[james]

james on September 10, 2020 at 6:58 am #20185

thanks Steve, as of yesterday am they were going to pull the Tach instrument out and O/H. They have talked to Century and that is what they felt it was. in the past 3 months, starting after I installed a new engine, it has been bouncing around until recently it would work during runup then drop 700 ish RPM and a couple days ago it failed until next start.
– 2 months ago they took the cable out and lubed the cable.
-3 weeks ago they unhooked the cable and ran a drill and the tach worked great for the 15 minutes they ran it.
– they have also did multiple runups with good success until I fly it.
-yesterday we talked and knowing that it was a risk on O/H tach and not fix the issue i authorized to swap parts from side to side. as of last night they called me and sent a video of the piece that separates the cable and converts it to electrical. both tachs work but the failed one worked better. So i authorized the owner to fly it 1/2 hour and see if either fails. i am now waiting to hear what happens today. I just hate to have the plane down for another 10 days to have Century O/H and not fix the problem.

Do you have any other thoughts? BTW it still could be the part on the engine where the cable goes into the gear reduction box?? but all of that looked good a few months ago and since it is the most expensive item they were leaning on the O/H tach due to costing and Century’s comments until I gave autho to spend labor swapping the cable to electrical connection. (what ever it is called)

[STEVE]

STEVE on September 3, 2020 at 8:47 am #20178

Hi James;
I am basing my suggestions on the information I have gleaned from the Seneca II manual and parts book simply because I do not have the Seneca III manuals.
The tach is driven by a rotating flexible cable that transmits the rpm of the engine to the tach. Two things can cause failure. First, the flexible cable connection can break. This failure is easy to troubleshoot- when discounting the struggles for access to the back of the tach instrument–by unscrewing the cable housing nut on the back of the instrument and pulling it far enough away from the instrument to see the cable, then watching to see if the flexible cable rotates when an assistant rotates the prop. If it rotates, then the problem is the instrument.
I suspect it’s the cable because these mechanical tach are pretty hardy. The most common fault with a mechanical tach is a weakened magnetic connection between the cable driven half of the instrument and the needle assembly, and increased drag on the needle part due to lack of effective lubrication.
The result is a tach that indicates low, sometimes as low as 200 rpm below actual engine rpm.
If the cable is good, I recommend that it be pulled and cleaned before re-lubing it and re-installing it. I recommend a graphite based light grease since this type won’t congeal in cold temperatures. Amazon has some, as does a good auto parts store.
If the cable it good, then it’s the instrument. You’ll need to send it to one of the instrument overhaul shops such as aircraft precision instruments (https://www.aircraftprecisioninstruments.com/) or one of the other well known overhaul shops.
Your mechanic may have an instrument overhaul shop he likes dealing with.
I hope this information is correct; I don’t know if Piper upgraded to electronic tachometer displays in the Seneca III. If it did, and your tach is dead, the fault is usually related to a wiring problem such as a broken wire. You can test the sending unit on the engine by swapping it with the sending unit on the other engine.
Please let me know what you find.
Steve

[james]

james on September 2, 2020 at 12:29 pm #20171

I have a 1982 Seneca III (PA-34) and looks like my tach just went down. does anyone have opinions on new vs. used, where to purchase, or should I O/H?