I have the standard Rotax 912 ULS setup with Ducati regulator/rectifier and 22,000uF capacitor. I’m having some issues with a small amount of smoke coming into the cockpit, along with an electrical smell. It is very sporadic, but most often happens during every flight. I have noticed it more often with higher RPMs during takeoff but also at intermittent times throughout flight. I've had several people look over the plane with no definitive diagnosis. Concerned that it might be the regulator rectifier, I ran some tests with a voltmeter and used the following website as a guide:

With the recorded values entered below, I’m suspicious there may be a wiring issue and that the regulator/rec is fine.

To start, I individually connected each terminal of the reg/rec, R, B+ and C, to the ground with voltmeter and the master switch “on” but the engine off. Values were R 12.3V, B+ 12.3V and C 12.0V. Next, turned off the master switch essentially disconnecting the battery from the circuit but still had a voltage reading of 12.3V on R and B+ but nothing on C. This is where I’m concerned there is a wiring issues, unless the capacitor is playing a part here. So question is, should all three poles of the reg/rec have the same readings, or are there other factors at work to explain this discrepancy?

Next, on to the regulator rectifier. With the same voltmeter set up (reg/rec output to ground) and with the original output wiring still connected to the reg rectifier, I tested the voltage output of R, B+ and C with the engine running.

C: Seemed to function correctly. Engine running with alternator off and master switch off, output was 0. With alternator switched to on, voltage in high 13s/low 14s. When alternator switched off, voltage dropped to 0 and then master switch turned on, voltage climbed to 12s.

B+: Engine on, Alt and master switch off, no voltage reading. Master switch turned on, no reading (would think this should be 12.3V from battery). Master switch turned off and alternator turned on. Voltage jumps to 17s, but then slowly corrects to 14s. With engine still running and master and alt switched off, reading stays at 14.5V, similar to R. Any ideas on where this voltage is coming from?

R: Had some unexpected voltage readings that I can’t explain. With engine running, alternator off and master switch on, voltage was reading 37V (would think this should be 12.3V from battery). When master was then turned off, the voltage stayed at 37V. When the alternator was switched to “on”, voltage corrected to 13.9V, as expected. But then with Alt switched off, voltage stayed in 14s (plane meter dropped to 0). Not sure why. Any thoughts?

R and B+ had correct readings when the alternator was switched “on” with voltage in the 14s. Is this enough evidence to prove the rec/rectifier is functioning properly and that the awry values are likely from wiring issue/master switch issue?

Another argument for a correctly functioning Reg/Rec is that the electrical system seems to be charging the battery properly and the voltmeter in the plane shows accurate numbers. However, could the C terminal keep up the charge on the airplane for the most part, while the other poles may be causing the problem? I've heard the regulator is solid-state, either working or not (no middle road), but should there be the difference of each of the wires, as mentioned above?

Thanks in advance for your help!


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Did you test the capacitor and its connection? That could explain the 37 volts.

How is the regulator mounted on the airplane?



It seems to me that your regulater is working. The charging system is working hardest just after take off.  This is when it is recharging the battery from engine start etc. If its going to get hot that would be the time.  Check for properly sized wire and good crimps on wire terminals.  Look closely at connectors for discoloration and other signs of heating.

The stray voltage you were reading may of been a charged capacitor.



Thanks for the response!  I can go on a two hour flight, and I'll get the electrical smell and smoke out of the blue, in the middle of the flight.  Normally, I switch off the alternator and it dissipates.  Yes, it most often happens during or just after takeoff, but it also happens throughout longer flights...maybe every 15 minutes.

Please let me know if any other thoughts,


Hi Francois,

Thanks for the response!  I'm confident that the capacitor is not the issue because I had a professional install it, according to Rotax and Zenith diagrams.  Also, this electrical issue has been going on for two years, since I purchased the plane.  I had the capacitor installed just a year ago, thinking that may be a possible fix, but it didn't make a difference.  Please let me know if you think of any other possible checks.

Thanks, Art.

I'd look for a loose ground. Engine to airframe, alt., or dynamo, check regulator too. If you can smell it during flight I promise you can see it. If there is enough current flow to cause a burning smell an no breaker/fuse pop my money is on a ground

I agree, sounds like the regulator is working. I'm not familiar with the wiring but will assume when the alternator is switched off the battery is disconnected allowing the capacitor to function as a small accumulator.  If that is the case there may not be enough load on the regulator to keep the system voltage from spiking and over charging the capacitor.  The addition  of a load resistor 25 ohm across the capacitor may stabilize the regulator during the "off " state.  Still doesn't help the main issue, burning smell.  Like Tim said, inspect wiring and connections.  If something is getting hot enough to smell it should show signs of overheating.

Assure that a separate ground wire goes from regulator case to ground bus. Don't rely on structure grounding.

Definitely ground the regulator case,  thanks Bob.  The smoke you smell may be a piece of your avionics trying to deal with high supply voltage.


I had a similar issue, strong smell of burning insulation, during takeoff.  This resulted from poor electrical connections between the stator and the regulator, so that the insulation was melting.  High current demand with high rpm (and corresponding alternator output) from having switched on all of the electrical systems (avionics, strobes, landing lights, etc) early in taxi (depleting battery) caused this.  I replaced the cabling and connectors, and modified my takeoff checklist to avoid powering up electrical systems until I was ready to roll.  I will say that the two-year duration of your issue makes my scenario unlikely, but thought it would be worth mentioning in any case.


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