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Fluctuating voltage, panel lights bright-dim-bright, ammeter needle swinging


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Classic symptoms of a failing flux capacitator.  

It could be the classic Mooney electrical demon- the master switch or the contacts for the master switch.  My 85K had the same issues and was so bad that the AP would disconnnect or have other strange

Thanks, Clarence!  I'll need to check the logs to see the service hours of this alternator but I do recall seeing at least one instance of alternator replacement in my review of the logs. Please

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I have had similar symptoms and resolved by three things all of which are relatively quick and easy

1) as other have said clean the connection on the master switch

2) remove and disassemble the master switch.   Take note when you take the switch apart so you can get the parts back together correctly.  There are little springs and other pieces in there. You can have carbon buildup on the contacts inside of the switch.  If so clean or replace the switch.

3) disconnect your battery, clean the terminals, and connectors and reattach.

good luck.

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4 hours ago, Piloto said:

FlyBoy

I noticed that your lower cylinder baffles are held by safety wire, I did the same on mine but added a steel washer to keep the safety wire from cutting into the aluminum baffle.

José

I put AN bolts with drilled heads and lock nuts. I put the Safety wire through the drilled heads.

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On 8/27/2018 at 9:40 PM, FlyBoyM20J said:

Thanks 81X - re the master switch, are you referring to internal contacts of the rocker switch (so, disassembly required)?

I'm afraid it will turn out to be a compound issue since everything else to do with this plane has been caused in part by at least 2 things!

Mine was just the contacts on the rear of the switch and not the internal contacts.  However, as Piloto said, contact cleaner can also do wonders.  Good luck, electrical demons are annoying.  

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26 minutes ago, Oldguy said:

Prior to cleaning my ignition switch:

image.thumb.png.3eac8aec68f56fea03559610a65e3f24.png

After cleaning:

image.thumb.png.e5d52386e061c6f28bb674bcb0e7fd01.png

 

Very cool!  I have about 50 flights of data from my EDM700 that recorded the fluctuating voltage...and perhaps 3 flights now that may show that it's gone.  I haven't had a chance to analyze it yet but will do a similar comparison this weekend.  I hope to see nice things even though the EDM700 sample rate is dreadfully slow (every 6 seconds in my case) relative to the frequency of the anomaly (at least 2 Hz).  Undersampling leads to easy misinterpretation, but perhaps I can still tell the difference between a mess and a steady DC level.

 

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2 hours ago, FlyBoyM20J said:

Very cool!  I have about 50 flights of data from my EDM700 that recorded the fluctuating voltage...and perhaps 3 flights now that may show that it's gone.  I haven't had a chance to analyze it yet but will do a similar comparison this weekend.  I hope to see nice things even though the EDM700 sample rate is dreadfully slow (every 6 seconds in my case) relative to the frequency of the anomaly (at least 2 Hz).  Undersampling leads to easy misinterpretation, but perhaps I can still tell the difference between a mess and a steady DC level.

 

Excellent!  BTW, you know you can change the sample rate on the 700, right?  I think it's in the setup menu--hold both buttons, press the step button to cycle to the "END Y/N?", then press and hold both buttons again.  I forget the option name, but it's in the 700 manual as well

 

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3 hours ago, jaylw314 said:

Excellent!  BTW, you know you can change the sample rate on the 700, right?  I think it's in the setup menu--hold both buttons, press the step button to cycle to the "END Y/N?", then press and hold both buttons again.  I forget the option name, but it's in the 700 manual as well

 

No, I didn't think to check the manual for that setting, thanks!

Yeah, at least 2x oversampling is Nyquist and all, but 10x would be better.  I believe I was looking at fluctuations on the order of the time constant of the VR feedback loop which probably isn't all that fast.  700 sps would safely be at least 2x (if not 100x).

Edit: yeah, I was reading in a hurry, you never said 700 sps!  :)

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Hmm...here's p. 30 of the manual.  Sadly, seems the EDM 700 will go from samples every 2 seconds to samples every 500 seconds (not 500 samples/second).  That's OK...if this comes back, I'll just use a normal lab data logger on the system voltage to get a higher sample rate.  For now, I think I'm OK and even at samples every 6 seconds, I'll see the overall stability of the line.

 

image.thumb.png.e8720d637d2293d3b185bf53d2f0ecfc.png

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8 minutes ago, FlyBoyM20J said:

Hmm...here's p. 30 of the manual.  Sadly, seems the EDM 700 will go from samples every 2 seconds to samples every 500 seconds (not 500 samples/second).  That's OK...if this comes back, I'll just use a normal lab data logger on the system voltage to get a higher sample rate.  For now, I think I'm OK and even at samples every 6 seconds, I'll see the overall stability of the line.

Ha!  Yes, the sample rate is intended to track temperatures, which shouldn't change that fast.  But every 2 seconds is at least fast enough to see some of the voltage fluctuations in question, where as 6 seconds has no chance.  At 2 second intervals, it seems like there's still enough data memory to record several hours of flight before you have to download.

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  • 2 years later...

@FlyBoyM20J
Just wondering if people still see a slight oscillation of the ammeter even when the system is working properly (engine on and off)? 

I have had the intermittent full scale ammeter fluctuation in flight (including some flights where the alternator quits for varying lengths of time - sometimes automatically resets, other times it didnt).

I have cleaned the master switch connectors, tightened the field wire post on the alternator (was a bit loose), checked alt field cb resistance, etc. EDM830 voltage data looks cleaner now and I haven't had the wild random fluctuations in flight, but the ammeter still oscillates very slightly (1/2 needle width) even before engine start. Ammeter also jumps / oscillates a lot when systems are turned on (with and without engine running). I would expect a bit of a response when a load is applied, but the jump and oscillation seems disproportionately large (3/4 scale with some overswings) when low power systems are turned on. Is that normal or a sign that I haven't solved my problem?  78J

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I’m troubleshooting the same issue myself, 

You can confirm or deny it’s the Master by temporarily connecting in a toggle switch to the alternator wires, if the problem goes away. then of course it’s the switch.

‘As expensive as the switch is, I’m a little hesitant to take it apart or replace it without knowing it’s the issue.

Is the Master a switch breaker? I wouldn’t think it would be as the battery side only powers the relay, and we have an alternator field CB don’t we?

Edited by A64Pilot
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Thank you. That means a temp inistsled switch for trouble shooting isn’t a safety issue, and also opens up replacement possibilities.

‘Does anyone know if our Master is also used in other aircraft? Will a Piper switch fit? Cessna’s seem to have different mounting 

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Pretty much an off the shelf item used by several aircraft manufacturers...

Expect the relay to be acting up because they get dirtier with age until they stop seating well...

PP thoughts only, not a mechanic...

Best regards,

-a-

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The issue is either poor connection, bad switch, bad vr, internal break in wire. Somewhere along the path from battery to alternator there is a large drop of voltage, .1 or .2 volts will do it, causing the alternator output to go up, to increase output, then go down, off and on. Check each connection with volt meter engine off until you find where the drop is. Of course wires vibrate too. If no voltage drops, suspect vr or master.

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8 hours ago, carusoam said:

Pretty much an off the shelf item used by several aircraft manufacturers...

However the same switch has different mounting attachments, Cessna going off of memory uses a split switch thwt “clicks” in where my Mooney’s switch is held on by screws.

So I’m trying to find which manufacturer other than Mooney uses a DPST switch that’s held in by screws, but a split switch would also work

Edited by A64Pilot
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On 5/3/2021 at 3:42 AM, 65C_flier said:

@FlyBoyM20J
Just wondering if people still see a slight oscillation of the ammeter even when the system is working properly (engine on and off)? 

I have had the intermittent full scale ammeter fluctuation in flight (including some flights where the alternator quits for varying lengths of time - sometimes automatically resets, other times it didnt).

I have cleaned the master switch connectors, tightened the field wire post on the alternator (was a bit loose), checked alt field cb resistance, etc. EDM830 voltage data looks cleaner now and I haven't had the wild random fluctuations in flight, but the ammeter still oscillates very slightly (1/2 needle width) even before engine start. Ammeter also jumps / oscillates a lot when systems are turned on (with and without engine running). I would expect a bit of a response when a load is applied, but the jump and oscillation seems disproportionately large (3/4 scale with some overswings) when low power systems are turned on. Is that normal or a sign that I haven't solved my problem?  78J

My ammeter has a slight oscillation with the strobes on, but none with them off.  Voltage is steady. Even though they’re led strobes, they still oscillate.

More likely you have a slightly corroded connection somewhere.  Mine showed up as low voltage, like 13.3ish and fluctuating with or without strobes.  It took me 2 years of learning and 2k of parts replaced before I found and fixed a connection myself.

I used a DVM and tracked 14v steady from the alternator (carefully with engine running), through each step, cb etc, to my bus.  The jumper between buses ended up being very slightly corroded.

Edited by Ragsf15e
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3 hours ago, Smiles201 said:

The issue is either poor connection, bad switch, bad vr, internal break in wire. Somewhere along the path from battery to alternator there is a large drop of voltage, .1 or .2 volts will do it, causing the alternator output to go up, to increase output, then go down, off and on. Check each connection with volt meter engine off until you find where the drop is. Of course wires vibrate too. If no voltage drops, suspect vr or master.

VR’s don’t usually go bad this way, they usually just quit totally.

Being that’s it’s surging indicates that something is causing a variable resistance, testing with a multimeter may not show anything as when off the resistance is likely static.

‘Which doesn’t mean don’t test the switch, do so, just a test my not be conclusive.

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26 minutes ago, A64Pilot said:

VR’s don’t usually go bad this way, they usually just quit totally.

Being that’s it’s surging indicates that something is causing a variable resistance, testing with a multimeter may not show anything as when off the resistance is likely static.

‘Which doesn’t mean don’t test the switch, do so, just a test my not be conclusive.

I agree, Just be careful checking it with the engine running!  Alligator clips and a well thought out and practiced plan.

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