An ignition coil functions as an electrical transformer, converting the low 12-volt current supplied by the vehicle’s battery into the thousands of volts necessary to fire the spark plugs. This high-voltage surge is created through electromagnetic induction by rapidly collapsing a magnetic field built up in the coil’s primary winding. When the coil fails to perform this conversion efficiently, engine performance suffers. This guide provides a detailed approach to diagnosing a faulty ignition coil using a standard multimeter and a spark tester.
Common Symptoms of a Bad Ignition Coil
A failing ignition coil often presents several noticeable signs. The most common indicator is engine misfiring, which feels like a stutter or hesitation, particularly when the engine is under load or accelerating. This occurs because the spark plug in the affected cylinder is not receiving enough voltage to ignite the air-fuel mixture properly.
The engine may also exhibit a rough or jittery idle. This incomplete combustion leads to diminished power and reduced fuel economy. Furthermore, the vehicle’s onboard diagnostic system will often detect the ignition irregularity, illuminating the Check Engine Light (CEL) on the dashboard.
Safety and Preparation Before Testing
Before beginning any diagnostic work, safety precautions must be followed. Always ensure the engine is completely turned off and has cooled down sufficiently, as engine components can retain heat for a long time. Disconnecting the negative battery terminal is necessary to de-energize the electrical system and prevent accidental shorts or shocks.
Gather the correct tools, including a digital multimeter, a spark plug socket, and a dedicated spark tester. Locate the ignition coils, which are typically found directly on top of the spark plugs in modern coil-on-plug systems. Perform a visual inspection of the coil body and its connector, looking for physical damage such as cracks, oil leaks, or corrosion on the terminals.
Testing Coil Resistance with a Multimeter
The most definitive way to test an ignition coil is by measuring the electrical resistance of its internal windings using a multimeter set to the Ohms ($\Omega$) function. This test checks the integrity of the copper wire windings inside the coil, which are divided into primary and secondary circuits.
Primary Resistance Test
The primary resistance test measures the low-voltage input circuit. To measure primary resistance, set the multimeter to the lowest Ohms scale, often 200 $\Omega$, and touch the probes to the coil’s positive and negative terminals. A typical reading for the primary winding should fall within a very low range, often between 0.4 and 2.0 Ohms, though this value is highly specific to the vehicle manufacturer.
Secondary Resistance Test
The secondary resistance test measures the high-voltage output circuit, which consists of thousands of turns of much finer wire. For this test, move one probe to the high-voltage output terminal—where the spark plug connects—and the other probe to the positive or negative terminal. The multimeter must be set to a higher range, typically the kilo-Ohms (k$\Omega$) scale, to accommodate the much greater resistance. Secondary resistance values are significantly higher, often ranging from 6,000 to 10,000 Ohms (6k$\Omega$ to 10k$\Omega$). Always consult the vehicle’s repair manual for the exact resistance specifications, as a reading outside this range indicates an internal fault.
Testing for Spark Output
While resistance testing checks the coil’s internal electrical health, a spark output test confirms its ability to generate the necessary high-voltage discharge under operating conditions. This functional test requires a dedicated spark tester, which connects between the coil and a grounded point on the engine block. The spark tester features an adjustable gap that simulates the distance the spark must jump.
To perform this test, connect the spark tester to the coil’s output and ensure the tester’s body is securely grounded to a clean metal surface on the engine. Remove the fuel pump fuse or relay to prevent fuel injection, then briefly crank the engine while observing the tester’s gap. A healthy ignition coil will produce a bright, blue, and consistent spark that easily jumps the gap. A weak, yellow, or intermittent spark indicates that the coil is failing to produce the required voltage.
Understanding Your Test Results
Interpreting the results from the resistance and spark tests determines the next course of action for the ignition system. If the multimeter reading for either the primary or secondary winding falls outside the manufacturer’s specified Ohms range, the coil has an internal defect. A reading of zero Ohms indicates a short circuit within the winding, while a reading of infinity (or “OL” for open line) suggests a complete break in the circuit.
If the spark output test yields a weak, inconsistent, or absent spark, the coil is not generating the necessary voltage to fire the spark plug reliably. In either scenario—an out-of-specification resistance reading or a poor spark output—the coil is considered faulty and requires replacement. If both the resistance and spark tests pass, the coil itself is likely functional, and the diagnosis should shift to checking the wiring harness, connectors, or the electronic control unit (ECU) signal that triggers the coil.