Hello folks, had my tach become erratic the other day on a short flight. RPMS were back and forth with little or no power changes, it seemed better when taxing back to the hanger. Just wondering if anyone else has had similar issue? 1967 PA-28. Thanks


  • Hi, I'm assuming it's an old style tach. There are two causes: one, a bad tach cable. It's probably original and ready to be replaced. Two, your tach could be originial and ready for overhaul or replacement. Don't replace either with the same technology. If you need a tach, get this. I had two for 15 years and they're awesome and totally accurate. https://buy-ei.com/portfolio/r-1/

    Scott Sherer
    Wright Brothers Master Pilot, FAA Commercial Pilot
    Aviation Director, Piper Owner Society Forum Moderator and Pipers Author.

    Need help? Let me know!

  • Thanks Scott!

  • Hi Billy,

    I have had some experience with the mechanical tach in the Cherokee. Mine was never intermittent but when I installed a factory zero time engine, I sent the tach out for overhaul and calibration. I also wanted the red RPM restriction indicated on the tach removed as the new engine had a solid crank and no RPM restriction. Because it was the original tach and had a crimped on case, the shop would not overhaul it! So my only option was a replacement.

    Mitchell tachometer was the recommended replacement. They would also preset the tach time to match the original tach.

    So I asked Mitchell directly in a phone call what model they would recommend for my aircraft make, model and serial number and then placed the order. As I recall, the tach with custom preset hours was under $300. So far, so good.

    After flying with the new tach, which, by the way, did not look as "cool" as the original, it was apparent that it was racking up tach time WAY to quickly!

    As it turns out, no pun intended, a mechanical tach indicates "clock" time AT ONLY ONE RPM! So when the engine is turning slower, the accumulated tach time per minute of clock time is lower. When the engine is turning faster, the accumulated tach time per minute of clock time is higher as was the case with the new tach.

    I spoke to Mitchell at length about this and even though THEY recommended the incorrect model, they would not offer ANY financial relief on a reorder and I had to pay double to get the proper tach in the aircraft.

    So, if you want to preserve the original mechanical tach, be sure to get the proper model most closely matching the cruise RPM for your aircraft or take Scott's suggestion and perhaps update the technology.


  • Thanks Mike, I sent my back to Mitchell and they repaired and returned within a week! So far it works well, $185 for the rebuild and they gave me new bezel inserts and screws for the install.

  • Is this a power surge or is it a tach fluctuation? Scott is dead on with the worn cable. Believe it or not, they need some rigidity and the proper type instrument grease.The old tach is probably not helping either.

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