How to Test an Engine Coil

by Chris Stevenson

Automotive ignition coils function as miniature transformers that increase 12 volts to several thousand for the purpose of igniting the spark plugs. Without an ignition coil, the spark inside the cylinder would not have enough voltage to ignite the fuel mixture for combustion. Consisting of primary and secondary windings, some coils fire all the cylinders through one unit, while other designs incorporate the use of single coils for each cylinder or one coil for two cylinders. Single unit coils routinely have jacket cores filled with oil to reduce heat. Once a coil becomes defective, it can allow misfire in the engine or keep the engine from running.

Place the vehicle in park or neutral with the emergency brake set. Raise the hood. Disconnect the negative battery cable. Identify the type of coil your vehicle has by referring to your owner's manual. Connected to the coil will be a high-voltage, plug-like wire, which will lead to a cap or an individual spark plug. Examine the single unit coil, if you have this design, for an oil film or oil leakage on the coil body. If the coil casing looks deformed (bloated) or has an oil leak, it must be replaced.

Connect the negative cable with a socket and wrench. Connect a code scanner to your vehicle's input jack under the dashboard if you have 1996 or newer engine with an OBDII (multiple coil packs) ignition system. Start the engine and look for a P030X code, where "X" represents a faulty coil at a certain cylinder location. Any such code will indicate a short or break inside the coil.

Connect the battery cables. Use a 10 mega-ohm impedance ohmmeter to check your coil. Turn the ignition key to the "on" position. Place the ohmmeter red (positive) wire to the "+" (positive) side of the coil. Place the black (negative) ohmmeter wire on the "-" (minus) side of the coil and read the ohm resistance. The value should be between 0.4 to 2.0 ohms. Refer to your owner's manual for the exact value. Any reading inside or outside specifications will indicate bad primary winding in the coil.

Place the ohmmeter red (positive) wire on the "+" terminal of the coil. Place the black (negative) wire from the ohmmeter down inside the coil tower and read the value for the secondary winding. The reading should fall within a range of 6,000 to 15,000 ohms, but check your owner's manual for the exact value. Any ohm reading inside or outside the specified value will indicate a defective secondary winding circuit in the coil.

Use a socket and wrench to remove individual coil packs to test with the ohmmeter if your vehicle has this design. Refer to your owner's manual for the pin location of the positive and negative sides to the coil inside the connector. Test the coil as you would for a single-unit coil and record the readings.

Turn the coil over and examine the underside for the terminal contacts if they do not appear on the top of the coil. Test the coil with the ohmmeter as in Step 5 for the single-unit coil. Values outside or inside of specifications will point to a defective coil.

Items you will need

About the Author

Chris Stevenson has been writing since 1988. His automotive vocation has spanned more than 35 years and he authored the auto repair manual "Auto Repair Shams and Scams" in 1990. Stevenson holds a P.D.S Toyota certificate, ASE brake certification, Clean Air Act certification and a California smog license.

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