Intermittent charge

[varlajo]

Been noticing intermittent charge recently. The alternator appears to function properly during most of the flight, but sometimes would not kick in until main battery charge drops to 11.7V - but never lower than that. 

Last image for comparison shows an actual failure of a charging system due to a separation of the field wire in flight, where the main battery keeps discharging towards 10V. This seems not to be happening in the scenario above. 

Thoughts? 

Edited February 24 by varlajo

[Yetti]

Simple things first.   Check Belt condition and tightness.   Build a new wiring harness to the alternator.    Replace brushes in the alternator.  May be a voltage regulator.   Replace with Zeftronics.

[PT20J]

It looks pretty noisy even when it’s working. I’d first check all the connections carefully. O

[AdamJD]

Ditto for checking connections.  I had the same issue.  Would get low volt alert on JPI and then back up to 14 in a few mins or so.  Sometimes longer.  Replaced the voltage regulator and that did nothing.  Wish we had tested output from the VR before we replaced it.  Pretty easy to do.  But the mechanic was convinced it was the VR.  Anyway, it wasn't until I was hearing occasional popping in the headset that we determined a loose connection somewhere.  Turned out to be a loose connection between the main buss and the avionics buss.  After a few turns of the screw driver, the problem was solved. 

[Bartman]

In 20 years of Mooney ownership, I found that the connection at the alternator is weak. The constant vibration of the wiring harness causes the wire to break at the terminal where it connects to the alternator and mine broke a couple of times. I think it’s the field wire.  A while back we replaced the wire because it was just a bit short and left a couple of inches of extra wire looped and tied so that I would have fresh wire when the time comes.  
 

I’m down for annual right now and will use this thread as the reminder to install a new terminal. 

[cliffy]

Don't forget the connections on the circuit breakers also can be loose. 

Someone a while back was chasing an alternator problem and showed how they checked voltage drop

at every connection and just how much was lost due to high resistance/age in all the electrical connections

Most of the Mooney fleet has circuit breakers and connections that have 50 years of age on them without ever

being taken apart and cleaned. 

On another note- circuit breaker manufactures recommend cycling C/Bs occasionally. How many are out there that haven't 

been cycled  in 50 years?

[AdamJD]

I carry a multimeter with me. When I was having these issues, I used it with the engine running to check steady voltage to the power buss at the alternator circuit breaker. That’s how I figured out it wasn’t the alternator. From there it was a matter of following the connections after the alternator circuit breaker. If the alternator wasn’t steady at 14 or so at the breaker, I would have started looking at the field wire or voltage regulator.