Understanding your car’s ignition system might seem daunting, but the basics are quite straightforward. One of the easiest and most effective methods to diagnose ignition problems is by using a simple tool: a test light. This guide will show you how to use a test light to check your ignition coil while it’s still in your vehicle, whether you have a traditional points-type ignition system or a modern electronic ignition.
To generate a spark, your ignition coil requires two primary voltage signals. Let’s break down how to check these signals using your test light.
Essential Voltage Checks for Your Ignition Coil
To properly function, the ignition coil relies on two key electrical inputs. These inputs are crucial for creating the spark needed to ignite the fuel in your engine. Here’s how to check them:
Constant +12V Power Supply
Terminal #15 on your ignition coil should receive a constant +12V supply whenever your ignition key is in the ‘on,’ ‘cranking,’ or ‘running’ position. This power supply is essential for energizing the coil.
How to Test:
- Turn your ignition key to the ‘on’ position.
- Connect the clip of your test light to a good ground point on the vehicle (an unpainted metal surface works well).
- Probe Terminal #15 of the ignition coil with the test light.
- Expected Result: The test light should illuminate brightly, indicating the presence of +12V power. If the light does not turn on, or is very dim, you have a problem in your power supply circuit, possibly a blown fuse, a faulty ignition switch, or a wiring issue.
Pulsing Ground Signal for Triggering
Terminal #1 of the coil needs a pulsing ground signal as the engine turns over. This signal is what triggers the coil to release its high-voltage spark. In older vehicles with points-based ignition systems, this pulsing ground is created by the points opening and closing in the distributor. In electronic ignition systems, this signal comes from the electronic control unit (ECU) or ignition module.
How to Test:
- Connect the clip of your test light to a +12V power source (you can use the positive battery terminal or Terminal #15 if you’ve confirmed it has power).
- Probe Terminal #1 of the ignition coil with the test light.
- Have someone crank the engine.
- Expected Result: The test light should flash on and off as the engine cranks. Each flash indicates a trigger signal being sent to the coil.
Diagnosing Ignition Issues with a Test Light
By observing how your test light behaves when connected across Terminals #1 and #15, you can quickly pinpoint where the problem lies within your ignition system.
Testing Across Coil Terminals #1 and #15
For a comprehensive check, you can connect the test light directly across the two primary input terminals of the coil (Terminal #1 and Terminal #15).
- Connect one end of the test light to Terminal #15 and the other end to Terminal #1.
- Have someone crank the engine.
- Interpreting the Results:
- Test light flashes: This is the desired result. It indicates that both power and the pulsing ground trigger are reaching the coil, and the points (or electronic ignition) are correctly signaling the coil to fire.
- Test light stays ON constantly: This suggests that Terminal #1 is constantly grounded, or there’s a short circuit. The issue is likely before the coil, possibly in the points or electronic ignition module keeping the circuit closed continuously.
- Test light stays OFF constantly: This indicates a lack of power to Terminal #15 or a complete break in the ground circuit at Terminal #1. Again, the problem is likely before the coil in the power supply or triggering circuit.
Important Note: If the test light doesn’t blink, and you’ve confirmed that Terminal #15 has power, the coil itself is likely not the issue. The problem lies in the trigger signal to Terminal #1.
Testing Terminal #1 Trigger Signal Independently
You can also test the trigger signal at Terminal #1 more directly. It doesn’t matter whether you hook the test light to power or ground for this test because the points (or electronic switch) are essentially switching between power and ground as the distributor rotates.
- Option 1 (Test light grounded): Connect the test light clip to a good ground and probe Terminal #1 while cranking the engine.
- Option 2 (Test light powered): Connect the test light clip to a +12V source and probe Terminal #1 while cranking the engine.
- Expected Result (for both options): The test light should flash as the engine cranks, indicating the points or electronic module are switching the ground signal. If there’s no flashing, the issue is in the distributor points, condenser, or electronic trigger system.
Coil is Triggering But Still No Spark?
If your test light flashes correctly during the tests above, confirming that the coil is receiving both power and a trigger signal, but you are still not getting spark from the coil itself (no spark at the spark plug wires), then the ignition coil is likely faulty and needs to be replaced. At this point, no further testing of the coil itself is necessary. The coil only requires these two inputs to produce a spark. It does not even need a separate ground connection to generate spark.
What to Check If Spark is Present at the Coil But Not at the Plugs
If you have confirmed spark output from the coil, but you are not getting spark at the spark plugs, the problem lies in the secondary ignition components that distribute the high voltage to the plugs. Focus your attention on these parts:
- Spark Plug Wires: Check for damage, cracks, or loose connections.
- Distributor Cap: Inspect for cracks, carbon tracking (internal spark paths), and moisture.
- Rotor: Look for cracks or burning, particularly at the rotor tip.
- Coil Wire: The high-tension wire running from the coil to the distributor cap can also fail.
Diagram showing distributor cap and rotor, key components in spark distribution.
Why Spark Plug Grounding Against the Block is Misleading
Avoid testing coil output by simply laying a spark plug against the engine block and cranking the engine. This method is unreliable and can be misleading. Here’s why:
- Voltage Under Compression: An ignition coil is designed to produce enough voltage to jump the spark plug gap under engine compression. When there’s no compression (like when testing in open air against the block), the voltage requirement is much lower.
- Inaccurate Test: A spark might appear to jump in open air, but the coil might still be too weak to fire the plug reliably under the high-pressure conditions inside a running engine cylinder.
To get a more realistic idea of coil output without specialized equipment, you would need to significantly widen the spark plug gap (double it or more) to increase the voltage demand and get a better indication of the coil’s strength. In some demonstrations, a healthy coil can jump a spark gap of up to half an inch or more in open air, indicating strong output.
Points Ignition Specific Checks
For vehicles with points-type ignition systems, here are additional checks related to the points and condenser:
- Points Open Test: The voltage checks described above are typically performed with the points open. If the points are closed, you will not get the same readings at Terminals #15 and #1.
- Suspect Points or Condenser? If your test results are inconsistent or unexpected, consider replacing the points and condenser as a matter of routine maintenance, especially if they are old or their condition is unknown. These components are common sources of ignition problems in older vehicles.
Test Light Resistance and Spark Output
It’s worth noting that while using a test light for diagnosis is very helpful, the test light’s own resistance can sometimes affect spark output. In some cases, you might see a spark when testing with the test light, but the spark might disappear or weaken when the test light is removed. This is due to the slight current draw and voltage drop caused by the test light in the primary ignition circuit. However, for basic troubleshooting, this effect is usually negligible and the test light method remains a reliable diagnostic technique.
Tracing the Spark Path: Beyond the Coil
Once you’ve confirmed the coil is producing spark, and you’ve addressed potential issues with spark plug wires, distributor cap, and rotor, you need to consider the spark plugs themselves.
- Spark Plugs: Fouled, worn, or incorrectly gapped spark plugs are a very common cause of misfires and poor engine performance.
- Distributor Cap Cracks: Even hairline cracks in the distributor cap can allow spark to leak to ground, causing misfires, especially in damp conditions.
- Ignition Wires: Damaged or deteriorated ignition wires can also leak spark. A simple way to check for wire leakage is to lightly spray them with water while the engine is running. If the engine stumbles or misfires more noticeably, it indicates spark leakage through the insulation of the wires. Another method is to use a grounded test light held near (but not touching) the ignition wires. If the engine stumbles when the test light is near a wire, it indicates leakage from that wire.
By systematically using a test light and following these diagnostic steps, you can effectively troubleshoot and resolve many common ignition system problems, getting your engine running smoothly again.
Diagrams illustrating points-type ignition and a CDI trigger system, showing the common principles of ignition triggering.
