Erratic rpms Rotax 912/Garmin G3X

Four years ago I had the same problem, which I solved with a noise filter using ferrite beads.
My problem was interference with the wiring. I haven't had any problems since (that was 4 years ago), so I consider it resolved.
However, each problem must be carefully analyzed before determining the solution.
It would be very helpful to know what signal you have at the output of the tachometer coil and what signal should be applied to the Garmin.
It is necessary to visualize the signal from the Rotax coil with an oscilloscope, and also the signal that reaches the Garmin.
Then, input the signal required by the Garmin documentation, possibly by adding a corrective circuit.
Regarding the diagram provided with diodes, it limits the signal amplitude to +0.6V and -0.6V.
There's no harm in trying, but in terms of electronic solutions, miracles don't exist; there's always a reason.
One person's solution is not always the solution for another.
For example, I had a problem with a friend's device. The radio worked on the ground but not at altitude! The solution was to separate the radio antenna cable from the transponder antenna cable... Not always immediately obvious...😳
Analyze your problem and you will find your solution (which will probably be unique because your circuit is unique).

Jacques

Bert Sparrow wrote:

Hi Paul, unfortunately it never did.  I did try two big ferrite surpressors, one near the engine and one further back, plus I added more complete shielding to the generator wires, in fact all the way inside the engine compartment the two gen cables are shielded, and it made zero difference.  Where I am now is here:

I decided a $20 dollar workaround was to by a Tint Tach, wrap it around #1 sparkplug, and run the little readout gauge to underneath my instrument panel where I stuck it with 2-way tape.  So now I get a 100% accurate reading of all rpms at all rpm ranges.  

What I now find with my Garmin in comparison is from idle (1700-1800) up to say 4600 rpms or so, BOTH read identical rpms numbers.  But when I go higher rpms than 4600, the Garmin rpm readout becomes erratic, with constantly shifting rpms, like 5000, 6500, then 6100, then 5200, etc, changing every second.  Of course my Garmin flashes the red X for rpms for anything above 5800, so it's annoying.

But at least my cheapo $20 Tiny tach gives me the correct number. It's part of my pilot visual scan now, lol.  I spent so much for all that work right and it doesn't but oh well.  Maybe one of these days I will wire in a 3rd resistor and see if that helps.  

Bert

Did you ever try with a single "mag" off?  Mine works minty without one (should be the left one now), and like yours it does seem to be interference from the generator coil wires (isolated to gen coil wire on ignition module "A")  

I guess I may need to consider trying a new 011-02348-00 which I think is like a Garmin filter for these things, calls it a Rotax kit in the wiring diagram.

Worst case Ontario it could be a bad coil that's throwing too much interference into something that's bleeding into my tach trigger or getting into the GEA through other wires.  But that would be team no fun

I had a very similar erratic tach quite a few years ago. I had neglected to read the Rotax, or maybe it was the Dynon installation manual where it suggested a simple resistor (don't remember the ohms) be installed in the signal line. Worked like a charm, with no issues since.

No I don't believe I ever tried mag off.  Attached is the pic of the Rotax-specific part that consists of diodes and a quarter watt 300 ohm resistor between wires.  I bought it pre-made from an Aircraft Company I think somewhere in Virginia, and all I had to do was handle the input and output wires.  

Bert

I read the Garmin documentation for Rotax-specific engines.  The docs have a specific part number with two diodes, one is a zener, and there is to be a quarter watt 300 Ohm resistor between both yellow Generator wires.  I do have that.  I guess I could go look at the Rotax Installation manual and see what it mentions.  I think I did years ago, But it seems the Garmin manuals would take precedence.  See attached pic.

Bert