Alternator / VR issue in M20K 252

[shawnd]

Hey folks, sorry been lacking in posts here. Long story to follow once the gremlins have been cleaned out of 2XM. But need some help with this issue that I am currently digging into. Doing test flight and after some maneuvers, my low voltage indicator came on (HI/LO VOLTS blinking). Cycled ALT FLD switch and checked CBs. Luckily I was above my home airport so landed safe and sound. Same results when I repeated the tests on the ground. Did some research and then some tests and here are the results:

- Alternator FLD SW Connectors (SW in ON position) = 0.9 ohms (should be 0.0 to 0.1 ohms?)
- Voltage Regular (harness plugged in) Pin 7 (FLD) to Pin 9 (GND) = 16.2 ohms
- Alternator FLD terminal and GND (Alternator GND wire & Engine GND) = 16.5 ohms

- Voltage Regulator w/  ALT FLD SW on, pin 5 POWER  IN = 25.7V
- Voltage Regulator w/  ALT FLD SW on, pin 7 FLD = 25.2V
- Voltage Regulator pin 8 SENSE is getting BUS voltage

FLD wire disconnected at Alternator:
- Alternator B+ terminal w/ battery ON = 25.5V
- Alternator FLD terminal w/ battery ON or ALT FLD SW ON = 0V
- Alternator FLD wire w/ battery ON =  0V
- Alternator FLD wire w/ battery ON, ALT FLD SW ON =  25.3V

Field terminal w/ battery/alt fld/engine OFF  = fluctuating between 14-16 ohms with hand propping

Jump 28V from B+ terminal to FLD  at Alternator, battery ON = NO magnetic pull

Engine running:
- Jump 28V from B+ terminal to FLD  at Alternator, engine ON with RPMs from 500-2000 = BUS VOLTAGE stays at 25.5V
- Feed bus voltage to FLD at VR shows no ammeter change with engine running at 2000  RPM.

Alternator:
- White plastic fan can be spun freely by hand (fingers)
- Hand propping the engine doesn't show any movement on the fan

Wanted some second opinions on this. Seems like my coupling and possibly diodes in the alternator have gone bad. The coupling was replaced with an overhauled unit last year during the after-purchase-fix-up mx. The fan shouldn't be moving freely I am guessing and should be moving when the prop is turned manually. And given the lack of magnetism around the shell - seems the alternator needs  TLC as well?

Thank you!

 

[carusoam]

Lots of detail Shaun!

Which alternator and VR do you have?

If Diodes have gone MIA... you might hear rpm related whining in your headset...

There may be a procedure for setting the VR output... expect that you should be seeing something a bit higher than 25v on a healthy system...

PP thoughts on getting the conversation started... not a mechanic or electrician...

Best regards,

-a-

[shawnd]

Alternator p/n 653344, 28V 75 Amp.

VR: 800270-501 (non-adjustable Mooney). Contacted Zeftronics, the right model # is A25EEAM but has to be bought from Mooney. Saw other posts - planning on sending it to Don for IRAN.

Yeah the battery volts of course kept dropping as we continued testing with load  on/off etc. Fully charged battery is higher at the beginning.

[EricJ]

From what you posted it looks like the VR is working, but the alternator is not.

 

[shawnd]

Yep - I agree, but wouldn't mind sending the VR in for IRAN since its from 1991. Is the alternator fan supposed to be movable by hand? It should be bound to the engine gears so only prop moving it should move the fan correct?

[MIm20c]

If this is a gear driven alternator I’d be very cautious going forward. Have a mechanic pull the alternator and see what’s going on internal to the engine. Make sure the unit is tested properly/fixed and an insured shop reinstalls the unit.  NO way I’ll ever let a random IA/A&P touch my alternator again. 

[LANCECASPER]

44 minutes ago, shawnd said:

Hey folks, sorry been lacking in posts here. Long story to follow once the gremlins have been cleaned out of 2XM. But need some help with this issue that I am currently digging into. Doing test flight and after some maneuvers, my low voltage indicator came on (HI/LO VOLTS blinking). Cycled ALT FLD switch and checked CBs. Luckily I was above my home airport so landed safe and sound. Same results when I repeated the tests on the ground. Did some research and then some tests and here are the results:

- Alternator FLD SW Connectors (SW in ON position) = 0.9 ohms (should be 0.0 to 0.1 ohms?)
- Voltage Regular (harness plugged in) Pin 7 (FLD) to Pin 9 (GND) = 16.2 ohms
- Alternator FLD terminal and GND (Alternator GND wire & Engine GND) = 16.5 ohms

- Voltage Regulator w/  ALT FLD SW on, pin 5 POWER  IN = 25.7V
- Voltage Regulator w/  ALT FLD SW on, pin 7 FLD = 25.2V
- Voltage Regulator pin 8 SENSE is getting BUS voltage

FLD wire disconnected at Alternator:
- Alternator B+ terminal w/ battery ON = 25.5V
- Alternator FLD terminal w/ battery ON or ALT FLD SW ON = 0V
- Alternator FLD wire w/ battery ON =  0V
- Alternator FLD wire w/ battery ON, ALT FLD SW ON =  25.3V

Field terminal w/ battery/alt fld/engine OFF  = fluctuating between 14-16 ohms with hand propping

Jump 28V from B+ terminal to FLD  at Alternator, battery ON = NO magnetic pull

Engine running:
- Jump 28V from B+ terminal to FLD  at Alternator, engine ON with RPMs from 500-2000 = BUS VOLTAGE stays at 25.5V
- Feed bus voltage to FLD at VR shows no ammeter change with engine running at 2000  RPM.

Alternator:
- White plastic fan can be spun freely by hand (fingers)
- Hand propping the engine doesn't show any movement on the fan

Wanted some second opinions on this. Seems like my coupling and possibly diodes in the alternator have gone bad. The coupling was replaced with an overhauled unit last year during the after-purchase-fix-up mx. The fan shouldn't be moving freely I am guessing and should be moving when the prop is turned manually. And given the lack of magnetism around the shell - seems the alternator needs  TLC as well?

Thank you!

 

Sadly, the fact that this was a one-alternator 252 was the reason it took so long to sell. Joe Frisolone used to write a column in the monthly MAPA log and he had comments about the 252 with one alternator so this is not something unique to N252XM .Mooney didn't make many of these, but many Mooney mechanics have pulled their hair out trying to chase gremlins on these one alternator 252s. They have a reputation for eating batteries every year, and these are the ones where the alternators and VRs are working well. . . .   If many people have worked on it, it's hard to say what all they've done that has compounded the problem. I would find the most knowledgable person in the country on it, maybe Don Maxwell, maybe someone else, and leave it with them, and patiently let them trace everything until they get it right. Hopefully the logs clearly show what's been done in the past. But paying someone else locally to learn about these isn't the answer.

[A64Pilot]

The alternator should be locked to the engine, it should not spin freely.

‘I woud start there.

[shawnd]

6 minutes ago, A64Pilot said:

The alternator should be locked to the engine, it should not spin freely.

‘I woud start there.

Thanks for confirming. I am guessing the drive coupler is shot.

[shawnd]

2 hours ago, MIm20c said:

If this is a gear driven alternator I’d be very cautious going forward. Have a mechanic pull the alternator and see what’s going on internal to the engine. Make sure the unit is tested properly/fixed and an insured shop reinstalls the unit.  NO way I’ll ever let a random IA/A&P touch my alternator again. 

Luckily - its not a random A&P and it's on my home field as well. If I could, I would take it down to Advanced in Troutdale, OR and have Greg do the work. But class B and C for KSEA and KPDX and an hour long flight relying solely on engine and handheld radio with no avionics doesn't sound very exciting  Even with ferry permits.

I am now curious to hear your story though!

1 hour ago, LANCECASPER said:

Sadly, the fact that this was a one-alternator 252 was the reason it took so long to sell. Joe Frisolone used to write a column in the monthly MAPA log and he had comments about the 252 with one alternator so this is not something unique to N252XM .Mooney didn't make many of these, but many Mooney mechanics have pulled their hair out trying to chase gremlins on these one alternator 252s. They have a reputation for eating batteries every year, and these are the ones where the alternators and VRs are working well. . . .   If many people have worked on it, it's hard to say what all they've done that has compounded the problem. I would find the most knowledgable person in the country on it, maybe Don Maxwell, maybe someone else, and leave it with them, and patiently let them trace everything until they get it right. Hopefully the logs clearly show what's been done in the past. But paying someone else locally to learn about these isn't the answer.

Well I think there were a few reasons why it took long to sell. Were you looking at this plane as well? Overall, it's not been bad at all. Sure there are few issues but electrical wise, it's been solid. And based on the data collected, it seems to be pointing to alternator and drive coupler.  Looking for some other opinions here in case someone spots something else.

[LANCECASPER]

11 minutes ago, shawnd said:

Well I think there were a few reasons why it took long to sell. Were you looking at this plane as well? Overall, it's not been bad at all. Sure there are few issues but electrical wise, it's been solid. And based on the data collected, it seems to be pointing to alternator and drive coupler.  Looking for some other opinions here in case someone spots something else.

Great, sounds like you're well on your way to figuring it out.

[MIm20c]

10 minutes ago, shawnd said:

I am now curious to hear your story though!

Sometimes when the coupler lets go it can cause a lot of damage…almost 30k worth.  Sometimes it causes fatal crashes like the a36 a few years back.  Next time ours gets worked on it will have a new alternator, NEW not overhauled coupler, and installed by an insured shop.  

[Guest]

There are a few different alternator drive coupling designs.  Some early 360’s had a multi part drive coupling with rubber drives similar to magneto drive cushions, there are also elastomeric couplings which are bonded between the metal halves.   Continental 520/550 permold series engines used to have a spring between the two halves, then they also went to elastomeric drive couplings.  All of these designs are known to fail in simple and sometimes dramatic ways.

For the OP, the cooling fan should turn with the engine, if you can hold it while turning the prop the drive is slipping.  Removal of the alternator is the only way to know for sure.

Clarence

[carusoam]

@shawnd

Check the latest messages....  looks like the alternator shouldn’t be able to turn while in the hangar...

I just read the news, and are sending a heads up in case your email is attached...

Best regards,

-a-

[shawnd]

Thanks @carusoam been following the thread through the day. Yeah, coupler is definitely "not coupled" @M20Doc is correct - need to get the pieces taken out for examination.

[shawnd]

We finally got the alternator out last night. What a major pain in the ****. Going to be contacting the folks that installed the coupling - my guess is that it wasn't torqued correctly? Placed an order with AeroTech in Louisville for OH alternator and new (surplus) coupling.

 

 

[carusoam]

Invite @M20Doc to have a look...(alternator drive on the 252...)

-a-

[Guest]

It looks like you’ve found the cause.  Those alternators sure are fun to remove!

Clarence

[shawnd]

So wanted to update the thread. This was a fun experience. After taking the alternator out and removing the castle nut from the first alternator, we discovered the part was missing a bushing that keeps it firmly in place around the alternator shaft. Previous shop was very cooperative and worked with me to get the problem corrected. Here's a photo of the first one:

 

We picked up an overhauled alternator from a MRO to replace the alternator & coupling above, and after the first 1.1 hours, it failed too! We took it out again and this time turns out the rear bearing overheated and ejected itself from the alternator. Yes - you read that correctly - ejected. The coupler failed as designed but ended up shooting quite a bit of the rubber bushing into the engine. We flushed/borescoped the engine multiple times till we were satisfied. The MRO did overhaul it under warranty and shipped it back overnight for free. OH'ed alternator is back in the plane and now it has flown for about 5 hours and things look good.

 

 

 

It was a fun and educational experience but definitely would have preferred to go through it only once. Most importantly, learned that it's very important to go with reputable companies for anything to do with airplanes.

[carusoam]

Nice work Shawn!

Somebody hands out tasks in order, based on how well you can handle said tasks…

You must be really strong with airplane tasks….  

Best regards,

-a-