How to Test an Ignition Coil with an Ohmmeter

How to check if an ignition coil is bad?

Identify Common Symptoms of a Faulty Ignition Coil

A failing ignition coil often causes noticeable driving issues. Look for these signs:
- Engine misfires: Rough idling, hesitation, or stumbling, especially under acceleration.
- Check engine light: The dashboard warning light may illuminate due to spark-related issues.
- Poor fuel efficiency: Reduced power and increased fuel consumption as the engine struggles to ignite fuel properly.
- Backfiring or popping sounds: From the exhaust, indicating incomplete combustion.

Perform a Visual Inspection

Inspect the ignition coil for visible damage:
- Cracks or physical damage: Check the coil housing for cracks, burns, or melted plastic.
- Disconnected or corroded wiring: Ensure all electrical connections are secure and free of corrosion.
- Arcing or sparks: Signs of arcing between the coil and spark plug wires indicate a breakdown in insulation.

Use an OBD2 Scanner to Retrieve Diagnostic Trouble Codes (DTCs)

Connect an OBD2 scanner to retrieve live data and stored codes:
- Look for P0300-series codes (e.g., P0301–P0308), which signal ignition system malfunctions.
- Monitor live data for misfire counts or spark plug circuit issues linked to specific cylinders.
- Clear codes after repairs to confirm resolution.

Test Ignition Coil Resistance with a Multimeter

Measure the coil’s primary and secondary resistance to determine functionality:
- Primary resistance: Test the low-tension terminals (typically 0.5–3 ohms).
- Secondary resistance: Test the high-tension terminals (5–15k ohms, varies by manufacturer).
- Compare readings to the vehicle’s service manual specifications. Values outside the range indicate a faulty coil.

How do you test a 12 volt coil with a multimeter?

Step 1: Prepare the Coil and Multimeter

To test a 12-volt coil, start by disconnecting power to the coil to avoid electrical shock. Clean the coil’s terminals with a wire brush to ensure a proper connection. Set your multimeter to the ohms (Ω) function to measure resistance. For coils with multiple terminals (primary and secondary windings), identify the correct terminals using the coil’s wiring diagram.

Step 2: Measure Primary Winding Resistance

Place the multimeter probes on the primary winding terminals (often labeled “+” and “-” or “battery” and “switch”). A healthy 12-volt coil’s primary resistance typically ranges from 0.5 to 3 ohms. If the reading is 0 ohms (short circuit) or infinite resistance (open circuit), the coil is likely faulty. Compare the result to the manufacturer’s specifications for accuracy.

Step 3: Test the Secondary Winding Continuity

Switch the multimeter to the high resistance range (e.g., 20kΩ or 200kΩ). Test the secondary winding terminals (often marked “output” or “spark plug”). A functioning secondary winding should show resistance between 6,000 to 12,000 ohms. If the meter reads 0 ohms (short) or no reading (open), the coil requires replacement.

Optional: Check Voltage Drop Under Load

To test functionality further, power the coil (ensure the circuit is safe) and set the multimeter to volts (V). Connect the probes across the primary terminals while activating the coil (e.g., by triggering the ignition system). A significant voltage drop (exceeding 1–2 volts) may indicate internal resistance issues. Always follow safety protocols when working with live circuits.

How to measure the ohms of a coil?

Gather the Right Tools

To measure the ohms of a coil, you’ll need an ohmmeter or a multimeter set to the resistance measurement mode. Ensure the device is calibrated and in good working condition. Avoid using a meter with auto-ranging features if unsure of the coil’s resistance range, as this can lead to inaccurate readings. Always disconnect the coil from any power source before testing to prevent damage or safety hazards.

Prepare the Coil for Measurement

Clean the coil’s terminals to remove dirt or oxidation, which can interfere with electrical contact. If the coil is part of a larger device (e.g., a vape mod or electronic circuit), disconnect it completely. For standalone coils, ensure no external components are attached. Safety first: never measure resistance while the coil is powered or connected to a live circuit.

Take the Measurement

Hold the meter’s probes by the insulated handles and touch the red probe to one terminal of the coil, and the black probe to the other. The meter will display the resistance value in ohms (Ω). For accuracy, take multiple readings and average them if the values fluctuate slightly. If the reading is “0” or “OL” (open load), check the probes’ contact or ensure the coil isn’t broken. Record the result and compare it to the manufacturer’s specifications if available.

Interpreting Results

A low resistance reading (e.g., below 1Ω) might indicate a short circuit, while a very high reading could mean a broken coil. Ensure the measured ohms align with the coil’s intended use—vaping coils, for instance, typically range from 0.5Ω to 3Ω. If the result is unexpected, retest after recalibrating the meter or using a different device to confirm accuracy.

How many ohms should a 12 volt coil have?

The ideal resistance of a 12-volt coil depends on its intended application and power requirements. Ohm’s Law (V = I × R) dictates that voltage (V), current (I), and resistance (R) are interdependent. For a 12V system, the coil’s resistance must balance the desired current draw and power output. For example, a coil designed for high-power applications like a car starter motor requires low resistance (e.g., 0.5–2 ohms) to allow sufficient current flow, while a low-power coil in an ignition system might operate at higher resistance (e.g., 10–15 ohms) to reduce current and prevent overheating.

Common Resistance Ranges for 12V Coils

Resistance values vary based on the coil’s purpose:
- Automotive starters: 0.5–2 ohms (supports high current for heavy loads).
- Ignition coils: 10–15 ohms (ensures controlled spark generation).
- General applications (e.g., solenoids, relays): 4–16 ohms (balances power and safety).
Always refer to the manufacturer’s specifications, as deviations can cause underperformance or component damage.

Testing and Matching the Coil to Your System

To determine the correct resistance for your 12V coil:
1. Calculate required current using power (P = V × I) and Ohm’s Law.
2. Use a multimeter to measure the coil’s actual resistance.
3. Compare results to the device’s rated specifications.
Mismatched resistance can lead to voltage drops, excessive heat, or failure. For instance, a 12V coil with 12 ohms draws 1A (12V/12Ω), while a 4-ohm coil would draw 3A—both valid but for different use cases. Always prioritize the manufacturer’s guidelines for safety and efficiency.