tomatl

Alternator failure M20K 231

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A couple of days ago I experiences a low voltage situation with my 231. The voltage warning light was blinking and the ammeter shoed a significant discharge (right on the line between 0 and -70A). I turned back home,

turned off as much as possible,

and reset the alternator. After a while the alternator came back online but quit again after 10' or so. Today, I ran up the engine and the electrical system was operating normally. The mechanics suggested to replace alternator and regulator. (The alternator is just a year old.) Does anyone have a suggestion what the issue might be and how to troubleshoot?

 

Thanks,

T

 

 

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Yes. The alternator in the 231 is a direct drive, in other words it is connected to the engine via a gear rather than a belt. To prevent a frozen alternator from damaging the engine there is a coupler, which is a rubber clutch inside the gear. The coupler is subject to failure. I am going to guess that when the new alternator was installed, they used the existing (old) coupler, or that the coupler that was used was defective.  I have run into many aftermarket couplers that fail in a short period. When the coupler starts to fail the system usually shows intermittent symptoms and works ok for awhile, but in a relatively short period will fail completely.  They are not cheap to replace. I would suggest that you have your mechanic check the coupler, and if it is the issue, then have them purchase one directly from Mooney if possible.  The Mooney part will be very expensive. But every aftermarket couple that was tried on my engine, and there were four or five of them, failed quickly. The Mooney coupler has given me several years of reliable service.

Also, when the coupler is installed by the mechanic, you really need to find an MSC. Whether an MSC or not, make clear to them that there is an internal bushing that must be installed. This bushing is sometimes left out by mechanics who do not know Mooneys and who do not use the service manual.  When the bushing is left out it allows the coupler to vibrate on the alternator shaft. The system is held together by a nut on the end of the shaft that has a tiny cotter pin. The cotter pin gets beaten to death, comes out, the nut comes off, and everything falls off into the operating engine.  If you are lucky the parts are found in the sump. If you are not lucky they make it into the oil system and you get damage to the oil pump or even to the engine. 

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Thank you! The installation of the alternator was done last year at a MSC but I did not have good experiences with them in the past. A faulty installation would not surprise me.


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Thank you! The installation of the alternator was done last year at a MSC but I did not have good experiences with them in the past. A faulty installation would not surprise me.


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45 minutes ago, tomatl said:

Thank you! The installation of the alternator was done last year at a MSC but I did not have good experiences with them in the past. A faulty installation would not surprise me.


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Which MSC did you use? If the drive coupling isn't the issue, there can be an issue with the alternator or voltage regulator. Having someone who understands how the charging system works will help prevent throwing money at a problem. I had a voltage issue last year (with the same symptoms as your's) and a mechanic at my destination convinced me that my alternator was the issue. He said because it failed near the end of my 2 hour flight, heat was causing my alternator to fail then get better on the ground. It turned out to be the voltage regulator that was heating up and causing the issue.

What kind of voltages are you seeing? My replacement Zeftronics has the voltage steady at 14.1 on the JPI.

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If it is the drive coupling get ready for one expensive rubber item.  The last one I replaced cost me $700 overhauled just for the part.

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6 minutes ago, Geoff said:

If it is the drive coupling get ready for one expensive rubber item.  The last one I replaced cost me $700 overhauled just for the part.

Or 40x that amount if the nut falls off and goes in the wrong spot. I’d get it checked out immediately by an insured shop. 

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My experience was the same when I had M20K 231. My service center recommended replacing both the coupler and the alternator at the same time. It only makes sense. That coupler will have the same hours on it as the alternator. It is just basically a rubber clutch. When the alternator freezes up it slips and prevents damage to everything else. Temperature changes effects the "gripability" of the rubber I found. Therefore sometimes it worked normal and other times it must have been slipping.  We used the following and worked well for 3 years so far. https://www.aircraftspruce.com/catalog/eppages/planePowerSAL_07-01321.php

Costs as much as alternator. Would have saved you labor costs to just do them together.

 

When we removed the old one you could almost turn the center and outer ring with your hand and it slipped. jlunseth has good recommendations as well.

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Which MSC did you use? If the drive coupling isn't the issue, there can be an issue with the alternator or voltage regulator. Having someone who understands how the charging system works will help prevent throwing money at a problem. I had a voltage issue last year (with the same symptoms as your's) and a mechanic at my destination convinced me that my alternator was the issue. He said because it failed near the end of my 2 hour flight, heat was causing my alternator to fail then get better on the ground. It turned out to be the voltage regulator that was heating up and causing the issue.
What kind of voltages are you seeing? My replacement Zeftronics has the voltage steady at 14.1 on the JPI.

I get perhaps 13.5 max when everything is working.


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As others have said, the alternator is driven by an internal gear coupling.  This coupling has an internal rubber slip clutch, which is known to slip after some time.

With the cowls off, you can stick your finger into the side of the alternator and hold the internal cooling fan, very carefully turn the propeller, if you can hold the fan without cutting off the tip of your finger the clutch is slipping.  
 

If it doesn’t slip you have further troubleshooting to do.

Clarence

43BEEDA4-739B-4636-B9FF-2260C76A3299.jpeg

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As others have said, the alternator is driven by an internal gear coupling.  This coupling has an internal rubber slip clutch, which is known to slip after some time.
With the cowls off, you can stick your finger into the side of the alternator and hold the internal cooling fan, very carefully turn the propeller, if you can hold the fan without cutting off the tip of your finger the clutch is slipping.  
 
If it doesn’t slip you have further troubleshooting to do.
Clarence
43BEEDA4-739B-4636-B9FF-2260C76A3299.thumb.jpeg.789d93ed221b224b5ebe321d4fac5af7.jpeg

Where in this schematics is this bushing that sometimes is not correctly installed?


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Thanks everyone for all the good advice. If possible, I'll try to ferry the plane over to the MSC who installed the alternator and will report back as soon as I know what's going on.


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38 minutes ago, tomatl said:


Where in this schematics is this bushing that sometimes is not correctly installed?


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The picture doesn’t reflect the different variations used over the years.  From the Continental manual here is more data.

Clarence

77C9580E-F0FB-439C-A461-8AFAD9F99D2A.jpeg

CF6C9F8C-D46D-4449-8169-C3CEA5D4F894.jpeg

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16 hours ago, Aspen2013 said:

My experience was the same when I had M20K 231. My service center recommended replacing both the coupler and the alternator at the same time. It only makes sense. That coupler will have the same hours on it as the alternator. It is just basically a rubber clutch. When the alternator freezes up it slips and prevents damage to everything else. Temperature changes effects the "gripability" of the rubber I found. Therefore sometimes it worked normal and other times it must have been slipping.  We used the following and worked well for 3 years so far. https://www.aircraftspruce.com/catalog/eppages/planePowerSAL_07-01321.php

Costs as much as alternator. Would have saved you labor costs to just do them together.

 

When we removed the old one you could almost turn the center and outer ring with your hand and it slipped. jlunseth has good recommendations as well.

In addition to temperature, it has been theorized that oil brand (specifically additive packages) have an effect on the clutch life as well.  

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16 hours ago, tomatl said:


I get perhaps 13.5 max when everything is working.


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I was seeing voltage in the 13.5 to 13.6 VDC as well. When my voltage regulator began to fail, the voltage would start to oscillate between 13.1 and 13.8 until it eventually went offline. Please let us know the final outcome.

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I also had difficulty locating the voltage regulator. So if you can't find it right up under the dash on the copilot side then look at the copilot side at the junction of the glare shield and the windscreen. Yes the glare shield has to come off. It was a son of a gun to get to and adjust. The easiest way is with a small screw driver in your left hand and then stand outside and reach through the cabin door and someone can run up the engine to adjust the voltage. Yes you will be standing in prop wash. You can kind of lean over a look for screw or look down through the wind screen. Fun. Mine was low all along and after new alternator we turned it up to 13.8-14.0.

This was not a zeftronics unit. Probably original.

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12 hours ago, tomatl said:

Thanks everyone for all the good advice. If possible, I'll try to ferry the plane over to the MSC who installed the alternator and will report back as soon as I know what's going on.


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As you can tell, there is considerable experience concerning the coupler and alternator on the 231. Two footnotes. First, the bushing is apparently not called out in the service manual as a separate, numbered part. It is there, but is considered part of the coupler. When assembled improperly, as by the newest mechanic in the shop, the IA cannot see or detect whether the bushing is present or not. So have a conversation directly with the guy who is going to do the work if you can, to make sure he understands that bushing must be correctly installed.  There have been too many cases where it was left out. It looks like Clarence posted the relevant manual section, very good guy to have around.

Second, if you are going to fly the plane somewhere to get it repaired, just remember your PPL training - you can fly the plane with the Master off. If you get to cruise and get power set up you can go a long way with no Master on. Its a way to save what is in the battery to extend the gear and use the radios when it is time to land. Not as scary as it sounds, but obviously VFR only. Its worth trying a reset of the Alt Fld breaker but the usual rule is try once and stop.

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As you can tell, there is considerable experience concerning the coupler and alternator on the 231. Two footnotes. First, the bushing is apparently not called out in the service manual as a separate, numbered part. It is there, but is considered part of the coupler. When assembled improperly, as by the newest mechanic in the shop, the IA cannot see or detect whether the bushing is present or not. So have a conversation directly with the guy who is going to do the work if you can, to make sure he understands that bushing must be correctly installed.  There have been too many cases where it was left out. It looks like Clarence posted the relevant manual section, very good guy to have around.
Second, if you are going to fly the plane somewhere to get it repaired, just remember your PPL training - you can fly the plane with the Master off. If you get to cruise and get power set up you can go a long way with no Master on. Its a way to save what is in the battery to extend the gear and use the radios when it is time to land. Not as scary as it sounds, but obviously VFR only. Its worth trying a reset of the Alt Fld breaker but the usual rule is try once and stop.

Thanks! I tried to do that today (flying with Master Off to the MSC). On takeoff, the plane did not seem to accelerate properly so I abandoned the plan and put it back into hangar. I hope it's not the "cotter pin and nut into the engine" scenario elaborated here. Definitely in need of a good Mooney mechanic in Western NY.


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If you are getting any charging at all on the ammeter, it is not likely the "cotter pin and nut into the engine" is the problem. That is usually a complete failure with all kinds of stuff falling in, including the coupler. The coupler would come loose and there would be no charging at all.

Get thee to a mechanic, and best of luck. 

 

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One thing you may consider: if you have the invoice from the alternator change out, check and see if you were charged for the coupler. I doubt it would be a freebie. If it is not on the invoice, might be a place to start looking.

Not any type of mechanic, but a pilot who wants to know what I spend money on that could have gone for 100LL.;)

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13 hours ago, tomatl said:


Thanks! I tried to do that today (flying with Master Off to the MSC). On takeoff, the plane did not seem to accelerate properly so I abandoned the plan and put it back into hangar. I hope it's not the "cotter pin and nut into the engine" scenario elaborated here. Definitely in need of a good Mooney mechanic in Western NY.


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Did you consider T/O without instruments? (Not recommended)

You should be able to check MP, and RPM, with FF as a back-up additional piece of information...

Power would need to be on in most newer Mooneys for this to happen...

With hot weather, performance will be down as the DA has changed....

 

Then there are those days... When things don’t feel just right... takes more practice to get over these kinds of days...

PP thoughts only, not flying mechanic...

Best regards,

-a-

 

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carusoam is right, takeoff with the Master off is definitely not recommended, I should be more clear. In the 231, setting the power on takeoff and then for cruise is too tricky, with the possibility of overboosting or overheating the engine. If you are aloft and in stable, cruise flight, so the engine is set up safely, then switching the Master off to save power for landing operations is a good choice. 

Here is a coupler (no bushing). If the coupler has gotten damaged the replacement is more expensive, there is usually a credit for return of the old part if it can be refurbished:

315674E1-1401-493D-925B-A207337E0955.jpeg

487AA165-A75A-4E77-96B3-59B33AEDC263.jpeg

Edited by jlunseth
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8 hours ago, carusoam said:

Did you consider T/O without instruments? (Not recommended)

You should be able to check MP, and RPM, with FF as a back-up additional piece of information...

Power would need to be on in most newer Mooneys for this to happen...

With hot weather, performance will be down as the DA has changed....

 

Then there are those days... When things don’t feel just right... takes more practice to get over these kinds of days...

PP thoughts only, not flying mechanic...

Best regards,

-a-

 

Thanks for the advice! I brought her home with Master OFF but on mine tach or TIT gauges, for example, don't work then. More importantly, I have next to the Aspens no backup airspeed indicator. I would not mind in cruise, but for T/O and Landing, I want a bit of a cushion before losing primary instruments. (The Aspen Emergency Backup Battery will last for a while though.) I am generally more on the conservative side and don't take off on shorter runways when something feels odd. 

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