Paul 5r4

Paul 5r4

Well Known Member
Hey all,

After my first flight of N729PG, I have a tach issue to work out. It is erratic and fluctuates. I've confirmed everything is tight with good connections. I have the Dynon D120. The manual states to install the 30K ohm resistors which is how I made the first two flights. The manual also states that for my kind of problem that you may have to increase the resistance to as high as 150K ohm. Here's the question. I have put two resistors in series, one is 22K and the other one is 47K. The adds up to 69K ohms. I've confirmed this with my volt meter. Is this the correct way to "increase" the resistance or should I have used a single resistor with a higher resistance or does it matter. I haven't run with the new resistors yet. That will come tomorrow.

Thanks in advance
 
Connecting resistors in series is a valid way to add the ohmage. But in my experience with D120s (I wired up an RV-10 and an RV-8 with them, both with Slick mags)is that 69K is still too small. I ended up using about 120K before the tach was stable at or above 2600 rpm
 
Resistance in series

Neal is absolutely correct - resistance is additive in series. One thing you can do is place a variable potentiometer in place of both resistors. If you connect from one end and the center, you have an infinitely variable resistor (within the max limit of the pot, of course) that you can use to find the sweet spot. Once you have done that, replace it with the closest value fixed resistor, and you're done.
 
Orange Beach/tach update

Yes Rob, I do like this part of the country. Except for the humidity and the hurricanes, I guess I'd consider this paradise. :)

Update on the tach... I flew this morning and can report that the 68000 ohm resistance isn't enough. The tach was still not correct in the top half. When flying, it did seem to smooth out some but at takeoff was indicating only 1800 RPM! Of course I could tell the engine was developing full power by the sound. I guess I'll jump to about 120000 ohm next go round. Will report back after that with an update.

Performance of the airplane this morning was good. I did a couple of full power runs in opposite directions and was indicating 149 Kts. I think that's pretty good for a fixed pitch prop. The true airspeed was 155kts. I don't have wheel pants yet so that'll kick it up some more too.
 
On the RV-8, I employed a terminal strip from Radio Shack to connect the tach resistors. That way I could easily change resistor sizes, using ordinary ring terminals crimped onto the resistor leads.

pRS1C-2266709w345.jpg
 
another update on tach issue

Today I took out the 68K ohm resistors and replaced them with 120K ohm resistor. I STILL have tach instability and at run up it will not read above 1800. In flight, it seems to bounce around anywhere from 1600 to 2500 or so. I've tried changing the pulse values on the dynon but any other number other than the "1's" I have in there now really throws the rpm off. I don't know what to do except to once again increase the resistor values. The dynon install manual states a value as high as 150K may be needed. Guess I'll try that next. Anyone ever have to go above that and if this does not work... what is plan B???

Thanks in advance!!!
 
Another problem?

In my Skyview installation the RPM would jump around occasionally using the 30k ohm resistor. Before I got around to investigating that I found a seal had failed in the right mag. Dissassembly showed the mag was due for a complete overhaul. The cap, rotor and coil tab were all severely worn. Reinstalled and the RPM has been steady ever since.
 
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