When you turn the key to start your car, a remarkable process occurs. Twelve volts of electricity are produced by a battery and converted to enough power to crank the engine. The coil takes that spark from the battery and sends a much larger spark to your plugs, and the engine roars. Without a working engine coil, there can be no spark and no roar. That is pretty critical to both you and your car. The symptoms of a failing engine coil are subtle. Once the coil has failed, you will know it.
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An ignition coil that is failing is a bit like a runner in a marathon without a full breakfast. He might get to the end of the race, but he will get there slowly. A failing coil may still produce a spark, but half what the car needs to run efficiently. Your car might move, but it will be sluggish.
Engine Runs then Stops
If the car is running when the coil fails, the engine will stop and not start again. When the coil fails, the spark from the plugs disappears. With it, the spark takes the energy needed to keep the engine running and the motor shuts off. Since the spark plugs deliver the ignition system the push it needs, and the coil gives the plugs the spark required, without it, the machine will not start.
Engine Will Not Start
The engine will not turn over because there is no current going to the spark plugs. A car battery produces 12 volts of electricity. It takes thousands to get that system running. When the engine coil is not functioning, the electricity produced by the battery is not converted. 12 volts are just not enough to get the job done.
If you put a spark plug wire against a good coil, ground it to bare metal and then start the car, your reward will be a nice, bright, blue spark. No spark, no electricity coming from the coil, this adds up to a faulty engine coil. The bench test for an engine coil is straightforward. Lay a flathead screwdriver against the coil and find a piece of bare metal, like a screw. Touch the screwdriver to the bare metal to ground it. Ask someone to start the car for you and watch for a spark. This will give you a sense of the power produced by the coil. Good spark, good coil--it is a basic concept. Be careful if you perform this test. Make sure you do not lean against any metal, like the fender. Keep your hand on the plastic portion of the screwdriver and your feet on the ground.
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