RPM Signal for CSU Prop

John,

Is your existing tachometer a stand alone unit wired directly to the tach pickup, or is RPM displayed on an EFIS?  If the latter is the case, what EFIS do you have?

I looked and this subject of a 2nd tach came up 6 years ago...this was the answer after a lot of disscussion.

Re: One Rotax 912ULS TWO Tachometers!

by Roger Spall » 6 years ago

Roger Spall

 

4 10 0

 

Bill,

As you identified, I currently ground the new Tach to the same ground that the EFIS is grounded to behind the panel - there is a single ground point that all electronics ground to (Star configuration) behind the panel.

Anyway, here is what I tried and appeared to work - haven't taken it out for a test flight yet...

I connected the 200 ohm Ballast resistor from the RDAC RPM input (where I connected the wire that also feeds the new Tach) to the ground post behind the panel. The resistor is very close to the RDAC side and a ground wire that runs through firewall to the panel ground post.

Both EFIS and new Tach now have identical and stable readings.

If this continues to work then I will probably leave as is.

I am guessing my aircraft has multiple grounds at different potentials with different noise.

Roger"

Jeff Blakeslee wrote:

John,

Is your existing tachometer a stand alone unit wired directly to the tach pickup, or is RPM displayed on an EFIS?  If the latter is the case, what EFIS do you have?

It’s on an EFIS. Garmin G3X Touch. 

My rpm indicator is an Aviasport stand-alone connected to the wire from the generator.  The original CSU wire was spliced into this wire and as noted didn’t respond very well and the tacho flicked at times but not sure why.  Tried different earthing systems and included the 200 ohm ballast also tried an anti static interference but nothing seemed to help so finally gave up.  Since then the Aviasport tachometer has not given any problems after disconnecting the CSU and going to the independent Airmaster prop control system has solved all my problems.  Can’t help but feel maybe the CSU control box was too sensitive to the occasional fluctuations from the Rotax generator or maybe it was because the CSU system itself which was designed for Rotax 912 engines and had to be modified to cope with the 914 extra power, don’t know?  This was 10years ago so hopefully more modern electronic CSUs have ironed out any problems and may well work fine with your glass cockpit.  Has anyone else used your CSU?

John Flemming,
The tach input for a G3X system connects to a Garmin GEA-24 (engine information module) and then is transmitted to the EFIS over the Garmin CAN bus. Specifically, the wire you are looking for should be connected to pin 8 of the J243 connector on the GEA-24.  At the point where the tach (+) pulse wire connects to the GEA-24, Garmin wants a diode, zener diode and a 300 ohm resistor to stabilize the circuit.  This is shown in the Rotax sensor interconnect examples in the G3X installation manual.

I think there is a good chance that connecting the CSU to the tach wire (before it reaches the diodes/resistor at the GEA-24) would not affect the tach function on your EFIS.  I would call Garmin Aviation support and tell them what you want to do, if they don’t know they will find the answer for you.  You can also email them at g3xpert@garmin.com.  Also, the manufacturer of the CSU controller must know the answer to this.  There is only one tach coil on the 912, so they must run into this every day. Their controller needs a tach signal, and they can’t expect you to disconnect your tachometer to do so.