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Tachometer or Tach Cable



 
 
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  #11  
Old October 12th 04, 04:28 AM
John_F
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Pull the center cable out and clean the old sticky oil off with a
solvent. Re coat with new motor oil and reinstall.
If you do this soon enough it will not break and you will not have to
buy a new cable.

On Mon, 11 Oct 2004 09:14:44 -0400, Rich wrote:

I've got a problem that seems to be cold-weather related, and I'm trying
to determine whether it is more likely the tachometer or the tach cable.
Happened a couple of times last spring, and once last week when we had
a cold snap in Michigan.

On the ground, I hear what sounds like an intermittant 'chirp... chirp'
sound. Sounds like a shaft vibrating in a loose bearing, kind of like
you sometimes hear when you fast-rewind a cheap VCR tape.

In the air, I get a drift toward an overspeed reading (2900 - 3000 RPM
instead of maybe 2700). Based on sound, power settings and airspeed, I
am convinced that the engine did NOT overspeed. After a few minutes
(5?) the tach settled down to "normal" readings and stayed there the
rest of the two leg flight.

Some people suggest it is the tach cable, tightening up and untwisting
due to either a kink or stiff lubrication. The tach, they say, is
reading the instantaneous velocity of the highest rotational speed.

Others say it is the tachometer (bearing noise) because they say "they
had that once and when they replaced the tach, the problem went away".

Tach cable is relatively cheap to replace, but difficult to thread
through the firewall and behind the panel. New tach is easy to replace
but more costly, and will add confusion to future record keeping in the
logbooks.

I sure would welcome counsel from people on this list who may have
encountered this problem.

Rich


  #12  
Old October 12th 04, 06:32 PM
John Kunkel
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"Rich" wrote in message
...

Hmmmm...
You've just offered a third alternative... replacing the inner cable
without replacing the cable and housing (I had been figuring on
replacing cable AND housing)...


I'll offer a fourth alternative, jerk that piece of junk from your dash and
replace it with a digital tach.

A cable driven tach is akin to a watch that's always fast or slow.


  #13  
Old October 13th 04, 05:35 AM
BTIZ
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Why do you have to thread it through everything? Tach cables come in two
parts: cable and housing. If you disconnect both engine and tach ends,
you can pull the cable out and thread a new one back in, using the old
housing. Be sure to use plenty of moly grease, to keep the cable
well-lubed. BTW, any speedometer shop can make up a new cable, with the
proper ends.


Question... is a tach cable made up at the local speedo shop an approved
aircraft part?
Does it need to be?
Or does a DAR have to approve it as a suitable replacement part?
Or can an IA do that?

BT


  #14  
Old October 13th 04, 02:28 PM
Rich
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BTIZ wrote:
Why do you have to thread it through everything? Tach cables come in two
parts: cable and housing. If you disconnect both engine and tach ends,
you can pull the cable out and thread a new one back in, using the old
housing. Be sure to use plenty of moly grease, to keep the cable
well-lubed. BTW, any speedometer shop can make up a new cable, with the
proper ends.



Question... is a tach cable made up at the local speedo shop an approved
aircraft part?
Does it need to be?
Or does a DAR have to approve it as a suitable replacement part?
Or can an IA do that?

BT


Well... maybe it requires a DAR... or maybe it could qualify as an
"owner produced part"... or maybe it could fall under the category of "I
don't know... it was like that when I bought it."

Anyway, I went out and looked at my cable and housing yesterday. After
fiddling around with it for some time, I concluded that that cable and
housing sure looked good to me. I ordered a new tach from Spruce which
should be here Thursday. When the cold weather hits, if it WAS the
tach, I will know it. If it WASN'T the tach, I'll know that for sure as
well. I would have preferred to do an accurate diagnosis first, but for
the price, I've done SOMETHING to advance the resolution process.

Rich

  #15  
Old October 14th 04, 02:54 AM
Dan Thomas
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(John_F) wrote in message ...
Pull the center cable out and clean the old sticky oil off with a
solvent. Re coat with new motor oil and reinstall.
If you do this soon enough it will not break and you will not have to
buy a new cable.


The inside of the old cable sheath gums up with dried-out grease
or oil and will cause problems with a new cable soon enough. Best
thing is to replace the whole thing, or if it's not broken, take it
all out and take it apart and flush it really well. A low-temp oil or
graphite is best so that cold weather doesn't make it sticky.
Old tachs often get a gummed-up bearing and will jam and break
the cable. A new cable will last ten minutes. Some of those tachs had
a tiny oil hole in the top of the bearing housing at the back of the
tach; a drop of light oil would keep it going.

Dan

On Mon, 11 Oct 2004 09:14:44 -0400, Rich wrote:

I've got a problem that seems to be cold-weather related, and I'm trying
to determine whether it is more likely the tachometer or the tach cable.
Happened a couple of times last spring, and once last week when we had
a cold snap in Michigan.

On the ground, I hear what sounds like an intermittant 'chirp... chirp'
sound. Sounds like a shaft vibrating in a loose bearing, kind of like
you sometimes hear when you fast-rewind a cheap VCR tape.

In the air, I get a drift toward an overspeed reading (2900 - 3000 RPM
instead of maybe 2700). Based on sound, power settings and airspeed, I
am convinced that the engine did NOT overspeed. After a few minutes
(5?) the tach settled down to "normal" readings and stayed there the
rest of the two leg flight.

Some people suggest it is the tach cable, tightening up and untwisting
due to either a kink or stiff lubrication. The tach, they say, is
reading the instantaneous velocity of the highest rotational speed.

Others say it is the tachometer (bearing noise) because they say "they
had that once and when they replaced the tach, the problem went away".

Tach cable is relatively cheap to replace, but difficult to thread
through the firewall and behind the panel. New tach is easy to replace
but more costly, and will add confusion to future record keeping in the
logbooks.

I sure would welcome counsel from people on this list who may have
encountered this problem.

Rich

 




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