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Friends,

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: http://pointsforpilots.blogspot.com/2012/12/testing-rotax-912914-generator-and.html

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!

Art.
  • Re: Electrical Problem Rotax 912 ULS

    by » 8 years ago


    Your voltage readings are not easy to understand unless you know how it's wired, you need a schematic.

    Some of your numbers are conflicting. The +B and R pin are jumped inside the regulator, so those measurements must match. My guess is you were measuring a G pin not the R pin.

    The C pin is an input, not an output. It senses the voltage and turns OFF the output (+B and R) when C is above 14 volts, below 14 volts it turns ON the output. If the C, +B and R pins are connected together, but are disconnected from the battery, the regular will self excite (the +B and R output drives the C input, the regulator can't be turned off). You will get higher voltage swings without the battery to stabilize the regular output. Without the 22,000 uf cap the swings would be much higher when the battery is disconnected.

  • Re: Electrical Problem Rotax 912 ULS

    by » 8 years ago


    Mike, thanks for your response! I will discuss your thoughts with my partner and see what other questions we have. The most intriguing thing you help us understand is why the C pin is different from Bt and R, because is it is an input. We can try it again and make sure, but I'm confident we were jumping the R pin, not the G. We did the G (generator) check in the beginning and it tested fine.

    Let me ask you this...do you know of a schematic out there from Rotax, which will help understand where the pins should be connected?

    Thanks, Art.

  • Re: Electrical Problem Rotax 912 ULS

    by » 8 years ago


    I have attached the schematic from the heavy maintenance manual (section 74-00-00).



    On my aircraft I have a simplified installation with no ammeter and no charge lamp so terminals R, +B and C are commoned.

    It may be worth switching your multimeter to ACV with the engine running just to make sure that you don't have a large AC component to the charging voltage.

    Kevin
    Rotax912ChargingCct.png (You do not have access to download this file.)

  • Re: Electrical Problem Rotax 912 ULS

    by » 8 years ago


    You have an unusual configuration with a Master and ALT switch. Both seem to have some effect on the system. There may be some other non-standard changes. It's likely this is a one off design. It would be best to make a drawing of the actual system than depend on something similar. With that said...if the system is changing correctly and there is no indication of heat damage in the regulator or associated wiring and devices, you may want to look elsewhere for the source smoke.

  • Re: Electrical Problem Rotax 912 ULS

    by » 8 years ago


    You may have a wire insulation rubbed through making contact on metal or the 22K uF capacitor plugged in backwards. The capacitor is polarity sensitive. It will smoke if swapped around.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


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