I have a 2010 390 DA and I thought...

[cbruck]

I have a 2010 390 DA and I thought I used to run the AC with just the ac connected.  I switched off the main 12 volt breakers.  The boat had been sitting for 2-3 months with no AC running since last fall.  I flipped on the ac breaker and my ac current meter was bouncing all over the place.  I then turned on the 12 volt main breakers and then the air conditioner and everything worked fine.  First, is the air conditioner supposed to run off of ac only?  If so, why did turning on the 12 volts solve the problem?

[wingless]

The problems may be only mildly related.

It may be a poor connection, such as on the shore power cord. How do all the AC cord blades and receptacles look?

Yes, the air conditioner only needs AC shore power for operation, such as is shown in this Cruisair manual.

https://www.dometic.com/assets/27/52/installation%20and%20operating%20manual_62752.pdf

[cbruck]

Thanks for commenting, I figured someone like you would know the answer immediately. Yes, all the cords plugs etc are in good shape and clean. The ac power was on and stable but I turn off the air conditioning over the winter. So I flipped on the air conditioning breaker and it was unstable, so I did it again and was again unstable. I couldn't remember for sure so I turned on the 12 volt main breakers and then went back to the air conditioning breaker and it worked fine. As you said, I think the air conditioning breaker was sticky from lack of use and the 12 volts has nothing to do with it. I'll confirm the air conditioning works solely on ac now without the 12 volts. Thanks again.

[CCH]

I agree that the first place to look would be your connections to shore. Take a look at how your battery charger operates. If you connect to AC and the batteries are low on charge from sitting some chargers will pull/push a lot to get them charging initially. This can give some volt meter flux until it settles down. I typically take a glance at the DC volt reading before bringing on AC so I have an idea of how the batteries are doing prior to charger coming on.

[cbruck]

Thanks, that is a good observation. I have a brand new charger in the boat and if it has been sitting without shore ac, it will charge 40 amps upon start up. Which is probably the limit on the charger and under that high current output it could be feeding noise back on the ac line.