Charging system

[mikey757]

'84 j model with 12 volt system. Began discharging at low rpm only. Still putting out above 14 volts at higher rpm. Belt is tight and in good shape. Connections are tight. Any opinions?

[LANCECASPER]

My 1st Mooney was a 14 volt 1983 K (231). At low rpm taxi speeds it would annunciate low volts until I brought up the rpms. 

In the mid-late 80’s when Js and Ks  were upgraded to 28v, it was a significant upgrade.

[gabez]

oh man, check the obvious the alternator and coupler. Usually the coupler craps the bed and starts slipping, I replaced both and it still happens sometimes. I read that some couplers from aircraft spruce don't last very long. Mine tends to do it more on a heavy rain, then it goes away. 

[N201MKTurbo]

If it was something that changed suddenly, you probably lost a phase in your alternator. Did you also notice an increase in alternator whine?  This is usually caused by vibration cracking internal solder joints in the alternator. Losing a phase (1 of 6) will reduce the efficiency and total output of the alternator.

Actually, there are two phase failure modes.

1. lose a diode by an open diode or an open connection and lose 1/6 of the output.

2. lose a stater phase and lose 1/3 of the output.

[1980Mooney]

2 hours ago, gabez said:

oh man, check the obvious the alternator and coupler. Usually the coupler craps the bed and starts slipping, I replaced both and it still happens sometimes. I read that some couplers from aircraft spruce don't last very long. Mine tends to do it more on a heavy rain, then it goes away. 

Apples to oranges.  You have a K with a Continental that uses a "coupler".  The OP has a J with a Lycoming that uses a V-belt.

[1980Mooney]

4 hours ago, mikey757 said:

'84 j model with 12 volt system. Began discharging at low rpm only. Still putting out above 14 volts at higher rpm. Belt is tight and in good shape. Connections are tight. Any opinions?

So you say that your ammeter is discharging at low rpm.

• Is this all the time or only after starting?
• How old is your battery and has it been tested recently? (Is cranking speed/sound the same or is it noticeably slower?)
• When in cruise, if you shut down everything that you can, does the ammeter go to zero? - ie not discharging and not charging because the battery is fully charged.
  • When you turn some load back on in cruise such as lights or pitot heat, do you see change in ammeter charge/discharge and also a change in voltage?

[mikey757]

Just began on my last flight. Cranking was normal. It dipped just below 12 on the taxi out and taxi in. There is a whine that I just recently noticed in the headset, so could be the alternator going. Thanks for the ideas

[LANCECASPER]

3 hours ago, mikey757 said:

Just began on my last flight. Cranking was normal. It dipped just below 12 on the taxi out and taxi in. There is a whine that I just recently noticed in the headset, so could be the alternator going. Thanks for the ideas

The whine is the obvious clue you’re looking for in the alternator going bad.

Just make sure that the whine isn’t coming from a passenger 

[MikeOH]

The whine will come from his wallet!

[Yetti]

On 3/11/2025 at 5:03 PM, MikeOH said:

The whine will come from his wallet!

Not really. If you search Mooneyspace you will find the number for the diode packs.  Spruce sells the brushes.   Or the Alternator shop can rebuild your air boat alternator.

[N201MKTurbo]

I have found the parts manuals for the Prestolite alternators on line. The diodes and brush holders have changed a lot over the years, so you need to make sure you are getting the right parts for your alternator.

[Yetti]