So yesterday when I went flying I kept seeing an intermittent low voltage light and the voltage would drop from 13 back to 12. I assume when this occurred the battery is not being charged. I see from this site and others that this may be a problem with the voltage regulator. Currently I have the Ducati 343620 which appears to be harder to find(new) than hen's teeth. I see that Leading Edge Airfoils has a regulator, part# 965 349 which is supposed to replace the one I have. Does anyone have any experience with this? Is the connector the same(not looking forward to having to replace the connector)? Is there anyway to test the regulator to verify if it is actually causing my issue?
  • Re: Intermittent low volts light

    by » 5 months ago


    This assumes we are talking about a 912ULS engine.

    The voltmeter will tell you what is going on.
    With the engine running...
    Above 13.8v the battery is Charging.
    From 12.6 to 13.8v, the battery is Floating.
    Below 12.6v, the battery is Discharging.

    A low voltage light at IDLE just after starting is to be expected.
    The battery was just under a heavy load and need a charge.

    Refer to Installation Manual Section 24-00-00 Page 8.
    Below 1800rpm the generator barely makes 5 Amps so the battery will likely be discharging depending on what is turned on.
    It will take at least 2500rpm to generate 10 amps and 5000 rpm to make 20 amps and get a good charge back in the battery.

    Rev the engine up to at least 4000rpm.
    If the voltage continues to increase and eventually tops 14v, all is as it should be.
    The generator and Regulator are good.
    If Not, Check the wiring first.
    Without good grounds on the battery and regulator, all bets are Off.
    Remove all the ground wires no matter how good they appear to be.
    Clean all connectors, replace any that show corrosion.
    Grease the connections and reassemble.

    Charge the battery.
    You should see 13+ volts on the battery immediately after removing the charger. (Engine OFF)
    You should see 12.5+ volts a day or two later.
    If not, the battery is weak.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    whertzel1@yahoo.com
    Clicking the "Thank You" is appreciated by all.

  • Re: Intermittent low volts light

    by » 5 months ago


    Yes it's a 912 ULS. I just had my annual 2 weeks ago and the mechanic tested the battery and it was fine. I'm use to the low volts at idle and while I'm taxing. However this happened while I was flying at 4800 RPMs. The light would come on, the volts dropped to around 12 then after a few minutes the volts went back up to 13+(less than what I'm use to) and the light would go out. Based on previous posts here and elsewhere it seems like it might be an issue with the voltage regular. I will check all the wiring and clean the connections.

  • Re: Intermittent low volts light

    by » 4 months ago


    Some additional troubleshooting. I see from other posts that people have had issues with the wiring harness that connects to the voltage regulator. As all the other connections on my installation appeared to be intact, I pulled the plug on the regulator and had a look. All the connections looked good. I cleaned them with some contact cleaner. When I went to test everything again, when I first start the engine everything works good. I ran it for a few minutes then started to taxi out to the run up area. As I was taxing I started to hear a buzzing sound from the right side of my instrument panel, the low volt light came on and the voltage dropped to just over 12. This persisted and I taxied out of the way and continued to observe what was going on. It appeared that the issue had resolved. I went back, did my run up and just as I took off the voltage dropped back to just below 12. I had planned to stay in the pattern anyway so I brought it around, landed and taxied back to my parking spot. The whole time the voltage is just below 12. When I get back to my spot I have a look under the hood(trying to figure out what may have been buzzing) but I see nothing amiss. Probably after about 10 minutes I start it back up and it seems to be working correctly. So the question is being the problem doesn't start to after the plane "warms up" a bit, does that point to the voltage regulator. Does it buzz when it's bad?

  • Re: Intermittent low volts light

    by » 4 months ago


    An update. After discussing my findings with my mechanic, he recommended we just change out the voltage regulator. Since I did not have the capacitor installed I ordered that as well. I got the wiring diagram from the install manual. While waiting for the parts I did some testing. I verified the voltage on the battery and the voltage coming from the engine stator were all good. I got the parts and the capacitor is huge. We hooked everything up yesterday but I was only able to make one flight in the pattern as it was a bit windy. No low volt light even while taxiing with everything(i.e. radio, xponder, lights, fuel pump)on. Soon as weather permits I'll do some more testing.

  • Re: Intermittent low volts light

    by » 4 months ago


    You left out the part where the Capacitor was never installed.
    Its purpose is to stabilize the regulator's output.
    It isn't an optional component.
    That could have been the cause of the Buzzing.

    The Capacitor Labeled as #14 in the installation drawings is mislabeled as a 1uF Cap.
    It is supposed to be a 22,000uF Cap rated at least at 25Vdc and install as close to the regulator as is practical.
    It should be located BEFORE any Fuses or Switches.
    When the ALT Switch is turned OFF, the Cap should still be connected to the Alternator.
    Within 12 inches of the regulator is ideal. Over 3 feet is too far.

    As you noted, it is NOT a small Capacitor.
    It will be about the size of a fist.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    whertzel1@yahoo.com
    Clicking the "Thank You" is appreciated by all.

    Thank you said by: RotaxOwner Admin

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