How do you test an ignition coil to see if it's bad?

To determine if an ignition coil is faulty, you can employ a range of diagnostic methods, from simple visual inspections to more detailed electrical tests.

Symptoms of a Bad Ignition Coil

Before testing, recognizing common symptoms can help confirm a suspected bad coil:

• Engine Misfires: This is often the most noticeable symptom, leading to rough idling or stumbling during acceleration.
• Reduced Fuel Economy: An inefficient coil can disrupt combustion, leading to more fuel consumption.
• Check Engine Light (CEL): The engine control unit (ECU) may detect a misfire or a coil issue and illuminate the CEL, often with specific diagnostic trouble codes (DTCs) like P0351, P0352, etc., indicating an ignition coil primary/secondary circuit malfunction.
• Rough Idle: The engine may vibrate or shake unevenly.
• Stalling or No-Start Condition: In severe cases, a completely failed coil can prevent the engine from starting or cause it to stall.
• Loss of Power: Weak or inconsistent spark can lead to a noticeable drop in engine power.

Testing Methods for an Ignition Coil

1. Visual Inspection

A quick visual check can sometimes reveal obvious damage:

• Cracks or Discoloration: Look for any physical cracks, particularly on the coil housing, which can indicate internal damage or exposure to extreme heat.
• Oil Leaks: Some older ignition coils are oil-filled; leaks can suggest internal failure.
• Corrosion: Check the electrical terminals for rust or corrosion, which can impede electrical flow.
• Swelling: A swollen or bloated appearance can indicate overheating and internal short circuits.

2. Inline Spark Test

This simple method helps determine if the coil is producing a spark, especially useful for older vehicles with a single coil and distributor. For a DIYer, this is a straightforward test:

1. Safety First: Ensure the vehicle is in park/neutral with the parking brake engaged. Disconnect the fuel pump fuse or relay to prevent fuel from entering the cylinders unnecessarily during cranking.
2. Locate the Coil Wire: Identify the main coil wire that connects to the distributor cap.
3. Position for Spark: Remove the coil wire from the distributor and position its metal end about a quarter-inch (6mm) from a grounded metal part of the engine block (e.g., an unpainted bolt or bracket).
4. Crank the Engine: Have a helper crank the engine briefly (for a few seconds).
5. Observe for Spark: Watch for a bright, blueish-white spark jumping from the wire to the engine block.
   • Strong Spark: Indicates the coil is likely firing correctly.
   • Weak/Orange Spark or No Spark: Suggests a problem with the coil or its power supply.

Note: For vehicles with coil-on-plug (COP) systems or coil packs, you'll need a specialized spark tester tool that plugs between the coil and the spark plug to safely check for spark at each cylinder.

3. Multimeter Resistance Test

A multimeter can measure the electrical resistance of the coil's primary and secondary windings. This test assesses the coil's internal health but requires knowing your vehicle's specific resistance specifications (found in a service manual).

Understanding Coil Windings:

• Primary Winding: A low-resistance winding that receives voltage from the battery.
• Secondary Winding: A high-resistance winding that steps up the voltage to thousands of volts for the spark plugs.

Performing the Test:

1. Disconnect Battery: For safety, disconnect the negative terminal of the car battery.

2. Remove Coil: Carefully remove the ignition coil(s) from the vehicle.

3. Set Multimeter: Set your multimeter to the Ohms (Ω) setting.

   • Primary Resistance Test:

     • Locate the positive (+) and negative (-) terminals on the primary side of the coil (usually the smaller terminals where the harness connects).
     • Touch the multimeter probes to these two terminals.
     • Record the reading.

   • Secondary Resistance Test:

     • For single coils with a coil tower, place one probe into the high-tension output terminal (where the spark plug wire connects) and the other probe on the primary positive (+) terminal.
     • For coil-on-plug (COP) coils, you may need to consult a repair manual for the specific pins to test, as the secondary circuit often routes directly to the integrated spark plug boot. Alternatively, some COPs have a separate ground pin for the secondary circuit.
     • Record the reading.

Interpreting Results:

Compare your readings to the manufacturer's specifications. While values vary by vehicle, here are typical ranges:

• Out-of-Specification Readings: If either the primary or secondary resistance falls outside the specified range, or if you get an "OL" (overload/open line) reading indicating an open circuit, the coil is likely faulty.
• Short Circuit: A reading significantly lower than the specified range can indicate an internal short circuit.

4. Professional Diagnostic Scan

For vehicles manufactured after 1996, an OBD-II scanner can be invaluable. Plugging a scanner into the diagnostic port can retrieve specific trouble codes (DTCs) related to ignition coil malfunctions (e.g., P0351 for Ignition Coil A Primary/Secondary Circuit, P0352 for Ignition Coil B, etc.). While not a direct test of the coil itself, these codes strongly point towards a coil-related issue and guide further testing.

By utilizing a combination of these methods, you can effectively diagnose a faulty ignition coil and restore your vehicle's performance. For more in-depth automotive troubleshooting, consult Automotive Troubleshooting Resources.