Tach problems

EvilEagle

EvilEagle

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EvilEagle
Anyone had a perfectly working tach that started to drop to zero then immediately back to normal? (no engine change) Mine did this for about an hour or so, then quit completely. Seems like a connection problem somewhere - where's the best place to start troubleshooting? Near the tach or at the mag?

Thanks!
 
EvilEagle said:
Anyone had a perfectly working tach that started to drop to zero then immediately back to normal? (no engine change) Mine did this for about an hour or so, then quit completely. Seems like a connection problem somewhere - where's the best place to start troubleshooting? Near the tach or at the mag?

Thanks!
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If you have a mechanical tach with a rotating cable driving it, chances are the drive cable went south. It's usually pretty easy to pull the cable off the back of the tach and see if you can rotate it. If it turns free it's broken, if it's stuck try cranking the engine (mixture in ICO) to see if it turns.
 
btw - even if the cable is apparently free to rotate, the mechanical
coupling could be bad. I had a mechancial tach needle that starting
large oscillations even though the cable spun freely.
 
EvilEagle said:
That's the idea - I don't know if it's electrical or mechanical.
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Well, at least tell us what you're flying so someone here can tell you whether you have an electrically or mechanically driven tach.
 
Clark1961 said:
Well, at least tell us what you're flying so someone here can tell you whether you have an electrically or mechanically driven tach.
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Ok, now I get what you are asking. I was confused! :mad2:

I've got a Glasair 1, O-320, mechanical tach.
 
The Tach cable probably went south. Replace the cable or put in a Horizon P-1000 for increased accuracy and reliability.
 
EvilEagle said:
Ok, now I get what you are asking. I was confused! :mad2:

I've got a Glasair 1, O-320, mechanical tach.
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If you have a mechanical tach and rotating tach cable there's no way it can be an electrical problem beyond the possibilty that an electrical cable shorted into the tach cable and burned through it (you'd know if that happened by various symptoms besides the tach failure).

So That leaves the tach adapter (very unlikely), the tach cable (most likely), or the tach itself (a possibility). See my earlier post to determine which has failed in your case.
 
EvilEagle said:
Anyone had a perfectly working tach that started to drop to zero then immediately back to normal? (no engine change) Mine did this for about an hour or so, then quit completely. Seems like a connection problem somewhere - where's the best place to start troubleshooting? Near the tach or at the mag?

Thanks!
Click to expand...

The best odds are that a strand of the tach cable came loose, caught on the outer sheath, stopped the whole cable, then as force mounted, let go, caught again, and finally caught permanently and broke the cable. Like somebody said, take the cable off the tach itself and crank the engine while keeping a light finger pressure on the cable. If it doesn't rotate, you've got a bad cable. If it does, you've got a bad tach.

Jim
 
Thanks for all the words - I finally got a chance to go work on it today.

I took the cable off from both ends and it spins fine. With the tach connected, spinning the cable from the engine side moves the tach needle. When the engine side is connected, the cable doesn't turn with the engine running (or by cranking the prop by hand). I'm confused. :frown2:

What would cause the cable to turn freely unless it's hooked to the engine? Is there something mechanical in the top section of the engine that could cause it not to turn the sleeve?

Thanks guys!
 
Well, the tach drive off the engine (the part the cable slides into) could have failed but it really shouldn't.

Pull the tach cable from the engine side, note the position of the drive and then turn the prop by hand. The drive should rotate with the engine. If it's hard to see, you should be able to feel it turn when the prop is rotated. Of course if the drive doesn't rotate, well, it's time to talk to the A&P. You may be able to cap the existing system and convert to an electronic tach.

One thing that happened to me when troubleshooting my tach problem was one time I assembled the tach drive at the engine side and didn't ensure full engagement. There is a right angle drive on mine and there's just enough play in the system that the drive can look fine externally but the internal shaft isn't engaged properly.

The easy way to test engagement is to loosen the tach cable from the tach and see if the cable rotates by hand. It shouldn't of course since the engine isn't turning and can't be driven by the tach.:smile:
 
Clark1961 said:
Well, the tach drive off the engine (the part the cable slides into) could have failed but it really shouldn't.

Pull the tach cable from the engine side, note the position of the drive and then turn the prop by hand. The drive should rotate with the engine. If it's hard to see, you should be able to feel it turn when the prop is rotated. Of course if the drive doesn't rotate, well, it's time to talk to the A&P. You may be able to cap the existing system and convert to an electronic tach.

One thing that happened to me when troubleshooting my tach problem was one time I assembled the tach drive at the engine side and didn't ensure full engagement. There is a right angle drive on mine and there's just enough play in the system that the drive can look fine externally but the internal shaft isn't engaged properly.

The easy way to test engagement is to loosen the tach cable from the tach and see if the cable rotates by hand. It shouldn't of course since the engine isn't turning and can't be driven by the tach.:smile:
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Loosening from the tach, the cable will turn one way but not the other.

Assuming the worst, what damage would be caused to the engine if the tach drive has failed? (if it was kept running)
 
Did you have a "tabbed tang" to drive the tach cable? There is a little "drive shaft" that transfers rotation from the engine to the cable. I'll see if I can find a picture of it.
 
Yes, on the back of the engine there is a port that is threaded and inside that there is a sleeve that the tach cable slides into. Could just that sleeve go bad?
 
EvilEagle said:
Yes, on the back of the engine there is a port that is threaded and inside that there is a sleeve that the tach cable slides into. Could just that sleeve go bad?
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It's possible for the tach drive on the engine to fail but extremely unusual. If that has happened, it's likely that serious problems exist in the engine. My guess is that the inner cable has sheered at the engine end, so close to that end that only last half inch has separated. IIRC, both ends of that shaft are square for about 3/4 inch of length. On the engine end, the shaft engages a square hole which is driven by a gear in the accessory case. You may be able to see the part containing the square hole with the cable removed from the engine and it should turn when the prop is moved in either direction. An inspection mirror may be needed for this.

It's also possible that the shaft "inner cable" is broken between the ends but the break is such that turning the cable slowly makes it appear that the shaft is unbroken. An easy way to confirm this is to disconnect both ends and pull one end of the shaft while someone holds the other end.
 
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How about just pulling the cable out and checking it? :rolleyes:

If the cable itself is not defective then now is a good time to lubricate it.
 
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Rotor&Wing said:
How about just pulling the cable out and checking it? :rolleyes:

If the cable itself is not defective then now is a good time to lubricate it.
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That's the plan for today. I've not pulled a tach cable before, but it seems straightforward - disconnect from the engine side and pull into the cabin. I'll regrease the cable as well.

Both ends of the cable can be seen, although the engine side of the cable isn't fully square - it's cylindrical with a notch in it. Maybe that piece is aft of the square end and the square bit is still in the engine accessory mount?
 
Rotor&Wing said:
How about just pulling the cable out and checking it? :rolleyes:

If the cable itself is not defective then now is a good time to lubricate it.
Click to expand...

Assuming you mean to pull the inner cable out of the shroud (vs removing the entire assembly), that is likely to be an easy task although I suspect that if the assembly has multiple sharp bends, it might be necessary to straighten it in order to re-install the inner cable. And R&R of the whole cable assembly can be difficult in many airplalnes.
 
EvilEagle said:
That's the plan for today. I've not pulled a tach cable before, but it seems straightforward - disconnect from the engine side and pull into the cabin. I'll regrease the cable as well.

Both ends of the cable can be seen, although the engine side of the cable isn't fully square - it's cylindrical with a notch in it. Maybe that piece is aft of the square end and the square bit is still in the engine accessory mount?
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I can't swear that the engine end should be square but that's what I remember, and yes a likely failure would leave a short piece of cable in the engine side. A strong, small magnet may help retrieve such a piece and it's important to NOT leave any pieces in the engine.

If you lube the cable (definitely a good idea) you don't want to use "grease", get some speedometer cable lube from an auto parts store and consider "washing out" the outer cable with solvent first if it's gummed up. Keep in mind that this might result in a mess if the solvent leaks from the cable along it's run.
 
gismo said:
Assuming you mean to pull the inner cable out of the shroud (vs removing the entire assembly), that is likely to be an easy task although I suspect that if the assembly has multiple sharp bends, it might be necessary to straighten it in order to re-install the inner cable. And R&R of the whole cable assembly can be difficult in many airplalnes.
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It's basically a straight shot - only about 36" from the back of the tach to the engine, so it "should be" fairly straightforward. I'll post in a few hours when I get it done - have to help put up the tree first. :D
 
gismo said:
Assuming you mean to pull the inner cable out of the shroud (vs removing the entire assembly), that is likely to be an easy task although I suspect that if the assembly has multiple sharp bends, it might be necessary to straighten it in order to re-install the inner cable. And R&R of the whole cable assembly can be difficult in many airplalnes.
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Pulling the inner cable out of the shroud should not be a difficult maneuver if the cable is in good condition. This is on a Glasair and should be a short and fairly straight cable.

This is another area most owners neglect lubrication "because it's too difficult" then wait for the cable to fail. They put up with a jerky tach that's giving every indication of imminent failure. It's far cheaper to lube the cable than it is to replace it especially if it damages the shroud when the inner cable fails.
 
gismo said:
My guess is that the inner cable has sheered at the engine end, so close to that end that only last half inch has separated. IIRC, both ends of that shaft are square for about 3/4 inch of length. On the engine end, the shaft engages a square hole which is driven by a gear in the accessory case. You may be able to see the part containing the square hole with the cable removed from the engine and it should turn when the prop is moved in either direction. An inspection mirror may be needed for this.
Click to expand...

I think that is a very good guess. I still think the inner cable somehow "seized" inside the outer shroud, whether from lack of lubrication or a strand of the cable catching on the inner diameter of the shroud, and at that point the engine side of the cable sheared off. As you said, a strong magnet may work or a very long narrow needle-nosed pliers. In any event, if you drop it into the case you've got a world of possible hurts ahead of you.

However, if you've run the engine since the tach broke and nothing came to a grinding halt inside the engine, you may have gotten lucky and that broken square piece may be down in the pan. If you don't find the broken piece, I'd suggest that a good oil change (NOT through the quick drain, but with the big bolt out of the case) through a strainer would probably be in order. In any case, you've got to find and remove the busted part or it will eventually find you.

Jim
 
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