As we discovered when we bought the airplane (a 1990 TB20 Trinidad Sn 1021) we are the sole operators of this kind of aircraft in Brazil. As a pilot I´m loving the experience of flying it! But we are having a very annoying problem… the tachometer started to show erroneous RPM indication in flight, at first it started after the cruise reduction, then started during the climb, then started just after takeoff… Two weeks ago I delivered the aircraft to the maintenance office for a 100hrs/500hrs inspection, prior to the inspection I bought a new RPM sensor as we thought the problem might be it. Last Monday I went back to pick the aircraft, the maintenance team said that after they changed the RPM sensor the tachometer worked just fine. Then I started the aircraft (RPM showing correctly!) and started the taxi, prior to takeoff, just as I approached the hold point, the tachometer needle went straight to ZERO…. So I turned back to the maintenance hangar. The mechanics checked again all the system (Sensor, cable, connectors, the tachometer…) they just found nothing… I started the engine again, the RPM needle worked for one minute and went to ZERO again… they checked again, I started the engine again and this time the needle stayed in ZERO… We stayed there two days in the hangar trying to figure out what was happening… They said that I should buy a new RPM gauge. But the peculiar thing is that the RPM gauge works just fine in the desk, the sensor is factory new, the cables are perfect. I´m sure that if I buy a new tachometer it will happen the same thing.
The question that I have for you is: Have you ever heard about something like this?
I would check the gap between the sensor and the “gear wheel”. This is from the IPC but right now I can’t find the spec on the gap
The tacho (depending on your S/N) is made by Noris in, I think, Swizerland. More details here. There is a US company called EI which makes a tacho which may be approvable as a replacement, and there probably are many others. Noris is a typical Socata supplier which is a dead end for any direct communications.
Dear all, today It was the first time I detected a zero RPM reading of the electronic analogue Tach of my TB20GT when starting the Engine. It still has the Factory installed Tach which didn‘t work at all after starting the engine today. Checked all CB‘s and switched Main and Alternator on and off without any reaction. Shutting the engine off and restarting did not resolve the zero reading with low RPM initially but with full throttle the RPM Indicator was jumping around and showing stabile max RPM at full throttle as he does subseqently in a flight, too but started to jump wild around below less throttle approx under 2200 RPM.
Would be thankful for any suggestion where to detect the problem?
@chris most moved to identical previous thread.
The sensor is positioned near the big gear on the front of the engine and maybe the gap there is wrong. There is a spec on that gap in the MM – see above.
Otherwise it could be bad wiring – just replace the wires, unless able to troubleshoot with an oscilloscope at the back of the tacho.
And finally it could be the tacho.
thx to Peter for your hints, finding a new Tacho working well after co-installing one from Noris (company in Nuremberg, Franconia 
) which indicates that either the sensor with wiring or the gap oviously is the problem. During current downtime getting an factory update of the Tacho now, however. Easy come easy go, on my return flight again the recently built in Roll-Servo KC271 in the KFC225 participation quit after 5 hours although I looked careful getting an exchange unit with Mod 10, so what the hell – this is another never ending story as you probably know
Glad you narrowed it down.
Which avionics shop “rebuilt” the servo? 
The servo s/n < 5999 I’ve got from SAE, Melbourne, Florida USA….😖 🙄
