voltage fluctuations

[mooneyflyfast]

I’m wondering what is causing my voltage to act like this.  The spike/drop at about 1:45 caused my Garmin 625 to reboot.

[PT20J]

I’d start by checking all the connections to the alternator and the brushes.

[Ragsf15e]

I’d also start where @PT20J pointed.  It’s also worth looking back a few flights to see if it was previously “rock solid” at 14.0.  If it’s been solid, something changed more quickly, like a wire to the alternator came loose or something is loose.  If it’s been slowly getting worse over time you could have a corroded connection.

Im scarred for life after my 2 year electrical issue, so I’ll give this advice.  Troubleshoot the heck out of it before throwing parts at it. You should be able to narrow it down to the exact cause.

[carusoam]

The voltage control wire is the field wire….

Check to see how clean its connection is…

The field wire is a popular failure point…

-a-

[mooneyflyfast]

It’s been like this for 6+ months but the flight that the graph represents is the first time the GPS  has gone off line.  Was on downwind at destination.  What has been done so far:  checked connections on the master switch, replaced alternator brushes.  The Voltage regulator was replaced a year ago.  Amps are jumping all around at low RPM, too.  Strange that both act up more at low power/RPMs.  Are bad alternator diodes a possibility?

[carusoam]

Let’s start with a couple of things…

1) What alternator do you have?

2) What voltage regulator do you have?

3) How old are they, years and hours…?

Because, if you are going to throw money at it… you may want to end up with some upgrades…. 

Are there any heavy loads running when the voltage noise appears?

It is possible that a motor going bad could be causing the charging system to attempt corrections…

PP questions only, not a mechanic…

Best regards,

-a-

[Ragsf15e]

It’s relatively easy with a digital volt meter and some wires with alligator clips to check the field wire with with the engine running and to check the alternator output with the engine running.  Mine looked similar to yours, but when I finally checked alternator output, it was steady at 14v.  Then I just went downstream from there until I found out where it went bad.

Edit: just do it carefully with a friend holding the brakes.  That big spinning thing on front is dangerous!

[shawnd]

@Ragsf15e is correct. Though when I did it with my A&P, we had wires connected to B+ and FLD and we ran them back into the cockpit for safety - than standing out there behind the spinning windmill. We also tested each wire for continuity and ensured we were seeing the right behavior when flipping ALT switches at the cannon plug by the VR.  I found the Zeftronics guide a good read to understand the system and diagnostic tips. Also, always a goo idea to clean your MASTER SW and ALT 1/2 SW terminals as well.

[Guest]

8 hours ago, mooneyflyfast said:

It’s been like this for 6+ months but the flight that the graph represents is the first time the GPS  has gone off line.  Was on downwind at destination.  What has been done so far:  checked connections on the master switch, replaced alternator brushes.  The Voltage regulator was replaced a year ago.  Amps are jumping all around at low RPM, too.  Strange that both act up more at low power/RPMs.  Are bad alternator diodes a possibility?

Have you tried bypassing the alternator field switch?  The external wires may be tight, but the internal contacts may be burnt and pitted.

Clarence

[Fly Boomer]

19 hours ago, mooneyflyfast said:

Voltage regulator was replaced a year ago

I wouldn't assume exoneration.  New and newly rebuilt components go bad with great regularity.

[mooneyflyfast]

Thanks for the suggestions. They were very helpful.  I cleaned the ship battery to airframe ground connection and now the voltage seems rock steady.  I’m keeping my fingers crossed.  It didn’t look too bad but i’d be willing to bet that it had never been cleaned.

Edited May 7, 2022 by mooneyflyfast

[carusoam]

nice follow-up MFF!

Go MS!

-a-