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Thread: Tach jumping

  1. #1

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    Tach jumping

    When a tach starts to "jump" around and varrious rpm, does it tend to be the fault of the cable or tach?

    larry

  2. #2
    8GCBC's Avatar
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    Tach jumps: it turned out to be a bad mag for me on a O-320! Could not feel anything in the engine (warning). Gulp.

    But, have replaced cables for a jumping tach too with success. Good first start. But, maybe check compression, timing etc. if it has been awhile.

    I always look a what can kill first. I have low insurance rates because of it.

  3. #3
    M1's Avatar
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    A tach cable that does not have enough lube or the correct lube is probably the most likely reason for a jumpy tach. Mine was like this once, and the cable was pretty dry, once it was lubed the problem went away.

    Mike

  4. #4

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    She was just timed, I'm doing the compression check next week when the kids go back to school. Where can I find direction for the proper way and lube for the tach cable? There hasn't been anything in the log about that. Not an indicator I'm sure, but......Thanks

  5. #5
    8GCBC's Avatar
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    There usually is a little hole on the thumb screw connecting it to the engine? I use ACF50, LPS 1 (T9, LPS2-3 to thick).

    You can remove/clean and reinstall if desperate.

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    8GCBC's Avatar
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    Clean with kerosene (or similar, LPS-1, very light oil), IF it is gummy. Not alot! Drip a little in. It may drip down cable to far and F-up the tach!

    Just thinking about what happend to me. Hope it is just needing lube and not a gummy mess of old plastic and oil like mine. If messy start shopping for new.

    But, get the tach right before departure!

  7. #7

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    Thanks guys, the cable is not leaking. I did clean it a couple days ago, but wasn't sure whether to lube it, so I just wiped it off, then lightly oiled it. No real improvement. Are the cable end designed to wear, or might it be the tach receiver piece that could be worn?

    Thansk

  8. #8
    8GCBC's Avatar
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    My experience has been cables are changed at 0 TBO. Unless you like to "dick with stuff" and change things as you drive. What is TT on the cable? If it is high maybe a change is good. I think we are on the same sheet of music. But, can't tell from here.

    Glad to see your hands dirty!
    Likes j3doc liked this post

  9. #9
    Mush's Avatar
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    Larry, my tach needle started jumping a couple months ago. It did it for a few hours until the tach frooze and broke the cable. Had to buy a new tach and cable. Aircraft Spruce had them both. Paid a little extra to have the range marked. You can even have them set it to your current time for a fee. Hope yours turns out to just be a lubrication issue.

  10. #10

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    If you swing your engine without unhooking the tach cable sometimes it get too sharp of bend in it under the panel which will cause it to jump. It needs a nice gentle arc to work properly.

    If it turns out you need a cable or tach you might take a look at the horizon electric model... They are WAY more accurate, lighter and will tell you in advance if you have a mag problem, I run them in everything.

    Dave

  11. #11
    Mauleguy's Avatar
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    Mine was jumping, put a new tach in 600 hours ago and it fixed the problem. I only had 538 hours on the tach when it went bad. Probably because I bought a cheap tach the first time.
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  12. #12

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    Most tachs have a small "boss" on the rear for the drive.

    Some of these have a port that allows lubing the tach itself.

    Remove a small screw to do this.

    3 in 1 is fine .

    Others still have the boss but no port.

  13. #13

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    Click image for larger version. 

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    I've got what I think is a Stewart Warner tach that is jumping. The cable looks good and is lubricated. I like the look of the old white faced, red needle instrument. I have a pretty original PA-12 and it goes with my other white faced instruments. Does anyone know if anyone overhauls these or if they can still be purchased? Or other options? I've been looking at the Cub tachs that Wag-Aero sells too that would look good. I wish I could find a non-recording one though. Any ideas?

    Thanks
    Mike Watson

  14. #14
    SteveE's Avatar
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    https://www.antiqueinstrument.com/

    Good company. I’ve used them


    Sent from my iPhone using Tapatalk

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    BC12D-4-85's Avatar
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    Mike, you might first try to rotate the tach (in the correct direction) with a variable speed drill. Not sure what it will take but likely something similar to the end of the cable between the two. Flattened nail end or whatever. See if it still flops around or is steady. If ok suspect something else. Or just a drop of oil on the input fitting parts may help.

    Edit: Forgot to mention drive the tach from the engine end of the cable then at the tach bypassing the cable. Just to see if there's any change.

    Gary
    Last edited by BC12D-4-85; 07-17-2021 at 09:47 PM.

  16. #16

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    Thanks for the link to the antique instrument company Steve. That may be a good option depending on price and turnaround time.

    The variable speed drill is a good idea too. I thought about that but wasn't sure if I'd damage anything. The tach jumps the worst in the higher rpm range. It usually fluctuates a couple hundred rpm but sometimes worse.

    Mike Watson

  17. #17

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    Ok, I guess I have to be the one who says it. Get rid of that old antiquated, inaccurate system and install an accurate electronic system. Once you do it you'll never look back. Oh, and you will actually know what your engine is doing.

  18. #18

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    Quote Originally Posted by bubb2 View Post
    Ok, I guess I have to be the one who says it. Get rid of that old antiquated, inaccurate system and install an accurate electronic system. Once you do it you'll never look back. Oh, and you will actually know what your engine is doing.
    Yeah I've been thinking of going electronic too. Still weighing all options. I see there are some electronic tachs that use a needle so as to look analog. I haven't seen any tachs lately that are non-recording. I use a hobbs installed in the panel but maybe it's time to bite the bullet and just get an electric recording tach.

    Mike Watson

  19. #19
    skywagon8a's Avatar
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    Quote Originally Posted by Supercruiser46 View Post
    I use a hobbs installed in the panel but maybe it's time to bite the bullet and just get an electric recording tach.

    Mike Watson
    Keep in mind a recording tach is not a clock, it is a revolutions counter. The number of revolutions an engine makes is more indicative to the amount of wear than the hours. A hobbs counts full hours from the time the oil pressure comes up or the master switch is turned on.

    If you are an FBO renting out your airplane you "sell" the time by the hobbs, but you do your 100 hour inspections by the recording tach. Any maintenance which may be required at a specific number of hours will come up more often with a hobbs keeping track of the time than it will with a recording tach. More $$$ sooner.
    N1PA

  20. #20

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    Quote Originally Posted by skywagon8a View Post
    Keep in mind a recording tach is not a clock, it is a revolutions counter. The number of revolutions an engine makes is more indicative to the amount of wear than the hours. A hobbs counts full hours from the time the oil pressure comes up or the master switch is turned on.

    If you are an FBO renting out your airplane you "sell" the time by the hobbs, but you do your 100 hour inspections by the recording tach. Any maintenance which may be required at a specific number of hours will come up more often with a hobbs keeping track of the time than it will with a recording tach. More $$$ sooner.
    You should consider recording the time on your aircraft based on regulatory standards. You're probably already following those standards by conducting any required Annual Inspections and 100 hour inspections. Aircraft maintenance is based on FLIGHT time. Install an air switch that turns the hobbs meter on when the airspeed reaches flying speed. Many operators will see up to a 10% reduction in recorded time on their aircraft verses a recording tach (obviously depending on your operation and the amount of taxiing you do). Operators using only a standard hobbs will see a substantial reduction.

  21. #21
    sjohnson's Avatar
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    My original tach lasted 60 years and thousands of hours (I'm pretty sure it was an AC). It finally failed, and I replaced with a Mitchell tach. It lasted less than 200 hours.

    I replaced it with an electronic UMA tach (TSO) and an UMA sender. No problems so far, after about 300 hours. You have to move your eyes level with the hour meter to read it, but that's ok as you don't need to in flight. It doesn't look antique, though. There is a version without an hour meter.

    https://www.aircraftspruce.com/catal...tachometer.php
    There are three simple rules for making consistently smooth landings. Unfortunately no one knows what they are.

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