What is acceptable voltage fluctuation

[Smokin_Joe]

According to the JPI 830 in my 231, the voltage during full power operation sits between 14 and 14.3v most of the time, but does vary occasionally  from 13.8 to 14.7v.

Can anyone tell me what the acceptable voltage range should be with the 6 cylinder Continental engine please?

The reason I ask is that recently I've been getting a howling noise in both my noise cancelling headsets after I level out in cruise and have backed the power off.  Sometimes the howling starts withing 10 or 15 minutes, or it can take an hour or longer before it starts.  The howling stops if I turn the noise cancelling function off on the headset.  I don't think the headsets are the problem as its occurring with two different head sets.  Pulling the alternator CB does not stop the howling.

Long story short:  another Mooney pilot here in Queensland had a similar problem and he spent a lot of money overhauling his alternator, only to discover that the problem was with his voltage regulator.  Hence my query as to what the acceptable voltage variation is and whether it could be the culprit.

Edited February 9, 2019 by Smokin_Joe

[ArtVandelay]

If it’s a modern electronic VR, and assuming no fluctuations of load, it should stay +-0.1V.
Check the tension on the alternator belt, lose belt could cause this.
Also the VR has adjustable pot internally to set voltage levels, it could be going bad.
Finally, check any connections for loose connections and spray electronic cleaner as well for good measure, especially if there is a cannon plug.


Tom

[Yetti]

Sounds a bit like a diode is going in the alternator.   @jetdriven has lots of posts on replacing the diode pack on the alternator.   Getting your air boat alternator rebuilt at the alternator shop is another way to save some money.

[PT20J]

I'd get a good DVM and check the bus voltage to verify that the JPI is reading right. You mentioned that pulling the alt CB didn't affect the headsets. Did you pull the alt FIELD CB? That will turn off the alternator. Check if that changes the headset noise or has any effect on the voltage. With the alternator off, you are just measuring the battery voltage. If that fluctuates, you have a bad gauge or a bad connection somewhere. Check all that before digging into the alternator/regulator.

[Smokin_Joe]

Thanks for the comments.

I'm not sure what version the voltage regulator is, but I suspect it is still the original (pre digital) style.

My radio guy changed the diodes when I first reported the problem.

It was the alternator field CB that I pulled and verified it was out by observing the ammeter showing a discharging current.