Tach going bad?

Big Ed

Big Ed

N50247 - '79 Super D
Joined
Jul 20, 2020
Messages
1,938
Location
Tampa, FL
Little voice told me my tach might be acting a bit funny on the last few flights before I dropped it off in the avionics shop for a new radio. Shoulda listened to the voice. Now it's definitely acting funny. RPM's bounce around during runup; can't get solid reading at 1800 RPM, so I sort of have to bracket it. Same with drop on mag checks. Once I get it up to 2500 in the air, it reads just fine, no bouncing.

How long can I tolerate that? Is it just an annoyance, or am I risking something? Will it deteriorate quickly, or stay like that for a long time?

Who fixes a Tach ... avionics shop, or regular maintenance shop?
 
Who fixes a tach? If you ask the authorities, they will say it has to be an approved instrument repair shop. But a tach is a re- purposed auto speedometer, and while it is illegal to have an auto speedometer shop work on an aircraft tach, they have the parts and the expertise.
 
Bob Turner said:
Who fixes a tach? If you ask the authorities, they will say it has to be an approved instrument repair shop. But a tach is a re- purposed auto speedometer, and while it is illegal to have an auto speedometer shop work on an aircraft tach, they have the parts and the expertise.
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After pricing new tachs, seems silly to get one repaired. Relatively inexpensive at $250, at least by TSO standards. Another $80 to get miles set and arcs painted on face.
 
To lub the tac cable go to where it. Attaches at the motor and disconnect it then the cable will pull out from there use wheel bearing grease on this cable and re install, if this doesn’t help have you called a salvage yard for a used one? Or maybe rainbow Ron would have a good used one just suggestions.
 
Big Ed said:
After pricing new tachs, seems silly to get one repaired. Relatively inexpensive at $250, at least by TSO standards. Another $80 to get miles set and arcs painted on face.
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Yes and no. Tach time is used for engine purposes, and when you change the tach you start over at "0000.0". That's inconvenient as it adds another set of numbers into calculating time on the engine, time since major, etc.

If it's a cheap fix, I'd fix rather than replace.

Also, if the issue is the cable, not the tach itself, just replace the cable if you can, if lubing it doesn't solve the problem.

If you do replace keep in mind that tachs are designed for a specific cruise RPM and hour meter will spin faster at higher RPM and slower at lower RPM. Consequently, you need a tach that is set for the appropriate rpm for your aircraft.
 
For 25 bucks the factory will set the hours on the new tach to match the hours on the old one.
 
That's what you want. Makes logbook entries lots easier.

Last overhaul we did was $160 including re-setting the window. Shame; I did it for the J4, and there is no room for the recording tach on that one, even with a 90 degree adaptor.
Got the tach on eBay for $35.
 
A&P on field is going to help me lube the tach cable, change the oil, and rotate the tires on Thurs. Gonna get an oil analysis done to start a baseline.
 
Lubed the tach cable. Very simple job. Unfortunately it did not fix the issue. Next stop, replace the instrument?

Any thoughts on switching to a 2 1/4" tach? Too small for old eyes? Would free up a 3 1/8" gauge space in the six pack for something useful like a DG.
 
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Just make sure your cable fits.
AC tachs are a dime a dozen - get one on eBay for $35 and have it rebuilt at the aviation department of your local speedometer shop.
2 1/4 instruments are easy to read. All the important instruments are that size or smaller.
 
One more thing before I replace it. The tach readings were much worse when I got the plane back from the avionics shop after my recent upgrades. Before, it bounced a little, maybe 50 rpm. After, it would stick and then jump several hundred rpm. When I pulled the tach today, the cable seemed somewhat oddly routed. Is it possible the avionics guys misrouted it when they were pulling my TC? Could that be causing the sudden degradation in accuracy?

I did not have a work light. Will go back in the morning and investigate further.
 
Looked at the tach cable. Not really any way to misroute it, but it appears to be about 6 inches too long. The logs show that the original owner moved the tach from the far left edge of the panel to its current location in the center "6 pack" about 30 years ago. Looks like they did not replace the cable with a shorter one. It has a fairly sharp bend where it comes out of the firewall. I tried to reposition it to take some stress off the cable housing so it sits more naturally. The removed TC sat right where the cable runs now; could it be that the TC housing acted as strain relief or immobilized it? Should I try zip-tying it to a panel support about halfway to from the firewall to the instrument, or does it do better when free to pick it's own position?

I'll probably still replace the tach, since they tend to go bad at about the number of hours I have. But don't want to replace the tach just to discover that the cable is the issue, or is still a problem. How difficult is it to replace the cable housing where it passes thru the firewall?
 
Certainly considering that option. I prefer the steam gauge look, but the accuracy of the digital tach is appealing. The digital would also free up a 3" hole in case I want to add something later like an electronic HI.

Cost of the gauge is about $100 more. However, I would need some help installing, and a sign off, so probably a few hours of labor. Total cost is probably $350 for a mechanical tach vs twice that for a digital. Normally that wouldn't be a deal breaker, but I just spent $7K at the avionics shop, so I'm less inclined to opt for the more expensive option. Also at issue is getting someone to do it on short notice.

The more I think about it, the more I think it is a cable routing issue caused by my recent avionics work.

Before I dropped off the plane 3 weeks ago, the tach was reasonably accurate, with about 50 RPM of fluctuation or bouncing at run-up. After I got it back, it was much worse. It seems to bind and release when accelerating from 1000 to 1800.

Highly unlikely that the tach would suddenly go bad. More likely the removal of the TC allowed the cable to reposition or move in a way that affected function. If I can troubleshoot and get it back to acceptable function, I can take my time on the tach upgrade.

So how much of a loose loop is acceptable for a tach cable? Should it be secured at all bends so it cannot flex or move?
 
Probably better unconstrained - just eliminate sharp bends. I just bought a tach for $35, had it overhauled for $160, with the J4 tach hours set - I think 2400 or so. You can have it for what I have into it, although my current plan is to save it and re-set the numbers when my next tach fails.

Get rid of the sharp bends first. Make sure there is no pressure on the tach from a too-long cable. Check to see that it goes properly into the accessory case, with no dirt keeping it from going in. The entire thing, including the internal engine receptacle, is standard auto speedometer.
 
Rerouting the cable resolved the issue sufficiently for now. It was pressing against the underside of the glare shield in a way that impeded function. Still a bit sticky/jumpy in spots, but good enough to fly. That buys me time to look at other options for the long run.
 
Following up on this thread. Installed new Mitchell mechanical tach from Spruce and shorter 25 inch tach cable from ACA. Much improved and IMO a safety of flight issue to be able to do accurate mag checks at run-up. Well worth the $$, which was not much by airplane standards.
PXL_20210115_171308884.webp
 
what did the tach cost you from ACA? having the logo face in the instruments is pretty cool and I've been kicking around the idea of buying from ACA if I don't go big on a Garmin flat panel.
 
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