Test A Spark Plug Wire On A Car

You can test your spark plus wires simply at home.

Whether you just want to be sure that the new carbon spark plug wires you purchased are up to par and that the resistance is correct or whether you want to test your current wires for resistance, you can perform a simple test to do so. Proper resistance in a spark plug wire is critical to providing optimum current so your car engine runs at its maximum ability, saving you fuel.


1. Remove only one wire at a time. If it is a used wire, start by wiping down the wire with WD-40, since your visual inspection will be easier with a clean wire. Be sure the metal clips within the boots at each end of the wire are clean, corrosion free and in good condition.

2. Prepare your tester and set up on the OHM test section on the meter at 20,000, or 20k.

3. Measure the length of the wire, since the length is a factor you must know to determine test results. To test, hold the positive, or red, probe into one end, being sure to have contact with the metal clip in the boot and the negative, or black, probe into the other end.

4. Watch the readings. The readings may jump around at first but should stabilize after a few moments. A shorter wire with a length of 12 inches may test out between 6.00 to 7.00 ohms and a longer wire of 30 inches may measure out between 9.00 and 11.00. A 24-inch wire may come in between 7.50 and 9.00 ohms. All of the above readings would put you in a normal range. Any reading of less than 6.00, regardless of the length, would represent low resistance, and that wire should be replaced.

5. Install the wires if your readings are satisfactory. Apply a small amount of dielectric grease on the inner portion of the metal clips within the boot to help ensure the boot will not get stuck to the plug and to prevent arcing.

6. Install your wire back to the plug and location on the distributor cap from which it came and be sure that the wire is secure at both ends. Tug on it a bit after installation to be sure you have a proper fit. If you have a problem securing a connection, work it with needle-nose pliers to reshape the metal clips within the boots for a more secure fit.