Tach issue

[Vendata]

Last June on a CC trip my tach went out. The needle pinged and eventually locked in position. Cessna 182K - O-470-R
When I got back, I purchased a new replacement Mitchell tach, had it setup, and installed it. A few flights later, I noticed the reading was about 175RPM low compared to my JPI800 tach reading.
It was still under warranty, so I sent it back. They said there was some oil in the gearing causing it to fail,, they replaced the internal gearing and sent it back. I then pulled the tach cable and inspected it with my mechanic. This was a new cable from my engine install the previous year. There was evidence of a real tiny amount of oil at the accessory case, and the cable itself had some grease from being pre lubricated.. -

We installed the new tach and same problem, about 175 RPM lower than the reading on my JPI. We confirmed the JPI against a handheld electronic prop tach as well.
I spoke to Mitchell about it and they were baffled, said there was definitely oil in the tach I sent back.
So I put a new cable in it.
Flew it today. Same problem.

To recap-
New engine 2 years ago, with new tach cable
Last summer the tach failed. Replaced with a brand new unit.
Reads 175 rpm low when compared to JPI
Sent back, apparently had oil in it.
Didn’t find much evidence of oil.
Reinstalled the new tach they replaced.
Same reading, 175 RPM low, validated against JPI & electronic prop tach
New cable again
Same reading.

Baffled. Any ideas?

Thanks!

 

[Dan Thomas]

The tach is bad. Its rotating magnet is weak, and no amount of fooling with internal gearing will fix that. Besides, the internal gearing only drives the hourmeter, not the needle.

 

[wanttaja]

Probably not the problem here, but on my Continental the drive fitting itself on the engine broke off.

This caused erratic operation of the tach. Rather than removing the engine and pulling the accessory case, I replaced it with an electronic tach.

It's obviously not the problem that your tach originally exhibited, but if the tach jammed, could the drive cable fracture the fitting on the engine like this?

Ron Wanttaja

 

[Magman]

Many engines have a “ drain hole” at the engine end of the Tach Cable.

Oil that gets by the seal is dumped overboard to prevent it from filling

the Tach with oil. IIRC there are some units that have the Cable twisted

in a particular direction for the same reason. Have you replaced the

Cable Housing as well?

 

[Dan Thomas]

Airplanes used mechanical tachs for a hundred years. A few worked on a centrifugal principle, using small flyweights that moved outward against spring pressure and drove the indicator needle as they moved in and out with RPM changes. I had one in my Jodel. Rare. Almost all others, if they didn't use electrical tach generators or an electronic tach, used the magnetic eddy current principle. The vast majority of the single-engine legacy fleet has them. This is the principle:


You can see that there is no solid connection between the input at the left and the pointer on the dial at the right. There is a magnet in a steel cup that is spun by the tach cable. There is an aluminum cup that fits closely between the magnet and steel cup, but does not touch either of them. Magnetic flux runs between the magnet, through the steel cup and around to the other pole of the magnet. It passes through that aluminum cup which, as we know, is non-magnetic.

BUT. Any time we move a magnetic field through a conductor, we generate electron flow in that conductor. The key is movement. No movement, no current. So that rotating magnet generates a current in that aluminum cup. We also know that any electron flow generates a magnetic field, and so that flow in the aluminum cup generates a field around the cup, and that field opposes the field of the magnet. The result is that the cup gets dragged around against the spring tension and moves the pointer. More RPM, more current, stronger field, more opposition, more movement. It works just fine until the magnet weakens with age and the tach starts to under-read. Very common, and in Canada we have a requirement that all magnetic-drag tachs be checked for accuracy every year, and they must indicate within 4% at the middle of the cruise RPM range. The tach gets placarded as to what the error is so that the pilot doesn't overspeed the engine with an under-reading tach. The law does not apply to centrifugal tachs, since there is no magnet to weaken. It also does not apply to electronic tachs, for the same reason.

This will give a steady, under-reading tach and it's the ONLY factor that will do so. All other defects, such as a bad cable or seizing tach input shaft bearing or a torn-out cable drive slot in the engine's tach output, will give erratic readings. Lots of oil in the tach will give wild readings. There is a tiny little extension on the pointer shaft, at the other end from the pointer, and it fits into a tiny hole in the input shaft. This keeps the pointer shaft concentric with the input and keeps the aluminum cup from touching the spinning stuff. Oil in that tiny hole can dry up and start grabbing the shaft, making the tach over-read and erratic.

The hourmeter is driven by a small worm gear on the input shaft. If that stuff gets seized, the plastic gears will fail or the load will break the tach cable. It will not create a steadily under-reading tach. You'd have to make the input shaft turn slower somehow, and that always results in a busted cable. The engine has some brutal torque available in its tach output. There is no slip designed into that system. It runs off the oil pump, and if that fails, the tach dies but so does the engine.

Mitchell did not calibrate that tach before it left the factory. They'd have caught the error.

 

[Vendata]

Thanks for all the responses--
Ron- my connector is definitely good...And i'll double check for a drain hole, but dont think there is one...
Dan that was great detail and thanks for taking the time to write that up...Looks like I'll be talking to Mitchell again this week. Will update as i go...

Thanks again!

Scott