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

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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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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Checked by AVG anti-virus system (http://www.grisoft.com).
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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

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.

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.

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

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.

"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.

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