Electrical Problem - Please Help

[MooneyRob]

Hey... been having a problem where the alt field breaker keeps popping in flight (M20R Ovation II).   I had a first mechanic look at it... he sent out the voltage regulator... mooney said it was fine.   Mechanic checked all the connectors, fuse, and batteries... all looked fine....  Replaced the diode... did not work... rebuilt the alternator... appeared to work for about 4 hours... then it popped again in flight.   Took it to a new mechanic.... he found chafing in the field coil wires. However, fixing that did not seem to fix things.   The new mechanic tested everything again... including the alternator.  He narrowed it down to the voltage regulator.   Sent it to mooney.... mooney checked it and said it was fine.  The mechanic is pretty sure it is the voltage regulator.   Has anybody had any odd failure modes with the voltage regulator that mooney did not find at first glance?   This is getting very expensive and my plane has been down for months... please help!   

[ArtVandelay]

Did they redo the windings on rebuild alternator, otherwise you would still have the short? 

why kinda of voltage regulator are you using? Is it connected via plug? If so I would replace the wiring harness, they can short out internally.

since it’s intermittent, you’re left with replacing components until you find the right one. Start with the cheapest (wiring, circuit breaker).

[Yetti]

did anyone check/replace the breaker?   They wear out.  This is what I found when I pulled the breaker panel.   Look at the 2nd CB down

 

[Yetti]

If you had wires rubbing it may have taken out a previously checked componenet

[carusoam]

The following is only PP thoughts... not a mechanic.

 

1) The field wire is not a load carrying wire is it?

2) this is the control for the alternator to charge or not charge.. depending on the voltage of the field wire...

2.5) The  Field wire voltage is supplied by the voltage regulator....

3) the field wire sees a lot of failures in the field across all airplanes... it often suffers from vibration ills and breaks...

4) I’m not sure if an ordinary mechanic has the tools to test the system while it is installed in the plane... but a volt meter connected to the field wire probably tells all the details you need.  After that, measuring the current in the line would tell the rest...

5) If something has rubbed through the insulation there is probably a burn mark on the wire where that is happening...

6) process of elimination starts with the field wire and it’s connections at both ends... trace the whole wire through the circuit breaker back to the VR...

7) Take pics of all the chafing you see...  

8) does your mechanic have a wiring diagram for the alt field wire? I would want to be chasing that one down from the CB to anything that comes after it...  looking for ordinary wear related items first...  sounds like a temporary Short may be occurring... the kind that may be caused by worn insulation...

9) testing a CB is also a possibility... they are known to mechanically wear and trip easier over time.  That usually takes about 40years on a High current line like the landing lights...

10) if the VR is sending too high a voltage... that would be interesting... it usually uses battery voltage at the high and zero volts at the bottom....  sounds unlikely to be able to accidently trip the CB...

again, just ordinary pilot knowledge. Not a mechanic...

hoping to possibly inspire an answer for you...

Welcome aboard, Rob.

Best regards,

-a-

 

Based on that