What If an Ignition Coil Has Low Secondary Resistance?

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The ignition coil in an automotive engine is a transformer consisting of two coils, or windings, of insulated wire. Low-voltage current from the battery flows through the primary windings and is transformed into high-voltage current required by spark plugs in the secondary windings.

Resistance

  • The electric resistance of the primary and secondary windings -- their opposition to the flow of electrical current -- varies from manufacturer to manufacturer. However, the resistance of the primary windings is typically very low (up to 1 or 2 ohms), while the resistance of the secondary windings is relatively high (up to 14,000 ohms or more).

Spark

  • Low electrical resistance in the secondary windings is often the result of a short circuit. In any case, low resistance will result in a very weak spark, or no spark at all, which means that it will be difficult or impossible to start the engine.

Testing

  • You can test the resistance of secondary windings by attaching a digital multimeter -- with its resistance range set to 20,000 ohms -- across the positive terminal and center terminals of the ignition coil. A meter reading of below 11.0 is usually indicative of a problem.

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