What should spark plug wire resistance be

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Automotive Industry. Automotive History. Welcome to AxleAddict! Testing spark plug wires helps you diagnose common engine problems, such as: cylinder misfires, rough idle, loss of power, hard-to-start issues, and an increase in gas consumption. Tools You'll Need: 1- or 2-foot jumper wire Screwdriver with a wooden or plastic handle Spark plug wire pliers, if necessary Clean rag Tape measure Digital or analog multimeter.

Read More from AxleAddict. EGR Valve Location. Exhaust Leak Diagnosis. Diagnosing an Intake Manifold Leak. That's Not the Campground's Job. Early spark plug wires were very simple—just metal that conducted.

Those simple conductors evolved to require insulation to shield the ever-increasing voltage from jumping to an unintended ground rather than through the spark plug. Not only did insulation join the features list for plug wires, increasing use of electronics as the decades wore on required the development of EMI and RFI suppression.

Electromagnetic and radio-frequency interference can be a real pain in the butt. The most common instance of radio frequency is the phenomenon that occurs when a home mechanic replaces the plug wires on a vintage car and afterwards the radio has a pinging noise whenever the driver turns on sweet tunes while the engine is running. That pinging noise is the radio-frequency interference from an uninsulated plug wire.

That electrical noise can wreak havoc on everything from AM radios to engine and transmission control computers. So if a little insulation is great, more must be better, right? Yet again, not quite. Like everything in life that is good, that insulation has side effects. The side effects are resistance and cost. The resistance of many wires is measured in ohms per foot. A wire with high resistance can be in the ohms per foot range for especially high-output ignition systems, whereas a low resistance performance wire could easily get double digit ohms per foot.

Lower resistance means more of the ignition coils energy is going towards bridging the gap or the spark plug. Also, all the shielding inside a wire is not free. The best spend of your money is to buy a wire that protects against just enough interference to keep the rest of your car functioning properly—but no more.

That is a tough balance to strike on modern computer-heavy cars, or even racers with sensitive data logging equipment. For stock applications, an OEM-spec wire might be best despite it having greater resistance than an aftermarket wire.

The Society of Automotive Engineers suggests the maximum resistance should be 12, ohms-per-foot. However, some OEMs have recommended different maximum resistances. Consult a tune-up manual for exact specifications. Attach the test light to a good engine ground. With the engine running, move the test light along the length of each spark plug wire. If a spark jumps from a plug wire to the test light at any point, this is an indication that the insulation has broken down, and the spark plug wire should be replaced.

Note: Try to avoid direct contact with metal parts of the vehicle when checking for shorts. Remove a spark plug wire from any plug. Attach a spark tester to the wire and to an engine ground. Crank the engine and check for a good spark at the spark tester gap. A good spark will be blue-white and will be plainly visible in daylight. If a good spark is present, the problem is probably not in the ignition system.

Weak sparks are orange or red and may be hard to see in daylight. If you did not see a spark, remove the coil wire from the distributor cap.