I have the standard Rotax 912ULS setup with Ducati regulator/rectifier and 22,000uF capacitor. I’m having some issues with a small amount of smoke in the engine bay that briefly enters the cabin and then clears. More often is an electrical smell, very sporadically during 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 test with a voltmeter and used the following website as a guide: http://pointsforpilots.blogspot.com/2012/12/testing-rotax-912914-ge...

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 volt meter 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 in R and B+ but not in 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 volt meter 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. But then 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; 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 the help! Art.

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Comment by Clive Richards on January 15, 2017 at 5:30am

I corrected my first post yesterday but it has vanished.

I said on reflection if R becomes disconnected I think for safety it would be biased high to turn output off this seems to be confirmed by you measuring 37 V this will come from the permanent magnet generator its output

voltage and frequency is determined. By engine speed

as David says I think your circuit is wrong when you say you turn off generator are you disconnecting R and B separately from your bus. Where is the capacitor connected if measuring the voltage wit digital voltmeter and it is the only load it will. take a while for voltage to decay Clive

Comment by David Josephson on January 15, 2017 at 3:43am

It sounds like things may be wired wrong, or you may have a faulty regulator. Do you have a diagram of how it's wired? Don't know how you have your master switch/relay wired so can't comment on what you've measured. R and B+ should be connected together inside the regulator, the voltage on those two terminals should always be equal. With the battery disconnected and no load, the voltage can rise, particularly when you are using a high impedance digital meter, not a cause for concern.

C is the control terminal. The regulator should turn off (stop charging the battery) if C has no voltage on it.

As noted the smell/smoke could be anything, but if you have the typical failure mode of the regulator that could be causing it too. I have opened several dead ones and they have the same problem -- component leads not cleaned properly before soldering, leading to high resistance contaminated joints, leading to melted solder.

Comment by Clive Richards on January 13, 2017 at 6:41am

Hi Art from your description of the circuit it dose not comply with the instructions in the Rotax installation manual I have wired up ours as the instructions but have not run the engine yet.

I am not clear if Rotax are correct as their was discussion on the Matronics Aerolectric-list  that C is control R is reference voltage B+ is battery positive L is lamp (optional) G is generator.

  Rotax state you should not open the connection between C & B on regulator with engine running I would have thought this should read R & B however I have connected C R & B plus capacitor together (Also shown in testing blog you quote) just in case as per  their diagram and use a relay from a split master to disconnect from bus. This will not stop generation however and wonder if one should also switch a G wire but rotax donot suggest this.  ( When you say you switch alternator on or off are you switching a G wire ? ) If R is the reference terminal and you loose its connection to B with alternator running I would expect output at B to go to max)

If you go to http://www.matronics.com/search and search the aerolectric list you will find discussions photos & circuit diagrams about the ducati regulator or post your original information to list for their answers.


Comment by Art Locklear on January 11, 2017 at 3:29pm

Chris, thanks so much for your response!  I can tell that mine is aggravated or controlled by the regulator, alternator and/or charging system.  So, I'm pretty confident it is not zip tie related.  However, I will be on the lookout for something like this.  Thanks, Art.

Comment by Christopher Jon Brown on January 3, 2017 at 7:12pm
I had smoke enter my cockpit and thought it was an electrical problem and went thru the whole electrical system searching around for something and found nothing wrong. Turned out to be a zip tie end that went into the heater shroud and when we climbed out melted and caused the small amount of smoke. Took off the heater shroud and got rid of every little piece of melted zip tie and alls perfect. Hope yours is just something like this. I have the Rotax in a STOL 750

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