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How to Check Coil Packs With a Voltage Meter

by Dan Ferrell

A faulty coil may fire intermittently, produce a weak spark or kill a cylinder completely. Coils are sensitive to heat and may produce an intermittent problem if their internal components become weak. Once the winding inside weakens or breaks, the problem is irreversible. However, you can test the coil pack on your car using a voltmeter.

Park your car in a safe place and open the hood.

Locate the coil pack in your engine by following the spark plug wires from the spark plugs; the wires are connected to the coil pack. Depending on your particular vehicle model, you may find the coil pack mounted on one side or the back of the engine.

Unplug the spark plug wires from the coil and label the wires if necessary for proper installation.

Unplug the pack's electrical connector and remove the coil pack from the engine, using a wrench or ratchet and socket if necessary. If your coil pack is mounted on the back of the engine, raise the front of the vehicle and safely support it on two jack stands to reach the coil pack from underneath the engine.

Set your ohmmeter to the 20000 ohms range. Turn on your meter and touch the spark plug wire terminals on one of the coils with the meter leads. You may get a reading between 5000 to 15000 ohms, depending on your particular model. This checks the secondary resistance.

Set your ohmmeter to the 10 ohms range, and touch the terminal B+ (usually the center prong on the coil pack electrical connector) with one of the meter test leads, and touch the corresponding coil prong on the electrical connector with the other test lead. You may get a reading between 0.3 and 1.0 or more, depending on your particular model. This checks the primary resistance on each coil.

Repeat steps 6 and 7 for each coil in the pack assembly, and compare your resistance readings to the specifications given on your vehicle service manual (see "Tips"). If your readings are out of range, replace the coil or coils as necessary.

Tip

  • Consult your vehicle's service manual to obtain primary and secondary resistance specifications and identify primary and secondary terminals on the coil pack used on your car.

Items you will need

About the Author

Since 2003 Dan Ferrell has contributed general and consumer-oriented news to television and the Web. His work has appeared in Texas, New Mexico and Miami and on various websites. Ferrell is a certified automation and control technician from the Advanced Technology Center in El Paso, Texas.

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Photo Credits

  • under the hood image by Lucy Cherniak from Fotolia.com