The International Cessna^® 170 Association

canav8 wrote:

spduffee wrote:Well, I assumed a tachometer was a plug-n-play device. I took out the original one, connected the new one and away I went. I was using the same tach cable as before. Was that not a good thing...?

This is not a owner/pilot operation. It is an A&P job.

Thanks for that. I will continue using the old one until I get through some other issues, then ask about what to do on the cable.

Wouldn't you know--the day after calling Keystone re OH of altimeter and tach, my tach packs it in! It must have been listening on the phone....

The current tach was installed in the 60's, and has I believe incorrect markings. From the operating limitations it seems green arc of 2200-2450, yellow of 2450-2700 and redline of 2700 is correct--like the photos in this thread. (53 B, O-300A) Am I correct?

The old tach hit 3500 RPM, screeched wildly, needle shook, and paint started flaking off within the instrument. Quite a wild demise...all of this at 20"MP, 100 mph cruise. Today's breakfast run proved to be more than a $100 omlette!

Are you sure it's the tach and not the cable? Tach cables are nothing but a long spring. If anything holds them back, they wind up and then when what ever is holding them back releases, all that energy in the wound spring unleashes. Wonder why your tach could show 3500 RPM? The spring was unwinding.

Tachs are pretty simple devices. The cable turns a magnet in a cup. On the other side of the cup is another steal cup which is biosed to O RPM by a spring. The spinning magnetic field acts on the steel cup the needle is attached to and moves the cup against the spring so far depending on how fast the cable is turning.

Also if this is a recording tach, which most of them are, the cable turns a gear train which turns over the numbers at a rate matching 1/10 at hour every six minutes at a given RPM.

So something stuck either in the cable or the tach and then released. If your tach went dead, as in the needle went to 0 and did not move after, then the tach cable is likely broke.

all tachometers do not have the guts as Bruce described. There is another type that uses fly-weights ans a spring. Ask an instrument shop.

I can not dispute what BL said but have not ever seen that type.

If this where my plane here is what I'd do.

• If the tack went dead and is resting on zero.
  • 1. In the same temperature as the failure, I'd remove the tach drive from the back.
    2. Start the engine.
    3. Observe the tack cable end.
  If the tach cable end doesn't turn you found your problem. Tach cable is broke.
  
  If it turns and changes speed erratically with no engine RPM change then something is holding the cable back in the sheath.
  • 1. Remove the tack cable from the engine.
    2. Pull the cable from the sheath from the engine side. (It may come out the tach side, I don't remember which. It will only come out one way as one end is larger than the sheath)
  With the cable removed, clean and inspect it's length for anything other than a smooth coil. If you find nothing lube the cable with petroleum jelly. (Petroleum jelly is what Keystone Instruments recommends as lub that will not get to viscus in cold operating temperatures. Don't just lub it with any grease)
  
  Even if the cable turns and seems to turn smoothly, I'd remove the cable, clean, inspect and lub it.
  
  Cleaning a lubing the cable will fix many cables. And it might even help or fix a cable that has a worn sheath. But if the sheath is worn to much, it needs to be replaced to fix the problem.

After addressing the cable and depending on what I found, I'd have to hook it all back up and see if the tach works.
If so great, I'd continue to fly. If the tach acts up again, you know the condition of the cable, it's probably tach servicing time.

If the tach doesn't work after I was satisfied the cable was not the problem, then I'd have to give the tach to my partner Leroy who would insist we open it up and fix it. There is no stopping him. He has a high rate of success with these types of adventures. You all would just shake your head in disbelief if you saw his "instrument shop" which also doubles as his small engine repair shop, his large engine repair shop, transmission overhaul center, house hold appliance repair station and just about anything you can drag into it, repair shop.

I do not recommend anyone open up their tach. You will screw it up. I have and would ship the tach off to Keystone Instruments. Personally, I'd just get an exchange from them which is what I did in order to get a fresh face with typical markings found in 170s. Tore up Leroy that we would trade in a perfectly good gauge just to change the perfectly good dial markings to more standard 170 markings.

I paint was flaking off inside the instrument, I doubt it's the cable...

I note reports below of bad tach cables...I was about to order a replacement from Aircraft Spruce. Before doing this, does anyone have experience with who is the best supplier? Or is it "take your chances"?

I went through this a number of years ago. Tach drive oil seal failed--ruined a tach; tach cable failed; tach itself failed and was a cheapo that a local Fort Worth instrument shop showed me the barrel to toss it into! After some expense and a lot of frustration I took the advise of a A&P friend and purchased a Horizon electronic tach. Cost a lot more than the mechanical ones and its not in keeping with being original (no issue for me) and I have been very satisfied with the operation of this type of tach. They are a lot more accurate, if that is important to you--it is to me and has no mechanical parts to fail.

When I was flying the Stearman. The tach cable split into a Y to drive the tachometer in both cockpits. The seal had failed at the engine and engine oil would flow down the tach cable housing to the Tachometers. Which would slowly fill the tachometer instrument with oil until such time it was full and then we would dump the oil out of the tachometer. But the reading was always stable.

Avee8or wrote:The current tach was installed in the 60's, and has I believe incorrect markings. From the operating limitations it seems green arc of 2200-2450, yellow of 2450-2700 and redline of 2700 is correct--like the photos in this thread. (53 B, O-300A) Am I correct?

I have not been able to locate any information that indicates the 170 tachometers, for C145 or O-300 engines, should have a yellow arc between 2450 and 2700. My belief this is a misnomer due to the "normal operating range" RPM of 2200 - 2450 and a redline of 2700, some people mistakenly assume the in-between must be yellow.

You mentioned "From the operating limitations..." Is that listed in the Limitations portion of the FAA-approved Airplane Flight Manual (the one-page, front-and-back, paper...not the Owner's Manual)? The one for my A model does not specify any yellow arcs on the tachometer. That is the limitation that applies to the engine/airframe combo.