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.
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).
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.
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.
