MGL Avionics User Forum

Hi,

I bought my first plane, but I have a bad reading in low RPM (Engine Rotax 100hp). In idle the RPM displayed on the screen is too low and when I increase RPM, reading is good.

I have just 1 cable connected to the MGL RDAC and in the documentation, I saw that can go to the magneto or ignition or ecu or pick-up.

This type of problem can be the cable or configuration on the RDAC?

I saw that I can ajust a potentiometer for the High Gain RPM1.

Another had this kind of problem?

Thanks

The pulse created by the Rotax on it's RPM pickup does vary in amplitude with RPM and gets smaller at idle.

The RDAC should be set with dipswitches: High Gain ON. Ballast ON, Filter OFF. The sensitivity trimmer for RPM1 usually ends up around mid position and is not very critical.

Thanks for the information.

For the ballast ON the documentation says:
used when the RPM 1 signal line requires a
220 ohm ballast resister to ground

Not sure if the installer guy put a resistor, can I break something if I don't have this resistor and I put this ON? Not sure what it is the ballast resistor and what does it do. Can you explain this part?

Thanks

dieuvolkanos wrote: ↑Wed May 11, 2022 9:50 pm Thanks for the information.

For the ballast ON the documentation says:
used when the RPM 1 signal line requires a
220 ohm ballast resister to ground

Not sure if the installer guy put a resistor, can I break something if I don't have this resistor and I put this ON? Not sure what it is the ballast resistor and what does it do. Can you explain this part?

Thanks

The RDAC XF (and the XG) have a built in 220 ohm ballast resistor. The dispwitch mentioned switches it into the circuit.

The purpose of this resistor is to provide a load to an inductive source - that source is typically a coil. A magnet flies past and this induces a voltage pulse. On the 912 in particular it's around 5-10V but there is a second one that is much smaller but tends to grow in amplitude as you get to about 3000RPM - as the RPM increases it becomes so big that it starts to get noticed by the RDAC and as result it counts twice the number of pulses per engine rotation. Giving the coil a load eliminates the second pulse while also cleaning up the main pulse.
Rotax rev counters have a 100 ohm resistor built in for this purpose - we found 220 ohms works as well and also more suitable for some other engines that need something like this.
We have had one case recently where the 220 ohm was too high - the installer eventually fitted a second resistor externally (giving a total of 110 ohms as they are in parallel). However this is the first time we encountered this over our 20+ years of history with these engines.

Ok thanks for the information. I will try that this weekend !!

Hi,

I tried High Gain ON. Ballast ON, Filter OFF and idle is ok, but when I put the throttle, it read 12000rpm. I tried to adjust the pot RPM1 without success. The gain is too high.

Any idea?

Thanks

Hello,

I have the exact same issue. My settings is High Gain On, Ballast On, Filter OFF.

When the configuration is for 1pulse per rpm with above settings the displays shows almost twice rpms (e.g. the engine sound like on 1800-1900 the mgl shows 3800-4000) rpm.
Please note that It's a brand new Rotax 912 ULS.

I guess the double counts is because per revolution by specification the rpm pickup generates one posive and one negative extermum.

Any idea how to setup this correctly will be much appreciated.

Thanks!

Hello,

I solved the issue with rpm reading.

My current settings are High Gain OFF, Ballast On, Filter OFF.

Pulse per revolution: 1

I hope that this will be helpful for someone.