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Tachometer or Tach Cable



 
 
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  #1  
Old October 11th 04, 02:14 PM
Rich
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Default Tachometer or Tach Cable

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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  #2  
Old October 11th 04, 03:23 PM
Jim Burns
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Default

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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  #3  
Old October 11th 04, 03:55 PM
Aaron Coolidge
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Default

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

  #4  
Old October 11th 04, 04:41 PM
Jim Burns
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Default

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



---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.775 / Virus Database: 522 - Release Date: 10/8/2004




---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
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  #5  
Old October 11th 04, 06:54 PM
Orval Fairbairn
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Default

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.
  #6  
Old October 11th 04, 07:20 PM
Rich
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Posts: n/a
Default

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

  #7  
Old October 11th 04, 08:26 PM
PaulH
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Default

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.
  #8  
Old October 11th 04, 09:14 PM
Doug
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Posts: n/a
Default

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.

  #9  
Old October 11th 04, 10:25 PM
Dale
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Posts: n/a
Default

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
  #10  
Old October 12th 04, 03:41 AM
Orval Fairbairn
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Posts: n/a
Default

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.
 




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