Mechanical tach fail

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Dave Taylor
This is the cable binding, not the tach failing correct?

 
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That would be my guess. Particularly since we can hear it. Replace the cable before the tach head is damaged.
 
Some older Tachs have a”Lube Port” above the cable attach.

Newer ones retain the boss but are undrilled.

A binding tach will give you issues as well.

On some engines there is a Tach Drive Seal and drain Hole.

The hole allows oil that has migrated past the Seal to go overboard

rather than fill the Tach.
 
Hard to say, cable needing replacement or lube is the cheapest first step. One can lead to the other.
 
Flew last night to reestablish my night pax carrying currency. Failing tachs must be going around. Although different failure mode for me. The RPM needle on my tach was fine and was stable on my selected RPM. But the recording (hours) part of my tach didn't advance at all. I'm guessing just an internal tach issue. If so, I'll order another set to the correct hours.
 
Flew last night to reestablish my night pax carrying currency. Failing tachs must be going around. Although different failure mode for me. The RPM needle on my tach was fine and was stable on my selected RPM. But the recording (hours) part of my tach didn't advance at all. I'm guessing just an internal tach issue. If so, I'll order another set to the correct hours.
ooo
flew through a time warp?
or, free flying?
 
Tach cable is pretty easy to service. About a 15 minute project.
 
T
This is the cable binding, not the tach failing correct?

That much flicker implies a seizing tach. Undo the tach cable nut on the back of that tach. See the short shaft sticking out. See if it spins freely. As Magman says, older tachs had a tiny oil hole just above that shaft. You can wick some really light oil (like sewing machine oil) into it and see if it settles down.
 
Just in case you you have to replace the Tach, please do everyone a big favor.

Regardless of what gets installed there must be a Log Entry.

Please don’t leave the ink in the Pen!

What I mean is YOU should assure the Entry has sufficient information and no abbreviations.

TT, TR, TIS should be written out.

Confusion over whether TT is Total Time or Tach Time ( or ?) is not unusual.

This is annoying at best and pricey at the worst.

Compliance with AD’s , overhaul periods, and even resale value are affected.

I ‘ve spent many years developing my sloppy handwriting skills.

On this item I try to assure folks other than me can understand the information

years later.
 
Just in case you you have to replace the Tach, please do everyone a big favor.
And the OP should do himself another favor. Don't buy another mechanical tach. There are good electronic tachs available that don't need that cable, they don't jump around, there are no moving parts to wear out, and they never go out of calibration. Here in Canada we have a requirement for an annual accuracy check of all magnetic-drag-type tachs, which is pretty much all of them, and the deviation limit is 4% of the reading in the middle of the cruise range. The magnet that drags the cup and needle around gets weaker with age, and the tach begins to underread. 4% of 2500 is 100 RPM. Replacing those old tachs gets around that annual check, and avoids the aging-magnet and worn-bearings and seizing shaft and chattery cable and leaking tach drive oil seal stuff.
 
Dan; are you really saying that some things don’t work as well with age?
 
Are there tachs that are not cable-driven?
 
Had the exact same failure on my Cherokee a few years back. Scared the heck out of my wife. We landed at the nearest airport which was run by complete a**holes. Overcharged me to put a new mechanical tach in which never read anywhere near correctly. Flew the plane home and had my mechanic install a digital tach immediately.
 
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