Symptoms of a Bad Coil:
* Misfiring/Rough Running Engine: This is the most common symptom. The engine may sputter, run rough, hesitate, or have a noticeable loss of power, especially under load. You might hear a distinct popping or backfiring sound from the exhaust.
* Difficulty Starting: The engine might crank but not fire, or start intermittently.
* No Spark: This is the most definitive sign. If you have no spark at the spark plug wires, the coil is a prime suspect.
* Weak Spark: This is harder to diagnose without specialized tools. A weak spark might be sufficient to start the engine but lead to misfiring at higher RPMs.
Testing the Coil (Requires Basic Tools):
1. Visual Inspection:
* Check for Physical Damage: Look for cracks, burns, or any obvious signs of damage on the coil itself.
* Check for Loose Connections: Ensure the positive (+) and negative (-) wires are securely connected to the coil terminals. Corrosion can be a problem here.
2. Testing with a Spark Tester or Test Light:
* Obtain a spark tester or a test light: These are inexpensive and readily available at auto parts stores.
* Disconnect the negative (-) wire from the coil. This is crucial for safety.
* Connect one lead of the spark tester or test light to a good ground. (Unpainted metal on the engine block is usually fine).
* Connect the other lead to the negative (-) coil terminal.
* Have someone crank the engine. You should see a bright spark across the spark tester's gap, or the test light should illuminate if using a test light. If there's no spark or weak spark, the coil is suspect.
3. Ohmmeter Test (More Advanced):
* This requires a multimeter capable of measuring resistance (Ohms).
* Consult your Mustang's repair manual or a wiring diagram: You'll need to find the correct resistance specifications for your specific coil. These values vary depending on the coil's design.
* Disconnect the coil wires.
* Measure the resistance between the primary terminals (usually the large terminals) and the secondary terminals (usually the smaller, high-voltage terminals). Your readings should match the specifications in your manual. Significant deviations usually indicate a faulty coil.
Important Considerations:
* Other Potential Causes: Before condemning the coil, ensure you've ruled out other possible problems like:
* Distributor cap and rotor: Cracks or worn contacts can cause misfiring.
* Spark plug wires: Cracked or worn insulation can lead to misfires.
* Spark plugs: Worn or fouled plugs can prevent proper ignition.
* Ignition switch: A faulty switch might not be sending sufficient power to the coil.
* Points and condenser (if applicable): Older points-type ignition systems can cause intermittent spark problems.
* Safety First: Always disconnect the negative battery terminal before working on any electrical components in your car to prevent electric shock.
If after testing you're still unsure, it's best to consult a mechanic experienced with classic Mustangs. Replacing a coil is relatively straightforward, but improper diagnosis could lead to unnecessary repairs.
