RH Alt Out Light

Home 2024 Forums Opening Section Operational Issues RH Alt Out Light

Viewing 10 posts - 1 through 10 (of 10 total)
  • Author
  • #84538

    Hi all, I have something weird going on I’m not sure how to solve and we have been chasing it now for about 3 weeks.

    Sometimes I can go on a 5-6 hour flight and never see my RH Alt Out lite turn on. I can also have 3-4 flights back to back with no light. Then I might take a quick IFR currently trip for a few approaches locally like I did last friday and on approach for an airport the light turned on.

    I cycle the switch and nothing. I then turn the RH alt switch off for about 15 min then back on and it goes out, sometimes for the rest of the flight but as in friday flight only for about 10 min.

    Last night went to take a night flight and right after I started it the light came on. Called my mechanic on the phone, we ran the engines up to higher rpm, cycles switches, for about 10 min and just as I was about to shutdown and abort the flight the light went out. stayed out for the entire 1.5 hour night flight.

    Here is what I know about when it turns on.
    Voltage is good and same as when it’s out (not lit)
    Amps are being produced by the RH alt when the light is on.
    I can cycles the RH alt switch and see an AMP drop on the meter, same with LH.
    I can cycles the LH and voltage stays the same, AMPS show charging on the RH side even while the Light show RH Alt Out.

    I’m now confused. Seems the RH alternator is working fine but getting a light all the time. There is some delta in AMP on the gauge between RH and LH amps. Usually about 3 amps.
    But its also there even when light is not illuminated.

    Anyone have any ideas?



    I had a similar thing going on, but different. My light would not light when the engine was not turning – some flights OK, others not OK. The push to test button would not light it either. But about 50% of the time it worked normally.

    I suggested it might be what sends the signal to light the light, so they did the R – to – L swap and it followed.

    I replaced: PN 5118335-10 Module Alternator and PN 9910094-3 Sensor Alternator. (this is a 340, but yours may use the same parts)

    Fixed the issue.

    My alternators and voltages were all normal too.



    Jim, thanks for the quick reply. That may be a good things for us to check next.


    I have a 414A and I am having the EXACT same problem you describe only my is the left alternator light. I constantly monitor the output and there is no problem so I have just learned to ignore it for the last two years. If you find a solution, please post it. I just didn’t want to pay a mechanic to go on a wild goose chase.


      Remove the light bulb and you won’t see it any more. OR–The alt out sensor is bad or one of the little wires that go to it are broken. Those wires sit out in a 200 knot breeze all the time so all those wires going to and around the alternator should have multiple tie-wraps holding them snug. I suspect you have a broken wire that is still protected by the insulation so you can’t see the broken part, but is intermittent when the engine is running and vibrating. The way to troubleshoot the wires is to cut all the tie-wraps and then twist each wire and see if a break shows up through the insulation. If you find the break, then put a splice in the wire. Sometimes the little wires are swaged wrong in the terminal ends so you need to tug each one with a good pull. The wires are #28 and the terminal ends are made for #20 or #22 wire so it is easy to mess us the terminal swag operation.


      I’m going to swap the Alt Out Sensor on the plane and see if the problem follows the sensor.. While doing that we will look for cracks.
      I’m starting to chase this in the shop and will let you know what I find out.



      I have the same problem as above.

      Left alternator OUT light occasionally comes on. Left alternator verified to be working with AMP gauge. (Nuisance warning). Maybe bad alternator fail sensor? P/N 9910094-3

      The alternator fail sensor looks like a small rectangular box mounted in the cowling on a flat baffle.

      The alternators are 100 amp Prestolite ALV 9610.

      I had my shop swap these boxes left to right to see if the problem went to the right side. It did.
      Now my right alternator OUT annunciator occasionally comes on.

      Am I sure the problem is with the sensor box? About 90% sure.
      The only thing is, the shop charged me $180 for 2 hours labor to swap the sensor boxes, but a new box is about $90! Doh! 😯

      Maybe I should have just bought one new box and had the shop put it on the naughty side! 🙄




        The alternator out sensor gets its sense from a special winding built into the alternator which comes out at the R terminal. This winding puts out about 10 to 12 volts, sometimes I have seen 14 volts. When that winding get below about 9 volts, the alt out box supplies a ground for the alt out light on the annunciator. The light always has positive volts on one side. One way to check if the special winding is acting up is to attach a long wire from the R terminal on the alternator and run it through the cowling and tape it to the fuselege and then through the vent window. Connect a volt meter to the lead and find a good ground in the cockpit. Then start up and see if the voltage is constant when the alt out light flickers or comes on steady. If you see the voltage dipping down below 9 or 10 volts and the alternator main voltage and amperage is normal, then there is a problem with the special winding in the alternator.

        The alt out sensors are prone to failing intermittently so that could be the problem. I have also found that the 327 bulb in the alt out light can get its contact tit worn down from vibration and cause intermittent ops until a new bulb is installed. Also where the wire attaches to the back of the annunciator could be loose, but that is a bear to get at. Or the wire is shorting out somewhere between the alt out sensor and the light, AND BECAUSE THE WIRE PROVIDES A GROUND, IT WON’T BLOW A CIRCUIT BRREAKER.


        An A&P found the problem with the sensor box system. Two of the wires comming out of the Lamar box had bare spots and were rubbing together! 😯

        The box itself was probably ok, but he put a new box on and added extra wire insulation sleeves.

        These wires sit out in the breeze, and have to endure a lot of vibration!


          quote :

          These wires sit out in the breeze, and have to endure a lot of vibration!

          If the wires are secured sufficiently, then the 200 knot breeze won’t bother them. Use lots of small tie-wraps.

        Viewing 10 posts - 1 through 10 (of 10 total)
        • You must be logged in to reply to this topic.