Tachometer or Tach Cable

[Rich]

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

[Jim Burns]

A friend of mine described exactly the same problem just last week in his
C170B.... then the tach cable broke. Replaced the cable and so far no
strange effects noticed on the tach.
Jim

"Rich" > wrote in message
...
> 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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[Aaron Coolidge]

Rich > wrote:
: 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.

If you do decide to replace the tach, any reputable instrument shop will be
able to reset the tach hours readout to what you specify. I wouldn't be
concerned about confusing the records in that manner.

I would try to borrow a known good tach, perhaps from your mechanic, and
plan an early flight on a cold morning to see if the problem changes.
--
Aaron Coolidge

[Jim Burns]

Nevermind..... now the tach won't turn, it's seriously messed up inside.
Jim

"Jim Burns" > wrote in message
...
> A friend of mine described exactly the same problem just last week in his
> C170B.... then the tach cable broke. Replaced the cable and so far no
> strange effects noticed on the tach.
> Jim
>
> "Rich" > wrote in message
> ...
> > 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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>
>


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[Orval Fairbairn]

In article >,
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
>

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.

[Rich]

Orval Fairbairn wrote:
> In article >,
> Rich > wrote:
>

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

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)... But what if there is a slight kink in
the housing?

Rich

[PaulH]

Had a similar problem on my Arrow a couple of years ago one bitter
cold morning - went up to 3000 RPM, then back to 2700 but no audible
change in the engine. I just decided (#*$& with this technology and
installed a digital tach. In addition to getting perfectly accurate
readings, the digital unit (Horizon) also monitors both mags
continuously.

[Doug]

I had my tach rebuilt for under $100. Works fine now. I think your
symptom could be either one. Check and lube tach cable, replace if
necessary. If it continues, rebuild tach.

Aaron Coolidge > wrote in message >...
> Rich > wrote:
> : 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.
>
> If you do decide to replace the tach, any reputable instrument shop will be
> able to reset the tach hours readout to what you specify. I wouldn't be
> concerned about confusing the records in that manner.
>
> I would try to borrow a known good tach, perhaps from your mechanic, and
> plan an early flight on a cold morning to see if the problem changes.

[Dale]

In article >,
Rich > wrote:

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


Was turning base to final one day when suddenly there was a horrible
screeching. A quick check of my wife and her friend confirmed that
neither of them were screaming at the moment. A peek at the tach and I
notice the needle going crazy!! Moving back and forth so fast it's a
blur..until TINK and the needle falls off and the screeching stopped.

I replaced it with a digital tach.

FYI if you buy a tach you can have the hours set to whatever you
want...makes the record keeping easy.

--
Dale L. Falk

There is nothing - absolutely nothing - half so much worth doing
as simply messing around with airplanes.

http://home.gci.net/~sncdfalk/flying.html

[Orval Fairbairn]

In article >,
Rich > wrote:

> Orval Fairbairn wrote:
> > In article >,
> > Rich > wrote:
> >
>
> >>
> >>I sure would welcome counsel from people on this list who may have
> >>encountered this problem.
> >>
> >>Rich
> >>
> >
> >
> > 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.
>
> 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)... But what if there is a slight kink in
> the housing?
>
> Rich
>

Generally, it is only the cable that frays and breaks, leaving the
housing intact. You can get the cable made up at a speedo shop for less
than $20 and have it in in minutes. If it breaks, then relplace the
housing, too, but I doubt that you will have to do that.

[John_F]

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

[John Kunkel]

"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.

[BTIZ]

> 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

[Rich]

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

[Dan Thomas]

(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