To measure spark plug wire resistance, you simply attach or hold a meter probe on each end of the cable and read the total resistance on the meter.
Measuring the resistance of your spark plug wires is a straightforward process that helps determine if they are functioning correctly. Excessive resistance can impede the flow of voltage from the ignition coil to the spark plug, potentially causing misfires or poor engine performance.
Step-by-Step Measurement
Testing a spark plug wire's resistance requires a multimeter capable of measuring ohms (Ω).
- Obtain a multimeter and set it to the resistance setting (Ω). Choose a range appropriate for measuring several thousand ohms, typically 20kΩ.
- Disconnect the spark plug wire from both the spark plug and the distributor cap or coil pack.
- Attach or hold one meter probe firmly against the metal terminal inside one end of the spark plug wire.
- Attach or hold the other meter probe firmly against the metal terminal inside the other end of the spark plug wire.
- Read the value displayed on the multimeter screen. This reading represents the total resistance of the wire in ohms.
Understanding Spark Plug Wire Resistance Specifications
The resistance value you read indicates how much the wire opposes the flow of electrical current. A certain amount of resistance is normal and necessary in some wire designs (like carbon-cored wires which act as resistors to suppress radio frequency interference). However, resistance that is too high indicates a problem, often due to internal damage or degradation of the wire.
Industry Standards and OEM Recommendations
Guidelines exist to help you determine if a wire's resistance is within acceptable limits:
- SAE Guideline: The Society of Automotive Engineers (SAE) suggests that the maximum resistance for spark plug wires should be 12,000 ohms-per-foot. This is a common standard for aftermarket wires.
- OEM Variations: It is important to note that some Original Equipment Manufacturers (OEMs) have recommended different maximum resistances for the wires used in their specific vehicles. Always check the vehicle's service manual for the most accurate specifications.
Compare your measured resistance value to the manufacturer's specification or the SAE guideline (considering the wire's length) to determine if the wire is good or needs replacement. A wire showing infinite resistance (or 'OL' on the meter, indicating an open circuit) is definitely bad.
