Diagnosing ignition problems in your GM vehicle can often lead you to the ignition coil. A faulty ignition coil can cause misfires, rough idling, and decreased engine performance. Fortunately, testing an ignition coil is a straightforward process you can perform at home using a few simple tools, primarily a multimeter and a spark plug. This guide will walk you through two effective methods to test your GM ignition coil: the spark test and the resistance test. By following these steps, you can confidently determine if your ignition coil is functioning correctly and save yourself time and money on unnecessary repairs.
Performing an Ignition Coil Spark Test
This test verifies if your ignition coil is capable of producing a spark, which is essential for igniting the air-fuel mixture in your engine’s cylinders.
- Disconnect the ignition coil output wire from the distributor cap. This wire carries the high voltage spark from the coil to the distributor.
- Attach a spark plug to the end of the ignition coil output wire you just disconnected. This spark plug will serve as a visual indicator for spark generation.
- Connect a ground wire to the threaded portion of the spark plug. This ground is necessary to complete the electrical circuit and allow a spark to form.
- Disconnect the ignition coil ground wire (typically a green wire) from the negative terminal (-) on the coil. This isolates the coil for testing purposes.
- Connect one end of a jumper ground wire to the ignition coil’s negative terminal (-). This will allow you to manually ground the coil during the test.
- Turn the ignition switch to the ON position. This energizes the ignition system, preparing it for the spark test. Do not start the engine.
- Tap the other end of the ignition coil ground wire jumper onto a known good grounding point, such as the battery’s negative terminal. Observe the spark plug for sparks as you tap the ground wire. The sparks should correspond to the frequency of your tapping.
- Evaluate the spark. A strong, consistent spark at the spark plug indicates that the ignition coil is likely in good working condition.
- Investigate further if there is no spark or a weak spark. Proceed to check for voltage at the coil. With the ignition switch in the ON position, use a multimeter to check for approximately 12V DC between the coil’s positive terminal (typically a black wire) and ground. You should also measure approximately 12V DC between the coil’s negative terminal (green wire) and ground. Lack of voltage indicates a wiring or power supply issue, not necessarily a faulty coil.
Performing an Ignition Coil Resistance Check
In addition to the spark test, checking the ignition coil’s resistance provides further confirmation of its internal condition. This test measures the resistance of both the primary and secondary windings within the coil.
- Check the primary coil resistance. Set your multimeter to measure ohms. Connect the multimeter leads across the positive (+) terminal (Black wire) and negative (-) terminal (Green wire) on the ignition coil. The resistance reading should typically fall within the range of 0.4 to 0.6 ohms. Refer to your vehicle’s service manual for the exact specification for your GM model.
- Check the secondary coil resistance. Connect your multimeter leads between the coil output terminal (where the spark plug wire connects) and the ignition coil’s negative (-) terminal. The resistance reading for the secondary coil should typically be in the range of 5000 to 7200 ohms. Again, consult your vehicle’s service manual for the precise resistance range for your specific GM vehicle.
Conclusion
By performing both the spark test and the resistance check, you can effectively diagnose the condition of your GM ignition coil. A healthy ignition coil will produce a strong spark and exhibit resistance readings within the specified ranges. If your coil fails either of these tests, it is likely faulty and should be replaced to restore proper engine performance. Remember to always consult your vehicle’s service manual for specific testing procedures and resistance values relevant to your particular GM model.
