Understanding how your car’s ignition system works might seem complicated, but testing it, especially the ignition coil, is actually quite straightforward. If you’re experiencing engine misfires, difficulty starting, or poor performance, testing your ignition coil is a crucial first step. This guide will show you the easiest way to Test An Ignition Coil while it’s still in your vehicle using a simple test light. Whether your car has a traditional points-type ignition system or a modern electronic ignition, these tests are applicable.
For an ignition coil to generate a spark, it requires only two primary voltage signals. These signals are delivered to the coil terminals. Specifically, Terminal #15 needs a constant +12V supply when the ignition key is in the “on,” “cranking,” or “running” positions. The other crucial input is at Terminal #1, which should receive a “pulsing” ground signal as the engine turns over. This pulsing ground is what triggers the coil to release its high-voltage spark.
To test an ignition coil effectively, connect a test light across the two primary input terminals of the coil (Terminal #15 and Terminal #1). Have someone crank the engine while you observe the test light. If the points or electronic ignition system are correctly triggering the coil, the test light will flash on and off. This flashing indicates that the coil is receiving the necessary signals to operate.
However, if the test light remains constantly ON or constantly OFF during engine cranking, it signifies a problem before the ignition coil in the circuit. This means the issue is likely not with the coil itself, but rather with the power supply to Terminal #15 or the triggering signal to Terminal #1.
If the test light doesn’t blink as the engine cranks and you’ve confirmed that Terminal #15 is indeed receiving power, then the ignition coil is likely not the source of your problem. The test light can be connected to either the power or ground side to check the trigger at Terminal #1; the connection polarity doesn’t matter for this test. The points (in older systems) or the electronic ignition module work by switching between power and ground as the distributor rotates. This opening and closing of the circuit is what the test light detects. So, by observing the test light’s behavior, you can verify if the switching action from the points or electronic trigger is occurring.
If you confirm that the ignition coil is receiving the correct input signals – indicated by a flashing test light – but you’re still not getting spark from the coil, then it’s highly likely that the ignition coil is faulty and needs replacement. At this point, no further testing of the coil itself is typically needed. The coil’s primary function is to produce spark when it receives these two key inputs. It doesn’t even require a separate ground connection to generate a spark.
Now, if you’re getting spark at the ignition coil but still have no spark at the spark plugs, the problem lies elsewhere in the ignition system. You’ll need to inspect the spark plug wires, distributor cap, and rotor. These components are responsible for distributing the spark from the coil to the individual spark plugs.
It’s important to note that simply laying a spark plug against the engine block and cranking the engine is not a reliable way to test ignition coil output. While you might see a spark in open air, an ignition coil is designed to generate enough voltage to jump the spark plug gap under compression within the engine cylinder. Under compression, the spark plug requires significantly more voltage to fire – often double the voltage needed in free air. This rudimentary check is therefore inaccurate and can be misleading when trying to diagnose a weak coil. To get a better indication of coil output using a spark plug, you would need to significantly increase the spark plug gap to something much wider than standard, but this is still not a recommended method for accurately testing an ignition coil. In a proper coil test, you should be looking for a strong spark capable of jumping a substantial gap, as demonstrated by the spark jumping close to half an inch from the coil to ground in the video linked in the original article.
These checkpoints in the diagram illustrate where to test your ignition system for power when the points are open. If the points are closed, you won’t have power at these test points. If your tests yield different results than expected, consider replacing the points and condenser in older ignition systems as they are common failure points. This same test principle applies to Terminal #1 of the ignition coil. When diagnosing a “no spark” condition, starting your diagnosis at the ignition coil is an efficient approach. Use a test lamp on Terminal #15 and Terminal #1. A blinking test lamp confirms power at Terminal #15 and that the points and condenser (or electronic trigger) are functioning to trigger the coil at Terminal #1. If you have no spark coming from the coil without the test light connected, but the test light flashes when connected, then the coil itself is likely defective. Be aware that you might experience a weaker spark, or no spark at all at the spark plug while simultaneously using the test light to check for triggering signals. This is due to the resistance introduced by the test light in the circuit, which can sometimes draw excessive current and reduce the voltage available for spark generation.
Now, let’s trace the spark path from the ignition coil to the spark plugs. The high-voltage current from the coil travels through the coil wire to the distributor. Inside the distributor cap, a small carbon button makes contact with the rotor. This button can wear down over time, leading to weak or no spark at the plugs. Cracks in the distributor cap can also cause similar issues by allowing the high-voltage spark to leak to ground. Next in the spark path is the rotor itself. Although less common in point-type ignition systems, it’s possible for the high-voltage spark to burn a pinhole in the rotor, causing the spark to ground through the distributor shaft. After the rotor, we come to the spark plug wires and spark plugs themselves. These are the most frequent culprits for engine misfires. Cracked distributor caps can also cause misfires. Spark plugs, wires, and distributor caps, along with lean fuel mixtures, are common causes of misfires. To isolate ignition components as the cause of a misfire, you can try swapping spark plug wires and spark plugs one at a time between a cylinder that is firing correctly and one that is misfiring. This process can often pinpoint the problematic component, although not always. Another method to check your spark plug wires is to spray them with water while the engine is running. If you notice increased misfiring when spraying water, it indicates insulation breakdown in the wires. Alternatively, you can ground a test light and hold it near (but not touching the tip) each spark plug wire. Test both sides of each wire. When the engine stumbles or misfires noticeably, you’ve likely found a wire with insulation issues. Visual inspection of the wires, distributor cap, and plugs can also reveal cracks, carbon tracking, or other damage.
Here’s a simple test you can perform to check for spark output directly from the ignition coil using just one wire if you have a points-type ignition system.
As illustrated in the diagram above, all ignition systems, regardless of their specific configuration, rely on a trigger mechanism for the ignition coil. While ignition systems vary in design, they all operate on the same fundamental principle: creating a low-energy magnetic field in the ignition coil’s primary winding and then rapidly collapsing it to induce a high-energy voltage in the secondary winding. This high-voltage electrical energy is then transferred to the secondary ignition system and ultimately to the spark plugs. All ignition systems are triggered by the primary ignition system. This trigger system varies depending on the vehicle manufacturer and ignition type (points, electronic, CDI, etc.), but the core principle of a low-voltage trigger remains consistent. This low-voltage signal is amplified and applied to the primary side of the ignition coil. This trigger signal is only present when the ignition key is in the “on” position and the engine is rotating. In a traditional VW distributor, the distributor cam rotates in sync with the engine, completing one revolution for every two engine revolutions. As it rotates, the cam lobes mechanically open and close the contact points. Each time the points open, the current flow through the ignition coil’s primary winding is interrupted. This interruption causes the magnetic field to collapse rapidly, releasing a high-voltage surge from the secondary coil windings. This high-voltage surge is directed out of the coil’s secondary terminal and into the coil wire. Now that we have generated the necessary voltage to fire the spark plug, it needs to be delivered to the correct cylinder at the right time. The coil wire connects directly to the center terminal of the distributor cap. Beneath the distributor cap is a rotor mounted on top of the rotating distributor shaft. The rotor has a conductive metal strip on its top surface that maintains continuous contact with the center terminal of the distributor cap. It receives the high-voltage surge from the coil wire and distributes it to the other end of the rotor. As the rotor spins on the shaft, it sequentially passes by each spark plug terminal inside the distributor cap. As the rotor aligns with each terminal, it sends the high-voltage pulse to the corresponding spark plug wire, and then to the spark plug itself. This secondary high voltage enters the spark plug at the top terminal and travels down the central core until it reaches the tip. Finally, the voltage overcomes the resistance of the compressed air-fuel mixture in the combustion chamber and jumps across the spark plug gap at the tip, creating a spark to ignite the mixture and start the combustion process.
Here’s a short video demonstrating the blinking LED test on an ignition coil. The LED is connected between terminals #15 and #1, and you can see the spark plugs firing in the video. http://www.youtube.com/watch?v=MlUHinc6wAU In this video, you can observe the spark coming from the coil wire when the test light is briefly touched to Terminal #1 while the other end is connected to Terminal #15. The spark momentarily disappears because of the test lamp’s resistance. This demonstration is using an Accufire electronic ignition, but the principle is identical for points-type ignition systems. http://www.youtube.com/watch?v=vUdjNG4u6po This video illustrates the effects of using test lights and LED lights to examine the secondary spark when connected between the primary terminals of the coil. In some parts of the video, the spark might not be visually apparent, but you can still hear the spark discharge. http://www.youtube.com/watch?v=XCEOLmr5ZSc
