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Thread: Erratic RPM reading

  1. #1

    Erratic RPM reading

    The last few flights on my AF-4500 I have started getting fluctuating RPM readings where the value rapidly goes up from where it should be (I have a LightSpeed on one of the mags with a digital display) by 500-700 RPMs and then back down again.

    On take-off the fluctuations (up and back again) are most pronounced with the cruise being smaller fluctuations.

    It has worked fine for over a year.

    I assume if it was a bad ground it would be fluctuating between zero and the proper value? In this case it is fluctuating upwards, not down.

  2. #2
    Advanced Flight Systems Shawn McGinnis's Avatar
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    Sep 2012
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    Canby, OR
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    We have seen many issues with RPM recently after spark plug maintenance. I would start by verifying the plug wires by removing the wire and giving it a light pull while measuring the continuity. Small breaks in the wire will still allow the plugs to fire like normal while also emitting a large amount of EMF. Normally enough to cause reading problems. If this was a new install we might look into routing but its normally linked to a maintenance.
    Shawn McGinnis

    Advanced Flight Systems
    support@advanced-flight-systems.com

  3. #3
    Something I and another building have found is the wires going into the RPM sensor can come loose. If you slightly pull or push on the wires where they enter the RPM Sensor in the magneto you can feel it slightly move/pop into place. I recently added some high temp RTV to help hold it in place.

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