Jumpy RPM readings

I am in Phase I flight testing with my ELSA RV-12 and am having a problem I need some help with.
The SkyView (conversion harness) shows reliable and steady RPM information for the Rotax only until it hits around 5000 RPM. As soon as it hits the "limit" (and the limit varies, sometimes it is down to 4800 but never below that) the RPM readings jump all over the place and never settle.With jumpy I mean vast jumps of +-500 and more RPM from one moment to the next.
At low cruise of 4500 - 4800 everything is steady and persistent but on take-off or climb, I cannot get a reasonable or reliable reading from the SV. The engine runs smooth, with low vibrations and has been balanced. The prop is still set at the initial pitch setting that Van's requested for first engine run.
The RPM sensor in the Rotax is a two-wire cable(white and blue) that is just hooked up to the FW forward harness(white/green and black) with a plastic plug, so no modifications or crimping had to be done to make this connection.
I suspected some resonant vibration at a certain RPM affecting the connection in the plug and cable-tied the connector to a hose ensuring it could not move. That didn't change the behavior one little bit.
I am pretty much at a loss as to what to do or try to identify the issue.

Any thoughts or suggestions?

resistors maybe??

I had this thought while reading your post: I believe the Dynon D120 had folks install resistors in-line in order to eliminate jumpy tach reading.

There are already resistors installed within the tach circuit when using the instrument panel components supplied by Van's but on rare instances the specified resistor value hasn't been high enough (slightly stronger than normal output signal from the tach sensor?)
I recommend a call to either Dynon's or Van's tech support. I think they will be able to help you out.

Looking at the schematics for the electric I saw that there is a 30K Ohms resistor installed going into pin 34 on the 37 pin EMS connector. I have added a 20K Ohms resistor in front of the firewall in the white/green wire that connects to the RPM connector coming from the Rotax. This increased the overall resistance to 50K Ohms and the problem is almost gone now. I only see it during the take-off run and the initial climb out but it is not so severe as it used to be. Once airborne and settled subsequent climb-outs and at cruise cover the range up to 5500 without any problem.
I will try a 30K Ohms resistor in place of the 20K Ohms when I take the cowl off for other maintenance work and see if this fixes it completely.

Please let us know how this works out for you. A couple of us RV-12/ROTAX 912 ULS builders are having similar jittery tach problems.

I had this same issue a few weeks back. I consulted with Dynon Tech support and their recommendation was to add a resistor to the tach input side, which is pin 18 on the firewall forward harness. The Skyview installation manual allows for the addition of up to 60K of resistance in line if erratic tachometer readings or "spikes" are observed(See Skyview System Installation Guide Revision P at 7-33, Trigger Coil Rotax 912). Since the value of the resistor installed in the Van's electronics is unknown, I started by adding a 30k resistor by removing the screen and accessing the firewall forward harness through the display opening. After removing the screen, the firewall forward harness is the closest harness to the opening, so you don't have to disconnect any other harnesses from the magic box to gain access to Pin 18. Pin 18 is a white wire with a green stripe. I added the resistor with crimp connections and heat shrink to keep everything tidy. Reinstall the harness and screen and your ready to go. My tach reading are now rock steady. As Scott said, this problem is pretty rare, so I wouldn't recommend this mod unless you have already run your engine and have observed erratic tach readings.
-Bruce

inline resistance

inline resistance is reducing current. from all above i understand the rpm sensor is AC voltage generator and at high RPM will overload input serquite
the correct way would be to connect potentiometer as voltage divider and when adjustment done replace it by standard resistor divider

http://en.wikipedia.org/wiki/Voltage_divider

It is not an AC generator but a solid state switch giving one puls per revolution of the crankshaft.
The resistor reduces the high impedance of the Skyview input circuit, making it less susceptible to noise (ignition) as the puls switch is open during most if the revolution.
In this one rev the ignition fires the sparkplugs introducing false pulses.

Erratic tach readings at higher RPMs

Almost identical to the discription at the top of this tread, while running the engine up to WOT yesterday to check for the proper prop pitch, I began getting highly variable RPM readings at or above 4,500 RPM. Settles down when the throttle is pulled back but its a little scary to see the tach needle bounce around like that with the engine running at high throttle. Had not ever seen this before - have run the engine up to full throttle 3 or 4 times in past. Contacted support at Van's today and according to Ken, this is so rare that they don't offer a canned "fix" although he had heard of the addition of a resistor. I wonder if this is more common than being reported?

It is a definite problem with at least two of the newly-minted SkyView-equipped RV-12s here at our airport -- including my RV-12. Lacking guidance from Van's, I guess I will proceed with the installation of a 30k resistor on the green/white TACH lead and see if that solves the problem.

Electrical Schematic Pre/Post 8/25/11

Noticed that the RV-12 electrical schematic for kits shipped pre 8/25/11 has a 100 K OHM resistor on the wire going into pin 34 of the 37 pin EMS module. However, the post 8/25/11 schematic has that changed to only a 30 K OHM resistor. Clearly there was a specific change made for some reason (Scott, do you have some background?). I'm beginning to suspect that the ideal resistor is actually between the two.

As Brent noted, my TACH wire had the original Van's 30K ohm resistor installed. New harnesses have a 100K in place of the 30K. I replaced the 30K with a 100K and my TACH now works great!