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Thread: Tachometer hookup for a Mosler 40 hp magneto driven

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    Tachometer hookup for a Mosler 40 hp magneto driven

    I am building a Lincoln Sport from plans and it will be powered by a 40 hp Mosler with a Fairbanks Morse magneto. No tachometer drive. I am looking for an original-looking tachometer that can be fired by the magneto or a magnetic pickup. Does anybody have a solution that they have used in an application like this. Any ideas would be greatly appreciated.

  2. #2
    Sam Buchanan's Avatar
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    Quote Originally Posted by iker1902 View Post
    I am building a Lincoln Sport from plans and it will be powered by a 40 hp Mosler with a Fairbanks Morse magneto. No tachometer drive. I am looking for an original-looking tachometer that can be fired by the magneto or a magnetic pickup. Does anybody have a solution that they have used in an application like this. Any ideas would be greatly appreciated.
    A TinyTach doesn't look vintage but is probably your best option. I strongly suggest you not use a tach that connects to the magneto P-lead. If the tach malfunctions and the signal lead to the tach goes to ground, it will kill the mag and you will be flying a glider......

    This is not an issue with a tach that uses a sensing lead wrapped around a spark plug lead (TinyTach)
    Last edited by Sam Buchanan; 12-02-2018 at 08:18 PM.
    Sam Buchanan
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    Quote Originally Posted by Sam Buchanan View Post
    A TinyTach doesn't look vintage but is probably your best option. I strongly suggest you not use a tach that connects to the magneto P-lead. If the tach malfunctions and the signal lead to the tach goes to ground, it will kill the mag and you will be flying a glider......

    This is not an issue with a tach that uses a sensing lead wrapped around a spark plug lead (TinyTach)
    I have had this debate before with others over this same subject. I agree with Sam that nothing gets hooked to the P lead but the kill switch. But then others tell me that if you hook a 1/4 amp fuse to said P lead and the tach is ran through this it will blow the fuse. I had a hard time believing this. Then my new airplane with a VW it came with a tach hooked to the P-lead and there is a 1/4 amp in-line fuse hooked to this. But IMHO a Tiny Tack would be a better choice. But many would ague that I am wrong and the tach hooked to the P-lead is fine. Only way I would know would be to short out my tach and see what happens. I am not doing this.

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    Dana's Avatar
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    The Mosler in my Fisher 404 had the tach connected to (and powered by) the P-lead, worked just fine. There was a fuse in the line as the tach manufacturer specifies. I don't see a problem with it.

    And if you have a shielded ignition as I did, a Tiny Tach wouldn't work anyway.

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    Sam Buchanan's Avatar
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    Quote Originally Posted by Dana View Post
    The Mosler in my Fisher 404 had the tach connected to (and powered by) the P-lead, worked just fine. There was a fuse in the line as the tach manufacturer specifies. I don't see a problem with it.

    And if you have a shielded ignition as I did, a Tiny Tach wouldn't work anyway.
    I agree the P-lead tach will work fine, I flew one for awhile on my Legal Eagle. But thinking about the possibility of the tach failing internally and shorting to ground was enough to prompt me to move to a TinyTach that eliminated that scenario. Having only one ignition causes us to re-evaluate single-point failures.
    Last edited by Sam Buchanan; 12-03-2018 at 07:58 AM.
    Sam Buchanan
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    The last thing I want to do is use the p-lead to generate the tach signal. I am more interested in magnetic or optical pickup. Has anyone used either of these on an engine without a flywheel? Maybe I could create a small "flywheel" to mount to the backside of the aluminum prop hub.

  7. #7
    Dana's Avatar
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    P-lead tachs have been around for a long time. Has anybody ever heard of one failing and shorting out the magneto? Seems a simultaneous failure of the tach and the fuse is pretty unlikely.

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