Intermittent Low Voltage Warning

[Jayhawk_aviator]

Have a 78 M20J.  We are seeing intermittent issues where the low voltage annunciator light flashes.  I don't have a full time voltage display, but I have checked it with a digital readout in the cig lighter as well as a page on one of the avionics.  It seems to be consistently fluctuating between 14 and maybe 12.8 or 12.9.  All should be above the low voltage threshold.  Alternator appears to be supplying power (if I pull the field breaker the ammeter goes negative right away).  Battery has been replaced with no real change in behavior.  Alternator is relatively new.  Seems to be loosely tied to load, as it's more likely to occur with a lot of stuff on (landing light, etc.), but it doesn't directly tie to loading (can also occur with minimal load).

I suppose voltage regulator could be suspect, but I'm struggling to tie all the evidence together with a failure there.  Any thoughts?

Thanks.

[ArtVandelay]

Voltage regulator or slipping alternator belt would be my first choices.

[carusoam]

Or the low level sensor can be adjusted..?  Check in with your mechanic.

Which voltage regulator are you using?

12.8V isn't very low.

PP thoughts, not a mechanic...

Best regards,

-a-

[N9201A]

My 1978 J started doing this. After chasing it, found tip here about adjusting low voltage sensor light. There are adjustment inputs on back of annunciator panel. It's in service manual and a search of posts here should disclose it, how I got procedure which my IA and I did last annual.


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[ArtVandelay]

1 hour ago, carusoam said:

Or the low level sensor can be adjusted..?  Check in with your mechanic.

Which voltage regulator are you using?

12.8V isn't very low.

PP thoughts, not a mechanic...

Best regards,

-a-

It will start flashing around 13.2, if alternator is working, it should never be less 13.8

[OR75]

tightness of the alternator belt would be my first guess (before going into the more expensive items like a voltage regulator etc

belt should be not too loose but not too tight either ...

target about 3/4 inch "play" for the belt

 

[cnoe]

That's a fairly common problem. The thread is "high/low voltage alarm" and can be found here http://mooneyspace.com/topic/17815-highlow-voltage-alarm/#comment-259016 and has been an easy fix for several of us.


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[nels]

Just a note on the alternator belt. The belt can be tight but the alternator pulley can be worn out. The belt gets its traction from the belt sides not the belt bottom. When the pulley wears out the belt drops down in the pulley visibly and you can see the minor diameter of the pulley being polished by the belt. This happened to me on my old E model.

[Jayhawk_aviator]

We cleaned and recalibrated the unit per the other thread to where the light comes on at 12.5 (per the service manual).  It still seems to come on in the low 13s or high 12s...initially shows up as a dull glow.

 

Anyone have access to a schematic for the IAI unit?

[cnoe]

The schematic I have is practically unreadable but if it turns out to be the annunciator they are still fairly well supported. Search for the thread Annunciator Overhaul Pirep for more info that may be of interest.


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