RPM readings Wrong and erratic on MGL E1 Help please

[gyroron]

I recently purchased a used Waiex with Jab 3300 engine. In the panel is a pair of MGL instruments, the E1 for engine monitoring and the Flight 2 for airspeed and altitude etc...

Honestly I really am not a fan. If I had built this plane I would have probably used a EIS for the engine and something different for flight monitoring, but this is what the plane came with...

The RPM readings are for sure wrong. Erratic as well. at low rpms I believe the gauge may be right, but in flight I see readings as high as mid to upper 4000 rpms and even at a set engine speed and airspeed the gauge just shows readings constantly going up and down.

I do not know what to check.

I also do not know if the Jabiru engines alternator is working. There is a voltage reading in one of the MGL gauges and also there is a stand alone instrument, a GT-50 that displays voltage. in flight the voltage displayed on the MGL was under 12 volts, and the GT-50 was showing the same voltage in flight as was shown on the ground with everything shut down and switched off. It is possible there could be a problem with the alternator or voltage regulator causing the RPMS to be off as well?

[peter anson]

I will assume that your RPM readings are taken off the alternator. You can also take them from the ignition coil but most people seem to use the alternator. It would be worth checking with the previous owner if it was a problem when you bought the aircraft.

The fact that you have any RPM reading at all would tend to indicate that the alternator is working but it is pretty easy to check. Disconnect the output of the alternator, two blue wires according to the circuit diagram that I looked up, and check the AC voltage. You will probably have to make some sort of messy arrangement of tape to hold the multimeter in place while you run the engine. I think with no load it should be about 30 to 40VAC even with the engine idling.

The in flight voltage should go up to a little over 14 volts so it seems most likely that the regulator has failed. I don't see how that could cause a high RPM reading but someone else might be able to shed some light on that. Certainly if the regulator is intermittently shorting to ground it would cause erratic RPM readings.

You can download manuals for the Jabiru engine from the Jabiru Australia web site.

Peter

[gyroron]

Thank you Peter. The previous owner Already screwed me on the planes battery.... And acted stupid when I sent him a email about it. I think he would deny anything was wrong with the plane if I asked him. The original builder, I have spoke to him but he sold the plane to the person I bought it off back in early 2017 so he doesn't remember every detail of the plane.

[gyroron]

I believe I found my problem. was at the airport yesterday. Sure enough, the wire coming out of the regulator that should be going to " Earth " was routed to the positive post on the battery.

I replaced the battery a few weeks ago and I am pretty sure I reconnected all the wires back to where they were when I took them lose from the old battery. So either myself, or the previous owner put that wire on the wrong post.

It would be easy to do... All 3 wires coming out of the airplane side of the regulator plug, they are all yellow wires. One going to fuse box, one goes to + on battery and one should go to ground.

Put the ground one from the positive to negative post, rolled plane out and tied it down and ran it up. Was seeing up to a little over 14 volts at 2500 and higher rpms. And tach was smooth and steady unlike before, and seemed to be no noise in the radio either. Fingers crossed this is now fixed.