Sticky Tach

[Kerbs]

My plane has sat for the last 6 months unfortunately as I have been working on my ticket. Now that I have it, I am getting some time in the t-craft, however have had an issue with the tach. It is really slow to read and come up. During run up, I can be well over 1,500 and it will barely read 1000. Full throttle at takeoff and again it only reads about 1000. It will eventually come up to operating speed but not until I am almost to pattern height. Last night I pulled the cable and the tach side knuckle. They were pretty gummed up, so we got them all cleaned out and re-lubed. Is there anything I need to do to service the tach being it has sat? I am planning to fly today and see if it is better since servicing the cable.

[Mark Bowden]

Re: Sticky Tach

One thing to help is to go get a "affordable, ~$20" optical tach from Harbor Freight or Amazon.com. Most will work through the windscreen and don't need a reflective sticker, some will need the sticker.

You can tape or Velcro it to your dash mount compass or dash.

It will tell you when you get your tach back close from servicing the cable and then how far off it is.

[Kerbs]

Re: Sticky Tach

Thanks Mark, that's a great idea. When you say windscreen, do you mean the intake vents on the front of the engine cowl?

[PA1195]

Re: Sticky Tach

I believe the tach cable turns correctly if still rotated by the engine. It may stick and jump around inside the sheath due to gummy bear stuff, but the engine drives it directly unless it breaks. The lag in tach display is probably due to internal problems in the tach...the display needle isn't driven directly by the input cable...magnetic transfer or whatever makes it move. Get the tach serviced or replaced if it lags.

Gary

[astjp2]

Re: Sticky Tach

There are 2 different wraps for the cables that go down the cover, clockwise and counter clockwise, you need the correct one for the application you are using it on...Tim

Oh yeah, tachometers are only allowed to be serviced by an instrument shop, A&P's should not be messing with the internals of any instrument.

[PA1195]

Re: Sticky Tach

There are 2 different wraps for the cables that go down the cover, clockwise and counter clockwise, you need the correct one for the application you are using it on...Tim

Oh yeah, tachometers are only allowed to be serviced by an instrument shop, A&P's should not be messing with the internals of any instrument.

Hey Tim what's inside a tach that makes it work? Never had one apart (legally).

Gary

[astjp2]

Re: Sticky Tach

There are bushing and bearings that wear and the magnets that cause the needle to move get weak and need to be re-magnetized.

[PA1195]

Re: Sticky Tach

Ok Tim thanks. I'd spin the inlet fitting and the needle would lag behind the input collar so figured there was some magnet or ? making it work. The TT must be direct via gears?

Gary

[astjp2]

Re: Sticky Tach

The hour meter is direct, the needle is spun up by a calibrated magnet. also the hour meter is set to a basic cruise RPM, wrong cruise RPM tach = wrong tach time

[Robert Lees]

Re: Sticky Tach

Thanks Mark, that's a great idea. When you say windscreen, do you mean the intake vents on the front of the engine cowl?

He means windshield (he's been flying with me much recently , and windscreen is our British term for the American "windshield"). Essentially, you put the optical tachometer on the coaming inside the cockpit to verify the aircraft rpm tacho against what the optical one says.

Rob
(England)

[Hank Jarrett]

Re: Sticky Tach

Funny how terms get changed with national borders. In aircraft engineering in the US we called the front glass a wind shield and the side glass a wind screen (when they were in sections in front). A side "glass" was called a window, as in door window where the air went parallel to the "glass".

I started with English cars and still call the "Trunk" a "Boot" and a "Hood" a "Bonnet". For me "Hood" will always be a really great ship that died way too soon.

[Mark Bowden]

Re: Sticky Tach

At my former, favorite OEM we used windscreen a bit more than windshield as windshields were for cars. Kinda depended on what part of the country you came from. Cockpit glass was a common term for electrically heated glass up front as the singles and 208 used Acrylic (Plexiglas).

Hank you forgot "transparencies" and "frost pane". lol

[Kerbs]

Re: Sticky Tach

Thanks for everyone's help. We flew today and it looks like things are back to normal. My guess is it was the way the cable housing and the 90 degree angle were gummed up with old cold grease. I was thinking about PA1195's comment about the cable still spinning. My only thought is that it was loading or twisting the cable up and then it would release inside the housing. That would cause the gear end to surge and I am guessing the magnetic stabilization of the tach was keeping it artificially low. But I could be way off base or totally missing something too. Anyway, just happy to have it working.