Tach Bouncing

[iflyvfr]

So my tach is bouncing around like that pilot landing the heavy iron in gusty winds somebody posted.

VFR, all steam panel. Should I put an EI digital tach in or can the old ones be repaired reliably? Personally, I like new stuff on airplanes and in this case, it's broke so I CAN fix it. If it helps, the tach cable on the back of the accessory case seems to be finger tight. Sounds like I could kill 2 birds with one stone, doesn't it?

I called EI and they said they provide a threaded cap that will plug that (possibly) leaky hole and a 3 1/8" adaptor for the panel hole so I can mount it there. What am I missing other than that 1/2 AMU hole in my wallet?

 

[Bob Noel]

Years back I put an EI digital tach in my 140. It works fine. But I kind of like the analog display better.

Make sure you check the depth of the EI tach - it should have fit in the original tach spot, but was about 1/4" too deep - I had to relocate it.

 

[bnt83]

You want to spend money just do it.


I personally grease the tach cable instead. I forget who fixed the tach when it broke but is around $200 with shipping.

 

[bnt83]

But I kind of like the analog display better.

Same here

 

[Jeff Oslick]

But I kind of like the analog display better.

+2. I flew an Archer with a digital tach for a few years, and hated it. Hard to read, had to focus on it too long so it messed up my panel scan flow.

Jeff

 

[flyingron]

I still have a mechanical tach, but it's way over on the other side of the panel. I set power with the JPI engine monitor. But frankly, I'd start by looking at your existing one. Chances are it's the cable (could just be loose).

I sent my old Navion tach out for overhaul at one point. Didn't cost much and came back with the face all cleaned up.

 

[Norman]

Lube the cable first.

 

[MtnMarcus]

So my tach is bouncing around like that pilot landing the heavy iron in gusty winds somebody posted.

VFR, all steam panel. Should I put an EI digital tach in or can the old ones be repaired reliably? Personally, I like new stuff on airplanes and in this case, it's broke so I CAN fix it. If it helps, the tach cable on the back of the accessory case seems to be finger tight. Sounds like I could kill 2 birds with one stone, doesn't it?

I called EI and they said they provide a threaded cap that will plug that (possibly) leaky hole and a 3 1/8" adaptor for the panel hole so I can mount it there. What am I missing other than that 1/2 AMU hole in my wallet?

Same thing happened to me except mine broke in flight. Replaced the old tach with an EI and couldn't be happier. I was a little concerned the display would be difficult to see given it is smaller than a lot of other options on the market, but it is just fine. I say go for it!

 

[iflyvfr]

Thanks for the feedback. I have an appointment with my mechanic to lube & tighten the cable. Except he specifically said he doubted the tach would behave that way from a loose cable. We shall see, my tach is also 'way over there' on the right side of the panel, so the comments about readability are appreciated.

 

[ahkahn]

Yeah, I just had the same problem. I looked into the EI digital tachs, but after watching a few videos with them in there I decided I like the old analog tach better. Bought a new tit-for-tat replacement tach.

 

[GlennAB1]

The indicator has a broken internal spring.

 

[iflyvfr]

Thanks Andrew & Glenn. That broken spring is likely because its old...or the result of a particularly forgettable landing I made at Cape Girardeau not terribly long ago.

 

[Dan Thomas]

Thanks for the feedback. I have an appointment with my mechanic to lube & tighten the cable. Except he specifically said he doubted the tach would behave that way from a loose cable. We shall see, my tach is also 'way over there' on the right side of the panel, so the comments about readability are appreciated.

The cable housing is full of dried sludge that makes it sticky. It needs to be clened out, and the cable itself cleaned off, before lubing. The bearing at the back of the tach also gets dry and will offer resistance that starts the cable winding up and snapping forward, making the tach bounce. A drop of really light oil will fix it. Some tachs even have a little tiny oil cup on top of the bearing boss.

 

[wanttaja]

On my C85, the tach drive in the engine broke. Would make intermittent friction with the cable, so the indication would bounce all over. I first tried a new cable, then borrowed a tach, until I could manever a camera into position and get a shot of the broken drive.

My solution was a "Tiny Tach"; $50 electronic tach sold for lawn mowers and the like. Works find, but you have to be Experimental of course.

Full story:

http://www.bowersflybaby.com/tech/tach.html

Ron Wanttaja

 

[James331]

If you're going EI or JPI, save up a little more money and just replace all your crappy primary engine instruments with one of their all in ones.

 

[flyingron]

If you're going EI or JPI, save up a little more money and just replace all your crappy primary engine instruments with one of their all in ones.

Crappy? People compliment me on my engine instruments all the time.

 

[jaybee]

+4 for lube the cable

 

[James331]

Crappy? People compliment me on my engine instruments all the time.

In comparison to most OEM gauges :wink2:

 

[iflyvfr]

Crappy? People compliment me on my engine instruments all the time.

Thanks Ron, I don't fly much in the NE airspace, so I don't have that issue. Seriously, thanks everyone, this was more educational than I hoped for, now it makes much more sense what we're trying to accomplish: clean, then lube.

 

[flyingron]

In comparison to most OEM gauges :wink2:

Actually, mine are OEM gauges, just for a different plane. My engine came out of a new A36 Bonanza that went through a turboprop conversion. I got an engine with 9 hours since new on it along with the exhaust, prop, and other random accessories not needed for the turbine version. All my engine gauges say "Beechcraft" on them.

 

[iflyvfr]

The indicator has a broken internal spring.

Well, today we removed the gauge and the cable. I now understand how most of it works, which is why I enjoy owner-involved maintenance. The cable and receptacle ends were remarkably clean and rotated freely with no binding. It was one of the simpler things I've worked on: quite easy to R&R it. The tach came out easily too and after cleaning it all up and lubing it with light grease, I flew her home. The tach is better but was still bouncing so I'm leaning toward the broken spring theory.

Also, while the tach was reading 2550 RPM, my iPhone app was only showing @ 2350, and the engine sound was closer to the lower than not. So, I think my tach needs overhauled.

Before I do so, is it worth it to replace the cable first? The length of the cable looked good, but the end that goes into the instrument has a bit of a burr on the very end of it.

On a side note, it was 66* in central Ohio today with some excellent winds: 17G23 variety, which was nice over the hills of southern Ohio. I was glad to see and hear a bunch of people flying, although there was only 1 other plane at Urbana when we arrived for breakfast. Today reminded me why I love to fly!

 

[Tom-D]

The indicator has a broken internal spring.

More likely a slipping drive cable. they will round off the square drive end and then drive the cable sporadically, causing the Hysteresis disk to bounce.

 

[Tom-D]

Thanks Andrew & Glenn. That broken spring is likely because its old...or the result of a particularly forgettable landing I made at Cape Girardeau not terribly long ago.

If you had a broken spring in the Tach, when you started the engine the needle will go to the max RPM side and not return to "0". that is what the spring does, "try to keep the needle at "0"

What you have is an old Hysteresis unit that is out of calibration, because it has lost some of the magnetic force it was made and calibrated with. but the bounce is caused by a slipping cable in its drive.

Get the electric one , they are direct replacement units.

 

[iflyvfr]

If you had a broken spring in the Tach, when you started the engine the needle will go to the max RPM side and not return to "0". that is what the spring does, "try to keep the needle at "0"

What you have is an old Hysteresis unit that is out of calibration, because it has lost some of the magnetic force it was made and calibrated with. but the bounce is caused by a slipping cable in its drive.

Get the electric one, they are direct replacement units.

OK, well it definitely doesn't do that. But it does now seem to be out of calibration. I thought you were pulling my chain (that doesn't happen around here much) but I found an exploded diagram of a tach and it does indeed have a hysterical unit. http://navyaviation.tpub.com/14009/1400900215.html

Decisions decisions...

 

[Tom-D]

OK, well it definitely doesn't do that. But it does now seem to be out of calibration. I thought you were pulling my chain (that doesn't happen around here much) but I found an exploded diagram of a tach and it does indeed have a hysterical unit. http://navyaviation.tpub.com/14009/1400900215.html

Decisions decisions...

I'd pull the drive unit, inspect it for being rounded out which will allow the cable to slip in the drive unit.
which engine do you have?

 

[nrpetersen]

OK, well it definitely doesn't do that. But it does now seem to be out of calibration. I thought you were pulling my chain (that doesn't happen around here much) but I found an exploded diagram of a tach and it does indeed have a hysterical unit. http://navyaviation.tpub.com/14009/1400900215.html.

That description looks to me like a lot of overcomplification to a simplex problem. I thought tach drives were simply a speedometer cable driven affair driving a rotation magnet which runs in a spring return magnetic cup. No synchronous motor etc - at least for most of us. The magnetic coupling rotates the cup against the spring which is the front indicator.

Tach factory calibration is done by varying the magnet strength/charge. That's why tachometers with age will generally only read low as the magnet strength slightly decays with time.

The OP problem is very likely a drive cable that needs (or needed) lube, tho Tom D noted that the cable drive ends can get mechanically messed up. Never seen that but Tom probably has.

And there's nothing hysterical (a big ha! ha!) about tachs. But some may be implying magnetic hysteresis - like old computer memory etc (but which it technically ain't either....)

 

[Tom-D]

I've also seen cases where the tach drive inner cable has a very minor kink in it, where it must stop, wind up the cable then break free spin over speed then catch again, wind up, break free, repeat repeat. resulting in a bouncing needle.

 

[Tom-D]

That description looks to me like a lot of overcomplification to a simplex problem. I thought tach drives were simply a speedometer cable driven affair driving a rotation magnet which runs in a spring return magnetic cup. No synchronous motor etc - at least for most of us. The magnetic coupling rotates the cup against the spring which is the front indicator.

Tach factory calibration is done by varying the magnet strength/charge. That's why tachometers with age will generally only read low as the magnet strength slightly decays with time.

The OP problem is very likely a drive cable that needs (or needed) lube, tho Tom D noted that the cable drive ends can get worn out . Never seen that but Tom probably has.

And there's nothing hysterical (a big ha! ha!) about tachs. But some may be implying magnetic hysteresis - like old computer memory etc (but which it technically ain't either....)

FTFY.

 

[iflyvfr]

Tom it's an O-300 engine, sounds like tach cable is next on my shopping list.

 

[Tom-D]

Tom it's an O-300 engine, sounds like tach cable is next on my shopping list.

It may not be the cable, it may be the drive, and that is a PITA to change because it is a part of the oil pump gear that sticks out of the rear of the accessory case.

When this is the problem, I simply alter the shape of the cable end and drive it into the worn out drive. so that it can not slip anymore. at next overhaul you get a new oil pump gear and new cable.
But first things first, remove the cable housing, clean and inspect.

 

[Tom-D]

Pictures of the parts for the tach drive in the 0-300, these are of a 0-200 accessory case, but it is the same in the 0-300

 

[iflyvfr]

We removed and cleaned up the cable and housing Saturday. I can believe the square end is worn out tho.

 

[Tom-D]

We removed and cleaned up the cable and housing Saturday. I can believe the square end is worn out tho.

Squish it a little bit with a vice grip so as to make it bind a bit as you push it into the drive see what happens.

Turn the square into a diamond. sorta.