or coils include.
Loss of engine Power. This is one of the first symptoms of ignition coil failure.
Check Engine Light On.
Poor Fuel Economy.
Backfiring.
Misfiring Engine.
Hard Starts and Stalling.
Spluttering and Coughing Sounds.
Jerking and Vibrating.
A high-performance ignition coil enhances engine performance.
It allows for a wider spark plug gap, which leads to a more robust first flame kernel at the beginning of combustion, resulting in increased voltage.
If your engine misfires or stalls when you are idle or when you stop suddenly or accelerate, there could be a problem with your ignition coil.
A misfire, which can sound like a coughing or sputtering noise or feel like a jerk or strong vibration means that one of the engine's cylinders is not firing correctly.
