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After having an issue with a tachometer bouncing so much that it fell off, I now have the opposite problem with the replacement (Mitchell D1-112-5025): The tachometer is sluggish. When I open the throttle from 1,000 to 1,700 for the runup, the tachometer takes about 6s to show 1,700 RPM. The delay seems even more pronounced when I reduce power. The smaller the change in RPM, the longer it takes the tachometer to update.

My A&P says that it just needs breaking in, which could possible because the sluggishness has reduced since the tachometer was installed; however, I was under the impression that modern tachometers should work without sluggishness immediately.

The counter increments as expected.

How should I approach this problem? Is it common for tachometers to need >15 hrs to break in? Is it possible for a drive cable to cause this issue (continue to spin, but in such a way that the tach speed is sluggish)? Could some kind of installation issue have caused this problem?

UPDATE Mar-22: Removed the tach cable, inspected outer and inner cable, washed, dried, lubed with graphite, and replaced. Cable seemed good, but the tachometer is still sluggish.

UPDATE Mar-24: Tachometer is now beginning to exhibit the same symptoms as the previous tachometer before the needle spun off; the tach is still sluggish, but the needle is now bouncing considerably.

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  • $\begingroup$ I remember your last question, it seems to me you may have a problem with the tach cable rather than the instrument itself. The bouncing you had before sounded like the cable was binding, and now you have slow response, which could be another symptom. Did your A&P check that? $\endgroup$ – GdD Mar 21 at 9:18
  • $\begingroup$ @GdD: No. He seemed convinced that it wasn't the drive cable. He seemed to think they are pretty much indestructible. $\endgroup$ – Zaz Mar 21 at 13:28
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    $\begingroup$ I think you need a second opinion. That doesn't sound right to me and the statement that instruments need "breaking in" is suspect too. Instruments have to function properly out of the box. $\endgroup$ – GdD Mar 21 at 13:30
  • $\begingroup$ @GdD: We removed the tach cable, and my A&P thought it looked good. Other than the very slight kinks in the outer cable, I thought both looked good too. We cleaned, lubed with graphite and reinstalled. Now it's bouncing as the last one did. You think I just need to get a new cable? My A&P thinks the new tach just happens to be broken also. Is it possible that the gears on the engine are stripped? Is there anything else that could be causing this? $\endgroup$ – Zaz Mar 24 at 22:28
  • $\begingroup$ If the gears were stripped then you'd be getting consistent behavior @Zaz, the fact you're getting different results every time you try something new with the tach cable convinces me more that it's the issue. If the cable was fine then lubing it wouldn't change anything. A visual inspection of the outside won't tell you if it's worn inside. $\endgroup$ – GdD Mar 24 at 22:47
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No you don't have to "break in" flight instruments.

When a TSO'ed instrument is new it will get an Acceptance Test Procedure (a functional test) from the factory done before it is released and it has to perform to specification. There is no such thing as a break in in service.

That being said, sometimes parts get by the ATP and malfunction in service, so you never know. In my years in the regional airline industry, "bad from stock" is not uncommon even with super fancy Transport Category components.

Problems with the flex drive could be a factor, but a flex shaft would have to be binding and winding up a hell of a lot to create that kind of lag. It's still something that could be checked.

Before that though, you should call Mitchell's tech support and describe what it's doing and see what they say. Maybe they'll tell you there is a break in, but I'd be pretty surprised if they did.

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  • $\begingroup$ I emailed Mitchell; will probably call tomorrow if there's no response. Is it possible that the gears on the engine are stripped? Is there anything else that could be causing this? My A&P inspected the inner and outer cables and said they are fine - any chance he is wrong? $\endgroup$ – Zaz Mar 24 at 22:30
  • $\begingroup$ I would say there's something going on with the instrument internally. There is a little gearbox inside that drives a signal generator, who's output drives the needle which is pretty much just a voltmeter. Lag would suggest high friction in a bearing or bushing. If they tell you that's normal, well in that case I'm stumped... $\endgroup$ – John K Mar 24 at 23:05
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It turned out to be the tachometer cable after all. After replacing the cable with a new one, the tachometer works as expected.

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    $\begingroup$ Good to hear it's solved. I got a JPI EDM900 about a year ago, electronic display, sensor picks up RPM from the magneto or something, no more spinning cables. Had a 150 many years, ago, 1994-1996. $\endgroup$ – CrossRoads Apr 1 at 17:57
  • $\begingroup$ @CrossRoads: Thank you. I considered the EDM 900 but didn't want the cost and hassle. $\endgroup$ – Zaz Apr 2 at 23:51
  • $\begingroup$ I understand. Sometimes inexpensive is the way to go. $\endgroup$ – CrossRoads Apr 3 at 1:12

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