View Full Version : Tachometer questions

Glen Geller
11-16-2007, 08:05 PM
Hi All, on my 55 PA22-150, I have a new problem and want some suggestions please. The story:
Took off from my home field one cool morning early this month, heading for nearby EAA Pancakes, when about three minutes out I started to hear a buzzing sound, like a fishing reel spooling out really fast (think how it sounds on a TV show-- zzzzzzzzzzip!).
Engine was making it's usual immense power, climbing steadily, no indications of trouble and all instruments in the green, EXCEPT the tach. Normally will read about 2450 in full climb, but the needle was rapidly vacillating +/-300RPM. Reducing power, it would be steady at 1500/1800, then start bouncing at ~2000, and really bouncing >2300.
Called the tower and told them I had a flakey tach and want to RTB, they cleared me, asked if I needed assistance, and I landed no problems (Thanks Hillsboro Tower Dude!) Ground run up after landing had same results as in flight. Done for the day!
A quick look under the hood showed no obvious problems at the engine where tach cable mounts. So I locked up and went to breakfast. Next day went on a biz trip via Alaska Air, so now I'm back home and want to find what's wrong. My theories so far:
1) Gummed-up tach cable, will remove from engine end and clean, lube and try again. If no joy:
2) Tach has "gone west". Try another tach. Buddy has an extra tach that reads ~75RPM off but otherwise is thought to be OK. (If this solves the problem I'll order a new tach and have lots of fun doing the service. Remember in 55 the panel was low, my tach is 1" above the right control column shaft, surrounded by other instruments. Looks like a real adventure!)
Any recommendations for replacement tach? ACSpruce has Mitchell unit for ~$171. I should probably go the extra $30 for a new cable as well, huh? Merry Christmas!

IF still no joy what is next, what in the engine needs examination?
Any helpful suggestions are appreciated.

PS, If I install a new tach with zero hours displayed, does that mean I have 2000 more hours till O/H?

11-16-2007, 10:53 PM
you pretty much know what to do by the sound of it but one thing you might check is the end of the cable where it goes into the drive at the engine. It might be rounded off and the cool morning causing it to slip and notch or two then it catches and drives again??? A badly gummed up cable might be adding to the problem? If it is worn to the point where it jumps you might be able to change the core without the outside case. A speedometer shop should be able to make up the right length. Of course this is something just to check before you start changing the Tac.

Geoff Newcombe
11-16-2007, 11:17 PM
"A speedometer shop should be able to make up the right length."

A problem I ran into several years ago when my cable broke. You CAN have your NAPA store make you up a new cable, BUT --- look closely at the cable for your PA 22 and an automotive speedo. cable. You will see that the cables are wound differently, one for clockwise rotation, one for counter-clockwise. Don't remember which is which but supposedly it DOES make a difference. A speedo cable will work but will not have much longevity, or at least that is my understanding. Aircraft Spruce made up my new cable which I lubed and slipped in the old housing after cleaning it out and it was still working fine when I swapped my old 200 RPM out of calibration tach for a nice, STC'd, Electronics International tach. More expensive than a Mitchell, which some have had luck with but I have not, and far more accurate and reliable.

Tom T.
11-16-2007, 11:56 PM
Hey Glen, I thought I lost [misplaced] my tack cable so I bought another one. Found it! If you want my old one, drop me a line and I will bring it down to you next Wednesday when I go to the big dinner.

Tom T.
11-17-2007, 02:44 AM
Glen, empty your mail box. I tried to send you a message and you are full up!!

Glen Geller
11-17-2007, 03:20 AM
Thanks all for the comments.
Tom, I cleaned up the mailbox so will check for your message.
I will try to inspect the cable on Sunday, so will let you know about that cable, that would be sweet if that did the trick.
What BIG DINNER are you talking about? I eat dinner most every night!

11-17-2007, 08:54 AM
HI, I should have mentioned that fact. I'm pretty sure the speedo shop should have cable wound both ways but the guy at Napa just didn't look close enough. we used to get cables made for various industrial applications some CW some CCW.

Glen Geller
11-17-2007, 11:05 PM
Hi Folks,
Tom, I replied to the private message. Thanks for the offer, I will see tomorrow if it's the cable or the instrument. And it took me just one day to figger out the big dinner involves turkey (at our house Tofurkey) Left my phone number in a private reply.
Doug, thanks for the NAPA suggestion, I think I'll either use Tom's spare cable or buy a new one from AC Spruce for a "legal" part.

Has anyone here any experience with the Mitchell tach? Same company as Mitchell radios?

Geoff Newcombe
11-18-2007, 11:50 AM
Put "Mitchell tach" in the Search box. When it finishes, read "PA22 Tachometer". There has been discussion about these tachs before.
My former employer had very poor luck with 3 new Mitchell tachs (as have others), finally went to the Horizon electronic tach. Steve Pierce, as you will see, has had good luck. I have an Electronics International electronic tach in my PA 22/20, STC'd and I am very pleased with it. The Horizon tach is not STC'd (or wasn't a couple of years ago) for our airplanes but could probably/maybe get field approved without too much trouble.