The International Cessna^® 170 Association

My tach was a little erratic on the last flight. For a few flights lately it has been slow to register engine RPM after the initial start of the day. I figured it was due to the cold. It would take anywhere from a few seconds to a few minutes for it to come off the zero peg. On the last fight it was a little erratic (+- 50 to 100 RPM) while in flight, but the engine was steady, level flight and no changes of the throttle (so I know it was the guage, not the engine actually changing RPM).

Any helpful hints? Is there a connection I could check to possibly solve this or does this look like I'm going to have to pry the wallet open and pay someone to fix it?

Thanks

Mike,

If what you say is correct, then how do you do a Mag. check?

I didn't T/O with out a tach reading, or a stable one. On the last couple of flights, after initial engine start, it didn't read until a few seconds to a few minutes after the start ... then it was normal (and stable). I wasn't sure what to think after it read normally. On the last flight, in cruise, it was a little jumpy during cruise at cruise RPM. I haven't flown it since then, but started suspecting a loose cable attachment or, if it is like my old 1965 MGB, the cable is all gunked up (technical term) inside with old crankcase oil or lube and just needs a little cleaning/TLC. I thought maybe someone else had this same malady at some point in their ownership.

Never had it happen (yet) to me in the plane so it's probably apples & oranges, but I had this happen twice on my motorcycle and both times it happened just before the tach cable broke. I'd have a look at your cable before I pulled the tach itself.

My $.02

Good input, thanks Doug.

The original tach system on this airplane is nothing but a speedometer with a drive cable. The cable is made of an outer sheath with an inner, twisted wire drive cable. That inner cable can become worn and when a strand or two breaks then they wad up and start dragging. The result is an erratic reading on an otherwise serviceable tach indicator. Aircraft Spruce sells replacement cables. 877-477-7823

I've had the same problem when the weather gets cold. I've inspected and cleaned the cable with no change. The cable is not actually attached to the needle but drives a mechanism (sort of an inductor) and that causes the needle movement. A new tachometer will cure the problem.

Ouch!! A new tachometer? I went out to see what it would do today, the hour numbers at the bottom of the guage were ticking off correctly, but the needle stayed at zero for about 2-4 minutes, then it started working correctly (verified with a handheld electronic tach). Could this still be a cable issue or does this signify a tachometer problem?

Sounds like it needs cleaning/overhaul/replacement or all 3.

Sounds to me like a problem with the tach itself. I've had good luck with Rudy Aircraft Instruments in Arkansas,479-474-8759. Their ad doesn't list tach o/h prices,but I would guess between $100 & $200.

Eric

I had my tach repaired twice in the last 7 years because of it reading too fast and also had a "bounce" in it. It would settle down and read about right after running the engine on the ground for a few minutes like you explain. The shop on my field only took a couple days and it wasnt but $75 or so to clean and repair.

One other thing to check is to be sure you tach drive oil seal (on the accessory case) isn't leaking engine oil into the tach cable housing. If this happens and isn't caught pretty quickly the oil will work its way into your tach and ruin it. (Under extreme conditions you can end up with a bunch of oil in the cabin getting all over everything) I had this happen to me a couple of years ago. My tach would work fine for a portion of a flight, then I would notice the indictor hand was either all the way past any rpm markings or under zero. After I had shut down the engine for an hour or two and go again, it would again be OK for a while, then the same thing all over again. Finally one flight I just happened to catch the tach in the process of going bonkers and the indicator going around wildly. Sent the tach to a repair shop & it was too far gone for repairs. (Oil in the tach) Had to replace the tach drive seal, gasket, new tach cable (it ended up ruining it also) and a new tach.

Good luck with your efforts.

My tach first showed a problem by wavering a couple hundred rpm. Shortly after that it started jumping all around and making a horrible noise, kinda like a power tool was vibrating against the panel. Then it acted normally for a couple minutes before it did it again, and then it went to zero. I replaced the broken cable (probably the first time in 40 years or so) and it worked just fine for about 30 minutes. Then it repeated itself. Looks like I need to have the tach overhauled (and buy the second tach cable--I'm sure it also failed). Any good suggestions on overhaul--or replacement? I would actually like to get the tach face redone--it has this Mooney emblem on it (the plane lived in Kerrville, TX sometime in its life and I suspect the tach was replaced back then with something local).

Copied this from another post of mine. they do tachs also. Check out their site.

Keystone Instruments
http://www.keystoneinstruments.com
keyinst@cub.kcnet.org
PHONE: 570/748-7083
FAX: 570/748-4439        LOCATION ON FIELD: CTR
HOURS: 8:30 am to 5 pm wkdys

They are located at Lock Haven airport in Pa acrossed from Pipers old plant. They are capable of all sorts of repair including repainting/silk screening your original face of the instrument and make it look like new if that is what you want.
They do work for a lot of other resalers, I have not ever heard of an instrument including gas gages they can't or won't repair, rebuild or restore.

Bill, forget about overhauling your tach. Just buy a new one. Aircraft Spruce (ph# 877-477-7823) PN 10-24623 (Mitchel PN D1-112-5023, old AC PN RT-7) for $169.