Getting low voltage annunciator light if below 1100 RPM (1988 M20J)

[kortopates]

I was watching this thread as I don't recall seeing a low voltage light on my 1980 M20J when running low idle.  I checked this morning and I had 14.0v at 800 rpm so a low voltage at low rpm isn't always expected.

I concur with that as well. I have a student with a 1990 M20J 28v electrical system and although the light may come on at 800 rpm after awhile it’s never on by 1000 rpm let alone 1200 rpm. Makes me wonder if brushes are worn or even a weak battery. But the annunciator voltage light is adjustable too so it may be out of calibration as well.


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[PT20J]

I checked my 1994 M20J. The Service Manual indicates that the annunciator should indicate low voltage when the bus voltage drops below 26.5V. I notice that there is some hysteresis in the annunciator: when reducing rpm, it begins to blink at about 950 rpm and 26.5V; when increasing rpm it will stop blinking at about 1100 rpm and 27.0V.

Skip

Edit: My alternator was new with the rebuilt engine so it has about 450 hours on it.

Edited August 18, 2023 by PT20J

[Fly Boomer]

2 hours ago, PT20J said:

I checked my 1994 M20J. The Service Manual indicates that the annunciator should indicate low voltage when the bus voltage drops below 26.5V. I notice that there is some hysteresis in the annunciator: when reducing rpm, it begins to blink at about 950 rpm and 26.5V; when increasing rpm it will stop blinking at about 1100 rpm and 27.0V.

Skip

Edit: My alternator was new with the rebuilt engine so it has about 450 hours on it.

So to avoid the warning, you need to stay at about 1,000 or above?

[PT20J]

1 minute ago, Fly Boomer said:

So to avoid the warning, you need to stay at about 1,000 or above?

Yes. But you should be doing that anyway because extended idling below 1000 rpm is a leading cause of spark plug fouling since lower  cylinder pressures allow more oil into the combustion chambers and lower CHTs impede lead scavenging. I usually keep it between 1000 and 1200 rpm except when necessary to slow down while taxiing.

[Fly Boomer]

Just now, PT20J said:

Yes. But you should be doing that anyway because extended idling below 1000 rpm is a leading cause of spark plug fouling since lower  cylinder pressures allow more oil into the combustion chambers and lower CHTs impede lead scavenging. I usually keep it between 1000 and 1200 rpm except when necessary to slow down while taxiing.

And maybe when landing?