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Fluctuating voltage


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I have the fluctuating voltage issue that sounds identical to the problem many have had that is either the Master switch, or corroded spade connectors on the switch.

‘I first cleaned the spade connectors and Voila it seemed to fix it, but soon returned, but as it’s an intermittent issue maybe cleaning did nothing. So I next tried contact cleaner in the switch itself, no resolution. So I figured it’s the switch itself, but before spending $250 I temporarily disconnected the alternator wires from the master and connected them to a regular toggle switch, same issue it still pulsates sometimes, so I’m pretty sure it’s not the Master switch

‘By the way, where is the voltage regulator on this aircraft? What is the function of the silver box connected to the alternator output, mounted just behind the alternator? I assume some kind of noise suppressor?

Like most A&P’s I’m more of a mechanic than an electrician 

Brief search in the forum only turned up the switch issue and I believe one alternator with worn brushes.

81 J model

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Mooney hides the VR ;) On my 1978 J it was located behind the CB panel up high under the windshield. Big OECO unit, bitch to remove. On my 1994 J it is located on a shelf behind the copilot's panel near the bottom just to the right of the console. Smaller Mooney branded unit.

Might be worth verifying the voltage fluctuation with a DMM to rule out the gauge if you haven't done so.

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Posted (edited)

I’ve not, but the lights dim to the gauge and you can hear it in the intercom as well. So it’s real unfortunately. The amps sweep with the volts too, which of course they would have to if the voltage is changing.

Engine off, I have 11.8V at both sides of the switch and 10.48 at the field connection at the alternator.

‘I assume the drop is normal going through the voltage regulator?

I guess maybe I need to wire the alternator so that I can see if the field voltage is fluctuating in flight, that should tell me it’s not the alt anyway.

‘I removed and cleaned all the connections on the alt, the output seemed loose, but who knows? It’s so rhythmic as in steady rate that well I don’t know.

 

Parts manual seemed to indicate a shelf, so maybe it’s the shelf one

Edited by A64Pilot
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5 minutes ago, 201Mooniac said:

On my 1980 M20J, the VR is in the copilot footwell along the ride sidewall about the height of the bottom of the panel up near the firewall.

Thanks I’ll look there, I suppose it’s out so that it can be seen and not behind a panel etc?

I think I’m done for the day, it’s awful hot in the hanger.

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17 minutes ago, A64Pilot said:

Thanks I’ll look there, I suppose it’s out so that it can be seen and not behind a panel etc?

I think I’m done for the day, it’s awful hot in the hanger.

Yes, it is visible if you put your head in the footwell at look up towards the sidewall near the firewall.

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35 minutes ago, A64Pilot said:

Thanks I’ll look there, I suppose it’s out so that it can be seen and not behind a panel etc?

I think I’m done for the day, it’s awful hot in the hanger.

If your original VR has been replaced by a Zeftronics, most of them have a diagnostic LED lamp on the front face that is actually fairly useful for a few things.   It'll tell you whether it thinks the field is shorted, etc.     Many of them have been replaced over the years with a Zeftronics.

 

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Does anyone know if the wiring on the backside of the Master supplies power to the VR, or is it the field wire coming from the VR on its way to the Alternator?

It matters because if it’s just power to the VR, then it ought to remain steady, but if it’s the field current, then I’d expect to see a lower voltage, one that would change with electrical load.

In truth I can’t figure a logical reason to run power through the Master, but maybe a reason to run the field current through the Master, on an alternator system disconnecting the battery can cause damage to an alternator, so maybe it’s field current to protect the alternator if the master is turned off with the engine is running?

See I measured 11.8V at the Master and 10 something at the alternator, if the field current is run through the master, then I have a high resistance between the Master and the Alternator

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On my 77 J the field wire goes through the master switch.   It is shielded.

If the field wire didn't go through the master you might not have a way to kill the electrical system in flight.

 

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1 hour ago, Warren said:

image.png.2ca9ee02761e9c2e176a7adf089fee8a.pngOn my 1985K Model (12V), the A+ wire in the regulator comes from the master switch.  FLD goes directly to the alternator.

I can sort of confirm that, on run up a little while ago the voltage at the switch followed bus voltage exactly, so it’s not field voltage.

I don’t understand the logic of a DPDT switch though for a master with one half of the switch for VR power. Anytime the Master is on, the VR receives power, any time the switch is off, it doesn’t, you cannot operate one half of the switch.

So why not connect the VR directly to the bus? Nothing would change, anytime the Master is on, it’s powered, any time it’s off, it’s not, same function.

VR is voltage regulator 

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22 minutes ago, EricJ said:

On my 77 J the field wire goes through the master switch.   It is shielded.

If the field wire didn't go through the master you might not have a way to kill the electrical system in flight.

 

Running the field wire through the Master sort of makes sense to me. The reason is, if I understand an alternator, disconnecting the battery from a running alternator can cause damage because suddenly the load is removed, and an alternator may could continue to be excited by its own output I guess, of course an alternator unlike a generator uses an electromagnet to form the magnetic field so that you can generate electricity, so it has to have a source of electricity in order to make electricity, but once going can it use the power it’s making to keep the magnetic field alive? I assume it could, if it could then the only way to kill it is to remove that field current, so killing the field current the instant the battery is disconnected seems smart.

But, it appears my Master just supplies power to the VR, so how is that logical?

My field wire in the engine compt is also shielded, so I assume it is all the way to the VR, but haven’t looked yet.

‘So why is the field wire shielded?

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2 minutes ago, Warren said:

image.png.942719fd781274c53e7267c3c9a95033.png

After more investigation, the power to the ALT Field breaker.  So the master or the breaker will turn off the alternator field.

So what your saying I believe is that the alt field CB is not the alt field wire, but is really the VR power supply?

Power from master goes through the alt field CB to power the VR?

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2 minutes ago, A64Pilot said:

But, it appears my Master just supplies power to the VR, so how is that logical?

If killing power to the VR reliably kills the alternator field, it's just as good as disconnecting the alternator field wire, and potentially avoids issues due to voltage spikes from disconnecting the field suddenly.

 

2 minutes ago, A64Pilot said:

My field wire in the engine compt is also shielded, so I assume it is all the way to the VR, but haven’t looked yet.

‘So why is the field wire shielded?

Shielding the field wire keeps electrical noise off of it, for example from the ignition system as the field wire passes the plug wires.   Any noise on the field wire can turn into amplified noise in the generated output power, which then gets into *everything* including radios, navigators, instruments, audio panels, etc.

 

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22 minutes ago, A64Pilot said:

So what your saying I believe is that the alt field CB is not the alt field wire, but is really the VR power supply?

Power from master goes through the alt field CB to power the VR?

Power goes through the ALT FLD circuit breaker -to- master switch -to- voltage regulator.

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My electrical issue was slightly different, but these tidbits might help a lot...

1. There is a good troubleshooting guide on both the Zeftronics website and the plane power website.  Both those companies make VRs, so the troubleshooting guides work best for IDing problems with the VR or Alternator.

2.  10.5ish is normal at the alternator field wire connection.  You should check power into the VR and it should match your bus voltage.  Power out of your VR (field wire) will be 10.5ish.  It should be the same at the VR and at the alternator end.  Ie no drop through the field wire.

3.  My issue was only resolved when I started the engine with chocks in and had my wife stay on the brakes.  Then I very carefully checked voltage at the alternator and then each connector all the way to the bus.  Eventually I found a dropping point about .6v.  I cleaned those contacts and all is well.

*that silver box in your engine compartment with alternator wire into it is a noise filter.

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OK figured the silver box was a noise filter, thanks

I found the VR, actually pretty easy to get to, it’s a gold colored box but pretty sure it is not a Zeftronics or not a Zef branded box anyway. It’s mounted under a shelf right beside of the nose wheel well on the pax side. black plastic cannon plug, disconnected and cleaned the contacts, no effect. four wires, red, black etc.

Identical female cannon plug unused right beside of it, three wires to it, have no idea what it’s for.

‘So I’m real sure the Master switch side of the house isn’t it, unless somehow there is a bad female spade connector, never seen that, it’s possible but unlikely?

I’ve disconnected and cleaned all connections to the alternator, all terminals appear good and secure.

It could be i guess an alt field CB, but voltage at the master matched bus voltage, so that ought to rule that out.

Off to do some reading at the Zef site, been several years, can’t hurt.

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1 hour ago, A64Pilot said:

OK figured the silver box was a noise filter, thanks

I found the VR, actually pretty easy to get to, it’s a gold colored box but pretty sure it is not a Zeftronics or not a Zef branded box anyway. It’s mounted under a shelf right beside of the nose wheel well on the pax side. black plastic cannon plug, disconnected and cleaned the contacts, no effect. four wires, red, black etc.

Identical female cannon plug unused right beside of it, three wires to it, have no idea what it’s for.

‘So I’m real sure the Master switch side of the house isn’t it, unless somehow there is a bad female spade connector, never seen that, it’s possible but unlikely?

I’ve disconnected and cleaned all connections to the alternator, all terminals appear good and secure.

It could be i guess an alt field CB, but voltage at the master matched bus voltage, so that ought to rule that out.

Off to do some reading at the Zef site, been several years, can’t hurt.

I had the same cannon plug into mine. Disconnect it, turn on master, use dvm to check voltage on red wire (double check this with the zef guide).  They are labeled with very tiny letters on the cannon plug.  Should be your power into the vr.  It should match bus voltage.  If you have bus voltage into the vr and the proper field voltage at the alternator, the vr isn’t the problem.

It could still be the field cb.  Mine is labeled field cb also, but it is power to the vr.  If power into the vr is low or fluctuating due to an old cb, that will do it.

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7 minutes ago, Ragsf15e said:

I had the same cannon plug into mine. Disconnect it, turn on master, use dvm to check voltage on red wire (double check this with the zef guide).  They are labeled with very tiny letters on the cannon plug.  Should be your power into the vr.  It should match bus voltage.  If you have bus voltage into the vr and the proper field voltage at the alternator, the vr isn’t the problem.

It could still be the field cb.  Mine is labeled field cb also, but it is power to the vr.  If power into the vr is low or fluctuating due to an old cb, that will do it.

Makes sense, I may quit for the day, but just went out and ran it up with a wire connected directly to the field terminal on the Alt to measure voltage, after startup when it was fluctuating, voltage was so whacked out the Fluke couldn’t even measure it, actually it measured a widely variable AC current, later after it settled down it measured a steady DC voltage that would decrease with RPM and increase with load (pitot heat). Which is about what I’d expect and voltage was roughly about 5 VDC.

So hopefully that’s telling me that it’s not the alt, it’s either the VR of I’m feeding the VR unstable power. Just about every trouble shooting tree I’ve read goes there, high resistance connection to the VR and the VR getting unstable voltsge.

‘I’ll measure voltage at the cannon plug tomorrow and I may jumper the field CB by taking current from the cigar lighter and inputting it into the spade connector past the Master switch.

I’m pretty confident that it’s not the Master so I may not bypass it.

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2 hours ago, A64Pilot said:

Makes sense, I may quit for the day, but just went out and ran it up with a wire connected directly to the field terminal on the Alt to measure voltage, after startup when it was fluctuating, voltage was so whacked out the Fluke couldn’t even measure it, actually it measured a widely variable AC current, later after it settled down it measured a steady DC voltage that would decrease with RPM and increase with load (pitot heat). Which is about what I’d expect and voltage was roughly about 5 VDC.

So hopefully that’s telling me that it’s not the alt, it’s either the VR of I’m feeding the VR unstable power. Just about every trouble shooting tree I’ve read goes there, high resistance connection to the VR and the VR getting unstable voltsge.

‘I’ll measure voltage at the cannon plug tomorrow and I may jumper the field CB by taking current from the cigar lighter and inputting it into the spade connector past the Master switch.

I’m pretty confident that it’s not the Master so I may not bypass it.

Also, someone else said it before, but you gotta check your bus voltage to make sure it’s not an indication issue.  After doing this once, I’d start at the alternator output.  If that’s steady and 14 (or 28) vdc, then go to through and check each connection of alternator output up to the bus.

If the alt output is bad (shaky or low voltage) look at alternator or vr.  Potentially the vr input as @EricJ mentioned.

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When I pulled my alternator the first time the B plus lug fell off the wire.  Also you can get enough corrosion between a lug and the wire that electrons don't flow.

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