The Cessna 120-140 Association Forum

The tach in my 120 will occasionally fluctuate. Condition seems to be more prevalent in colder weather.
Yesterday, I flew for about 1.5 hrs. in morning - no issues.
Then flew about 30 minutes to visit friend, no issues. But during return flight, I experienced worst fluctuations seen so far......when cruising at approx. 2400 rpm, needle of tach started to fluctuate up to 3500 rpm. After landing and at idle, seemed to settle down. prior to this, needle would fluctuate only up a couple hundred rpm for brief period and then settle down.

Anyone have any suggestions and or ideas as to cause / cure?

Thank you

Larry B. in Indiana

1. Have your A&P lubricate the tach cable

2. Check for oil pot on back of tachometer, if it has one then use a very fine oil, your A&P oughta know, see pic....

The cable is sticking or the internal mechanism of the tach is sticking, or the magnetic drag cup is catching. All three of these are more prevalent when OAT's drop. The tachs are very similar to the cable drive speedometers in cars until they got electronic speedo's. I remember my 1965 Corvair speedometer squealing in the winter until I dis-assembled everything, lubed it up and that quieted things right down.

Thank you for input.

Getting with my A&P lube cable and I will look for oil pot this weekend.

I'm considering buying a TRU TACH as I just don't trust my mechanical tach. Also very hard to see accurate numbers on a mag check for example.

While waiting for my A&P, I pulled the inner tach cable out for preliminary examination.

Found that it is starting to unwind on tang end that enters engine drive socket. It also is a left lay.

From info on Aircraft Spruce site, I believed I observed clockwise rotation of drive socket - inner cable should then be right lay.

Did I make a mistake in my observation or has incorrect inner cable been installed in past?

Measured old Inner tach cable at 30 5/8".
Found a 32" cable assembly on Univair website.
Found a 38" cable assembly in Wagaero catalog.

Thinking the Univair cable is closest match; does any one have better suggestion?

Make sure you measure the cable the same way the manufacturer does.

(Found some of my notes.) Cable length is the length of the housing minus the nuts on each end. Tach drive rotates CW, so you want a right hand lay.

Aircraft Spruce, Wag-Aero and others who carry Mitchell Instruments should be able to order you a cable pretty much whatever length you want. Their part number is PE 000 0NNX where NN is the length in inches and IIRC, X is tenths of an inch. If I still was in the business, I'd offer to get it for you, and I'd probably remember exactly how to measure it, too. If the installation allows you to put a "joggle" in it then it's pretty easy to accommodate a little extra length. No way to make up for it being too short, though.

The cable I used was 36" long and the PN was PE 000 0360, so I still don't know about the fractions. It was right hand lay.

Thank you for your input, very helpful