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An Ohm meter, which is part of a multimeter, can only check a spark plug for a short circuit or a breakdown of insulation resistance. A good spark plug should show an open circuit between the center electrode and the tip. Any resistance at all would mean the plug is bad.
An Ohm meter, which is part of a multimeter, can only check a spark plug for a short circuit or a breakdown of insulation resistance. A good spark plug should show an open circuit between the center electrode and the tip. Any resistance at all would mean the plug is bad.
Test the spark plug ignition by disconnecting the spark plug wire from the spark plug. Hold the end of the spark plug wire close to a metal surface. If the spark plug is good, you will see a spark or you’ll hear a crackling noise. This means that voltage is getting through the wire to the spark plug.
When spark plugs are new, the resistance value is between 500 and 3000 ohms. As the plug ages, values below 5000 ohms are acceptable. But once the plug’s resistance value exceeds 5000 ohms, the plug should be replaced. If a spark plug falls off a workbench or is dropped, it’s possible for the carbon resistor to break.
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Test with a spark plug tester
Attach a spark plug tester to the ignition wire. At the other end, the tester connects to the spark plug as it sits in your engine. Turn the ignition ON. … If no visible spark or glow is present, then you can be sure that no spark is occurring at the all-important tip of the spark plug.
A spark plugs reach is calculated by the length of the threads. It is measured from the seat of the plug to the end of the threads; it does not include the electrodes, or an extended metal shell.
With the advent of CDI and electronic ignition the plugs have a built in resistor in them. So you shouldn’t be seeing full continuity. If it is reading dead open then it has to be bad, unless your ohmeter is set on a very low range.
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Under normal conditions, a spark plug can sit in a vehicle for years without going bad, even if you didn’t ignite it once during that period. If stored properly brand-new, it can virtually sit forever.
In general, a light tan/gray color tells you that the spark plug is operating at optimum temperature and that the engine is in good condition.
The spark ignition tester connects to the boot the same way the boot connects to the spark plug itself. It will usually look like a clear cylinder with a wire coil and a light bulb inside.
Pull the rubber casing off the spark plug. Set the multimeter into the ohms function, or just use an ohmmeter. Manually set the dial or button on the meter to the 40 k range. Do not use auto ranging, as it is unreliable with a magneto.
Most coils should read between 0.4 and 2 ohms. Zero resistance would indicate a shorted coil while a high resistance reading would indicate an open coil.
Failing spark plugs can cause the car engine to misfire and thus affect its performance. A single spark plug that fails to ignite the fuel-air mixture can cause a halt in the running of the engine. It can result in incomplete combustion and damage to the catalytic converter of the car.
When testing a coil or secondary ignition component it is important to use a tester that will create this much voltage. Without it, a high tension leak or a weak coil can be overlooked. … A bit repetitious, but you need to use the same diagnostic strategy each time you approach an ignition system problem.
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