The mechanical tach started indicating about a 25 rpm+ oscillation. At the same time it started reading 75 rpm too low. I sent the tach in for repair. They said it definitely was out of calibration, but did not see where it would oscillate. I checked the prop in flight with one of those optical tachs and the governor is holding the rpms rock solid. Can the tach cable be the cause of a 25 rpm oscillation? If so, I see that Textron no longer sells the tach cable, but there have to be lots of sources. I see that Spruce makes them to length, but the reviews of the product were aweful.

No, we don't want one of those electronic ones. Someday we will get rid of the mechanical engine gages and install a nice electronic screen (right now, the ones from JPI and EI are too small).

Maybe try just lubing your tach cable?

hotrod180 wrote:Maybe try just lubing your tach cable?

So, the tach cable can cause a 25 rpm oscillation and lubing the cable may fix the problem.

Lube the cable, make sure the cable is attached tight to the tach and engine, and check for any tight bends or movement in attach points in the run of the cable. All can contribute to the rpm bouncing, but it could still be the internals of the tach wearing out.

I know you said you don’t like them, but installing a Horizon digital tach has been one of the best upgrades I’ve done. Compared to buying a new tach and cable, the Horizon was around $200 more, but far more accurate. Installation was 2 hrs tops.

When I've experienced tach rpm oscillation it's usually been reduced by cable maintenance. With a mechanic's oversight remove the cable from the tach and engine attachments. Leave the outer casing through the firewall or remove it if not a big job. Pull out the inside spiral cable part that turns. They usually only come out from one end of the casing (engine side) and note if there's a felt washer seal on the engine end. Put it aside and reinstall later as removed.

I like to put one end of the casing in a container then flush the inside with solvent like brake cleaner or charcoal lighter fluid. Black stuff from lube and wear metal usually comes out. Clean the inside cable as well, inspect for unusual wear or broken metal strands, and if ok then wipe it with appropriate grease for the temps you operate in. I like some with moly added to the grease commonly sold as auto CV joint lube. It reduces metal to metal wear.

Reinstall paying attention to avoid any sharp bends and fit the raised portion of the cable end into the engine accessory case and the square end into the tach . Gradual curves prevent kinks of the inside cable. Sometimes this procedure helps providing the tach is ok.

Gary

After my tach cable up and died, I had the local shop make a new one. While it did work, the oscillations were 300-500 RPM's esp at higher RPM's. After pulling it apart and putiing it back together again a few times and adding a little bit of lithium grease, it works great now.

Here's some commercial references: https://www.aircraftspruce.com/catalog/ ... cables.php Note the spiral twist needed versus the direction of rotation.

I think what happens is the inside spiral wound cable can momentarily lag or twist then quickly resume rotation. Gets jumpy. When it does the tach responds with + or - rpm indications. Probably gets worse when there's a tight bend or clamp on the shield housing or high friction due to lack of lube. With the engine running grab the tach cable and move it sideways. Sometimes the inner cable can be felt rubbing on the outer.

Gary

Well, I removed the tach cable today. What a bear on the back of the IO470. The plan was to clean and lube with a moly grease. Wouldn't you know it, the way Cessna had the cable fabricated, the inner cable will not come out. The swage part prevents it. So, back to the replacement plan. Spruce makes custom lengths, but they have some pretty bad online reviews of this product. Not sure who else makes these things. Back in the day, there were lots of speedometer shops around that did this sort of thing.

Attached is a photo of the engine end. Notice how the swage end has about a 5 deg angle to it. Do you think it came from the factory with that, or did it develop a bend over the years?

Lucky for you, I threw some electronics at my IO470 and might still have the tach cable. I’ll look around for it and let you know.

If you spin the inside cable by hand does it feel sticky or rough? Same for pulling it in and out as much as you can. Maybe put a bend in it and see if it gets harder to turn. Just a way to see if it needs lube.

The cable can still be cleaned and re-lubed while intact. Hold or clamp it vertical and run some spray brake cleaner fluid through if you suspect old lube and wear metal is present. Make a small funnel over the end of the cable with foil or ? to let the solvent flow in. Spin the inner part while the solvent works its way down. Blow it out with air if that's available.

Then there's at least a couple of choices for lube. Plain oil can be run down the inside. Better than no lube. Let it drip out before installing the cable.

Better yet spray pressurized lithium grease is available at auto stores. It comes out as a liquid but will thicken as the solvent vaporizes. Stick the thin nozzle that comes with the can in as far as able and wrap foil or ? around it to make a tight seal. Spray to force the lube in. It may help to reverse the cable and fill from the other end as well. With time the grease will distribute itself.

Gary

A reminder as I remember
These Tachs are only come close to being accurate
at the 75% mark of the Physical scale or Arch limit-

The goofy part of rpm and tachs is the lack of direct connection with the engine. It turns 2500 and the tach estimates that via a slipping magnetic-coupled tachometer in most aircraft installations. That needle is not coupled to the engine except via magnetics in the tach. Not sure why. Modern electronic engine displays go direct to the source.

Gary

Down at the hangar I found the receipt for the tach cable I purchased in 1990 when it broke on my way up to Alaska. Back then Cessna must have published the supersession data as what I bought was a S2550-3, which is still available. Thanks for all the ideas.

When I bought my ‘58 182 this last summer that had been sitting for 15 years, everything worked when we started it except the tach, which was dead. We tried everything...replacing the tach with new, replacing the angle drive on the back and finally realizing that the cable had twisted itself beyond it’s ability to “twist” at the end of the 3’ run from the accessory case to the tach. A guy in the next hangar had a 36” long cable he had taken out of a C150. It was the correct length. We installed it and it has been working perfectly. I had my prop balanced last week and the mechanic said it was indicating about 50 RPM low. I will install the Horizon when I redo the panel this winter. No oscillation at all.

CenterHillAg wrote:Lube the cable, make sure the cable is attached tight to the tach and engine, and check for any tight bends or movement in attach points in the run of the cable. All can contribute to the rpm bouncing, but it could still be the internals of the tach wearing out.

I know you said you don’t like them, but installing a Horizon digital tach has been one of the best upgrades I’ve done. Compared to buying a new tach and cable, the Horizon was around $200 more, but far more accurate. Installation was 2 hrs tops.

This ^^^^

My OEM tach failed 2 1/2 years ago, on the way home from OSH. It was not the cable, although its initial indications of bouncing were like a sticking cable. It was the tach itself. On my IA's advice, rather than get a new mechanical tach, he replaced it with a Horizon. At first I was disappointed--its view angle was such that it was hard to read. I removed it myself, sent it to Horizon, and they substituted an angled bezel, which solved the problem--at no charge. I also acquired greater respect for anyone working under a panel, as my hands looked like I'd been in a cat fight after removing the tach. It was a bit easier to install, so I left less DNA here and there that time.

The nicer things about the Horizon, besides dead on accuracy, are its additional features, which allow knowing better how much mag drop there is, when a mag is failing, etc. Its indicator lights also indicate when there's an overspeed (which I usually get by a few rpm on take off) and help set the rpm for run-up. Although it's a couple hundred more than a new mechanical tach, it's so much better than a mechanical tach that I would never advise trouble-shooting a tach that appears to be failing. Best to just yank it and replace it with a Horizon.

Cary

And by the way, McFarlane is a good supplier of cables, all sorts of cables.

MTV