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How Can I Check RPMs With Multimeter

  1. Aug 17, 2011 #1
    I have a Fluke model 112 multimeter. Is there any way I can measure gasoline engine speed by using the Hertz measuring capability of the meter. If I cannot, why not?
     
  2. jcsd
  3. Aug 17, 2011 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF.

    You might be able to hold the + probe up to one of the sparkplug wires (ground the - probe) and pick up enough ignition noise to give you a reading. Start by putting the meter on AC volts, and see how big the signal is. If it is a reasonable signal, try the Hz setting.

    If that doesn't work, you could make a transformer to put the sparkplug wire through. Use a ferrite core that is big enough for the sparkplug wire and cap to fit through, and wind about 20 turns of insulated wire around the core. This will form a 1:20 transformer that you can connect your DVM leads to. You should get enough of a pickup voltage with the 1:20 transformer to be able to make your measurement.
     
  4. Aug 17, 2011 #3
    Thanks, I'll give both suggestions a try.
     
  5. Aug 17, 2011 #4
    I tried the easier one first where I merely put the lead on the insulated portion of the spark plug wire. I did this on a single cylinder 4 cycle lawnmower. The AC voltage was a little over 1 volt. When I switched to hertz, the idle reading was 210 hz. Evidently something is confusing the DVM. The hertz capability works fine because if I insert it in an AC outlet, it says 60.0.
     
  6. Aug 17, 2011 #5

    berkeman

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    210Hz would be 420 revolutions per second, so 25,200 RPM. Yeah, sounds high. There must be some ringing or noise that is adding extra zero crossings to the reading. You can try filtering the signal some by putting a capacitor across the DVM terminals. Go back to the AC V setting on the DVM, and try putting a 1uF cap across the terminals. Does that knock the signal down too much?
     
  7. Aug 17, 2011 #6
    All you need to do is put the dmm on the low voltage side of the sparking coil. If its on a lawn mower it will be a small inductive pulse every revolution. If its on a vehicle, there will be a pulse to ground every time the ECU sends for a spark.
     
  8. Dec 16, 2011 #7
    A flywheel coil ignition will fire every revolution (wasted spark). 210hz = 12,600 rpms I'm getting a crazy reading too.And I also get a steady 60hz from an outlet. I'm thinking it must be noise. I could just order an inductive pick up. Noway that would take the fun of it.
     
  9. Dec 18, 2011 #8

    jim hardy

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    presumably there's a magnet that goes past the ignition coil once per revolution.

    perhaps you could wrap some wire around a nail and mount it near flywheel, making your own independent speed pickup for the dmm?
     
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