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


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I bought a 1982 M20J 201 back in April and I took it for my first night flight the other evening.  I didn't notice anything unusual until I was in flight for several minutes, at which point the panel lights started changing intensity at a rate of perhaps 2 Hz...bright-dim-bright, a bit random. 

I then looked at the ammeter and the needle swings pretty wide left and right of zero in time with the panel light fluctuations.

I have a JPI EDM-700 and it logs various voltages from 13.6 to 14.5. 

The problem is also intermittent.  Most times it doesn't happen until in flight (I've made a point of checking for it every time I fly now), and it will just stop happening for many seconds or even minutes at a time, then starts up again.  Sometimes it happens on taxi to runway, sometimes not.  I can't yet figure out what causes it to happen..

It's also probably a long-standing issue that I only happened to notice the other night since it was the first time I'd actually flown with the panel lights turned up and visible.  I downloaded the last 50 flights from the EDM-700 yesterday and as far back as it has been recording (just since June- I'm in IR training in this plane), it has been recording variable voltage like this.

Nothing else seems wrong and only direct DC loads like the panel lights (and presumably the landing light) are clearly affected.

I just re-tensioned the alternator belt but that didn't help.  The belt is old and I got a good look at the inner surfaces between the ridges - it has some cracking and in general looks worn, but I'm not sure it could be slipping for minutes at a time and then just stop slipping for a while, etc.  And it's definitely under good tension as of this evening but a test flight after that revealed no change in the behavior.

I also inspected the wires on the back of the alternator.  The field wire (I think it's the field wire) is a bit damaged a few inches away from where it connects to the alternator (like, the insulation was nicked or something at some point), but it doesn't seem like it is a broken connection, just some exposed conductor for about 1/8" along the wire in free space there.  I decided not to disturb it since I was tensioning the belt and wanted to address one variable at a time.

Anyone else ever see a problem like this?

IMG_20180827_182335.thumb.jpg.dc6d1a689a5cacd379ca5824e8b1cb97.jpgIMG_20180827_182251.thumb.jpg.2e49ae77d6035e3d69f926dc9df678f7.jpgIMG_20180827_180546.thumb.jpg.1ecf60316c3b4a10cf33616c1b51037c.jpgIMG_20180827_180354.thumb.jpg.fbe9b6dec8d9c778b721c48837a6dcb8.jpgIMG_20180827_180438.thumb.jpg.09f6a2eab9a63536049ca059e08f937d.jpg

Edited by FlyBoyM20J
fluctuating, not fluctuation...
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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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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 behavior.  Cleaning the terminals took care of that issue for me to the tune of 0AMU.

On my Cherokee, it was a compound issue of corrosion on the voltage adjustment screw on the regulator and also the alternator itself.  Replacing both with Plane Power gear rectified that issue!  

 

 

Edited by 81X
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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!

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teejayevans, thanks, I did test the field wire by pushing it around and tugging on either side of the abrasion and it seemed solid, but that's definitely a suspect wire.

I'll try cleaning the master switch tomorrow and then the field wire if the problem persists.

 

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While you are working on the details...

Also look to see which voltage regulator you have.  That is the device that sends the signal down the field wire....

If it is 40 years old, There may be better ones available...

Best regards,

-a-

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i had a similar voltage issue with a swinging ammeter and flickering lights. It turned out that the big power conductor from the alternator was broken inside the boot and making intermittent contact. Cut the wire back to clean conductors and crimped a new terminal on; also secured the wiring better to prevent it from happening again.

I didn't see the issue until I pulled the boot back as the boot and tension on the wire really held it in place.

 

iain

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Well, I agree with the advice so far but there is another component to it, and that is draw.  The draw issue is particularly exacerbated in my model aircraft, which is the 231, known for poor amperage from the alternator at idle.  It is a matter of the design of the connection between the engine and the alternator (called the coupler).  Your design is different, but the draw issue remains.  The old incandescent electrical devices, particularly the wing tip strobes and the landing light, draw a hellacious amount of current.  It makes pattern ops at night a little difficult, because a good part of the time the engine will be at idle, and thus the alternator output will be low.  The old-style lights will then draw on the battery, and your buss voltage will drop.  The solution is to replace all the lights you can with LEDs.  I would start with the wing strobes and landing light.  I have an older LED landing light system, one of the first, it is not very bright.  There are better and brighter one's on the market now.  But it is at least as good as the old incandescent was.  I would be sure to get a good landing light, it my case it would make night landings much easier.  It helps to convert the panel lights also, there is a type of LED ring light that mounts around the gauges and substitutes for the small incandescents you now have, but I don't know how you would mount those.  Probably best to talk to an avionics shop, which is what I did.  Replace all the incandescents and your electrical system will be very happy during night pattern ops.

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Going to try cleaning the contacts on the back of the master switch before I fly again today.  Sounds like this is the answer, but I agree with all other suggestions.  Thanks, jlunseth for bringing up current draw - I'm definitely going to replace the landing light and the strobes with LEDs in the near future.  Panel lights made the list after that first night flight, too.

I'll update with progress on this.

Thanks everyone!

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FWIW, if it's the master switch, the fluctuating ammeter was associated with a warbling sound on the intercom which rises and falls at the same frequency as the ammeter, in addition to fluctuating panel lights.  And yes, cleaning the terminals fixed the problem for me.

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Yes, jaylw314, I noticed for the first time last night after I test flew (after tensioning alternator belt) that the COMs had a warble that moved in time with the ammeter!!!  I've noticed this warble before lots of times but only last night correlated it with the ammeter.

I simply cannot WAIT to get this solved...going to be at the plane in about 1.5 hours.

Edited by FlyBoyM20J
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I think the idea behind cleaning the master switch is that dirty or loose connectors produce more resistance in the alternator field circuit, that's what causes the fluctuations.  If happens again, stick your finger behind the master switch and jiggle the connectors.  If that temporarily fixes the problem, you have your answer. 

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Well, I think that just jiggling the spade connectors on the back of the master switch may have solved the problem.  I had about 2 minutes to do that before my flight lesson today and I didn't see any ammeter fluctuations for the whole 1.2 hour flight (though I wasn't watching it all the time).

I'll get back to the plane after work and actually unplug, re-crimp (if necessary), and re-plug all four spade connectors and then work out a good way to test...perhaps a flight after dark again.

This will be sooo awesome if it solves the problem!!!  Looks good so far...thanks, all.

Edited by FlyBoyM20J
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I had the same problem when I had my M20J and to solve the problem I took a long screw driver and just pushed the spade connectors in and made sure they were in tight and that solved the problem. Unfortunately you will find that your going to have to do this every few months but it should solve your problem.

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21 hours ago, FlyBoyM20J said:

I bought a 1982 M20J 201 back in April and I took it for my first night flight the other evening.  I didn't notice anything unusual until I was in flight for several minutes, at which point the panel lights started changing intensity at a rate of perhaps 2 Hz...bright-dim-bright, a bit random. 

I then looked at the ammeter and the needle swings pretty wide left and right of zero in time with the panel light fluctuations.

I have a JPI EDM-700 and it logs various voltages from 13.6 to 14.5. 

The problem is also intermittent.  Most times it doesn't happen until in flight (I've made a point of checking for it every time I fly now), and it will just stop happening for many seconds or even minutes at a time, then starts up again.  Sometimes it happens on taxi to runway, sometimes not.  I can't yet figure out what causes it to happen..

It's also probably a long-standing issue that I only happened to notice the other night since it was the first time I'd actually flown with the panel lights turned up and visible.  I downloaded the last 50 flights from the EDM-700 yesterday and as far back as it has been recording (just since June- I'm in IR training in this plane), it has been recording variable voltage like this.

Nothing else seems wrong and only direct DC loads like the panel lights (and presumably the landing light) are clearly affected.

I just re-tensioned the alternator belt but that didn't help.  The belt is old and I got a good look at the inner surfaces between the ridges - it has some cracking and in general looks worn, but I'm not sure it could be slipping for minutes at a time and then just stop slipping for a while, etc.  And it's definitely under good tension as of this evening but a test flight after that revealed no change in the behavior.

I also inspected the wires on the back of the alternator.  The field wire (I think it's the field wire) is a bit damaged a few inches away from where it connects to the alternator (like, the insulation was nicked or something at some point), but it doesn't seem like it is a broken connection, just some exposed conductor for about 1/8" along the wire in free space there.  I decided not to disturb it since I was tensioning the belt and wanted to address one variable at a time.

Anyone else ever see a problem like this?

IMG_20180827_182335.thumb.jpg.dc6d1a689a5cacd379ca5824e8b1cb97.jpgIMG_20180827_182251.thumb.jpg.2e49ae77d6035e3d69f926dc9df678f7.jpgIMG_20180827_180546.thumb.jpg.1ecf60316c3b4a10cf33616c1b51037c.jpgIMG_20180827_180354.thumb.jpg.fbe9b6dec8d9c778b721c48837a6dcb8.jpgIMG_20180827_180438.thumb.jpg.09f6a2eab9a63536049ca059e08f937d.jpg

The small frayed wires you see on your field wire appear to be the shielding wires.  Normally the shielding wire went to case ground on the alternator.  Also the shielding wire from the main output wire is missing, it too grounds to the case.

If the aleternator has 500 or more hours in service you might also consider pulling the brushes for inspection.

Clarence

Edited by M20Doc
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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 forgive my ignorance, but aren't shielding wires supposed to be grounded on one side only to prevent ground loops?  I don't have a good picture handy but I believe that I saw the shield braid for the main output had a pigtail bolted to the engine or the frame over to the left, similar to that other ground braid at the bottom of this photo.

If it is grounded on the other side, will that be sufficient, or are we basically looking at an oversight here when this alt was last serviced?

Thanks a million, and as of this evening, the base problem seems to be resolved.  I pulled all four spade connectors on the master switch, inspected everything, re-crimped all 4 (2 were pretty easy to pull off), and securely re-connected them.  I went for a 45-minute test flight and had no issues.  This is not conclusive yet but it is very, very encouraging!

 

Edited by FlyBoyM20J
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Additionally if the belt is off check the pulley for side to side / in and out play -

Mine was working well well but when I had the engine down for cylinders I checked it and it REALLY needed to be addressed.  

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Nice video...I find it interesting that your cluster gauges were also dancing.  Perhaps that's a way to differentiate between the situation I had (hopefully resolved) when it's the master switch vs. a panel grounding issue like you had (guessing)?

My ammeter needle swings were much wider than yours but I had no cluster gauge needles moving.

So, potentially (no pun), my problem had to do with poor conduction through the alternator field current loop, leading to an oscillation caused by the VR trying to boost voltage in response to this, but never locking due to the unstable field current...result was big swings in alt output, so system-wide.  But, anything that has a local step-down converter (e.g., the GNS) still has enough headroom and local PS filtering, so likely didn't care too much.  Direct loads such as landing light, panel lights, different story.  I do wonder about the cluster gauges - they're not that complex.

Hmm.

 

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