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Hi all - I've poured through a few other electrical problem threads. I'm far from an electrician and so a lot of the language about checking voltage at different parts along the system are totally confusing me - but I did take the first step by buying a voltmeter, cleaning solution, and sandpaper, so I'm one step forward :)

Here's the issue I'm having with my 1976 M20F:

Flight 1: Start-up and run-up (incl full electrical load check) were fine. Just before takeoff, I saw the VOLTAGE annunciator lit up solid (not flashing). Ammeter was either zero or showing positive (charging). Cigarette lighter voltage meter showed ~13V, falling to 12 then 11V. I recycled master/alternator (single switch) and also pulled and reset the ALT FIELD circuit breaker. Problem cleared, but then after 5 minutes it returned. I cancelled the flight.
 
Flight #2: Start-up and run-up fine. Waited 15 minutes on the ground - all electrical (ammeter, annunicator light, voltage meter) looked fine. I flew for 1 hour - no issues. On the last 5 minutes just before landing, the problem repeated but with a twist: VOLTAGE annunciator lit up solid. Ammeter showed discharge (different from flight #1). Voltage was steady around 12V. I landed and tried reseting the master/alternator switch, the ALT FIELD, BUS, and main FIELD circuit breakers. The problem disappeared, but I shut down 5 min later and suspect the problem is still there.
 
A few things:
- The alternator field wire was replaced in September 2022 
- The VR was adjusted at annual last year to achieve steady 14V. I normally see 13.9-14.4V now.
- The annunicator light flashes when I pull the ALT FIELD cb - which makes me think when it is solid (in this problem), its showing an OVERCHARGE condition, not undercharge (maybe)
- I have noticed some electrical interference when I run the ELECTRIC BOOST PUMP. When I turned it on yesterday, I had a handheld radio on, and it picked up electrical noise. Maybe totally unrelated
- alternator and VR are both >10 years old (I'd guess >20 years old - need to re-check logs)
 
Local A&P thought it could be simple: corrosion after all the rain; or a faulty circuit breaker; or a faulty wire behind the master/alternator switch. Or it could be my alternator or VR are dying.
 
Trying to get some initial diagnostic ideas (or tips on what I should check myself), so that I can point either my A&P or local avionics shop toward possible issues/solutions
 
Thanks!
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16 minutes ago, FlyingCanuck said:

Local A&P thought it could be simple: corrosion after all the rain; or a faulty circuit breaker; or a faulty wire behind the master/alternator switch. Or it could be my alternator or VR are dying.

 

That pretty well sums it up.

I would probably start by pulling the brushes and inspect them and the brush holder. It is easy and doesn't cost anything. I have had more trouble with the brush holder than the brushes. The binding posts, which the wires are riveted to, get loose from vibration. If that doesn't show any problems, you just have to troubleshoot it. 

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Blinking light is low voltage; steady means that the overvoltage relay has tripped. Usually the latter is a voltage regulator problem because lot's or faults can cause the alternator to quit, but only too much field current causes an overvoltage. But, I would check all the wiring for security because that's easy.

Why did the VR need adjusting?

For diagnosing electrical problems, I have found avionics shops to be better than most mechanics and they often have a lower shop rate.

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2 hours ago, PT20J said:

Blinking light is low voltage; steady means that the overvoltage relay has tripped. Usually the latter is a voltage regulator problem because lot's or faults can cause the alternator to quit, but only too much field current causes an overvoltage. But, I would check all the wiring for security because that's easy.

Why did the VR need adjusting?

For diagnosing electrical problems, I have found avionics shops to be better than most mechanics and they often have a lower shop rate.

Skip

VR needed adjusting because it was set too low and was consistently putting out around 12.8-13.2V. An east coast MSC adjusted it at annual last year and (other than a faulty field wire on the alternator) the electrical 'appears' to have been fine for the last 100+ hours.

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Which VR do you have…

We have two types around here for the most part…

1) Zeftronics 

2) Sticks and stones and springs…

If unsure… look under the cover… :)

I don’t think anyone adjusts the first one…(?)

Replacing #2 with #1 is probably lower cost than adjusting #2….

PP thoughts only, not a mechanic…

Best regards,

-a-

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6 hours ago, vik said:

Alternator field circuit breaker.

Not sure what you mean by that. If the alternator field circuit breaker pops due to a fault, it will cause a low voltage indication. Different voltage regulators handle an over voltage differently. Some have a relay that removes field excitation. Others have a crowbar circuit that shorts out the field circuit which will cause the field circuit breaker to pop thus removing field excitation.

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Your symptoms sound very similar to the voltage regulator failure on my 75 F. My resolution was a two step process. The maintenance shop (I was traveling when this happened) thought it was a failing alternator. Replaced the alternator, voltage was around 13.8 for about half the flight home. Then it dropped to 12 volts.

Replaced the regulator with a Zeftronics. Been solid for the past 3+ years.

If there isn’t obvious issues with the wiring, odds are the VR or alternator is the issue.


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1 hour ago, PT20J said:

Not sure what you mean by that. If the alternator field circuit breaker pops due to a fault, it will cause a low voltage indication. Different voltage regulators handle an over voltage differently. Some have a relay that removes field excitation. Others have a crowbar circuit that shorts out the field circuit which will cause the field circuit breaker to pop thus removing field excitation.

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I can confirm, no CBs were popped. Only when I manually pulled the ALT FIELD circuit, and then reset it, did the system temporarily return to normal. 

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This problem sounds familiar.  A long time ago, in the '80s, I was experiencing something like this in a '77 J.  At random times the Over Voltage annunciator would solidly illuminate when the charging system tripped off line.  Pulling and resetting the Alt Field C/B would usually take care of it until the next time, which could be anywhere from minutes, weeks or months.  Couldn't relate it to any single action like heavy current usage (landing light, gear motion).  Usually just out of the blue in cruise but thought maybe rain or moisture would exacerbate it.  Chased the problem for the longest time.  Changed alternators, VR, checked every wire, cleaned and reseated every connector along the path of the Field circuit.  Nothing fixed it.

Finally called Mooney and spoke with none other than the legendary Bill Wheat.  I remember him saying that the factory would occasionally experience the same thing.  He told me to go to Radio Shack and buy an inexpensive general purpose diode like a 1N4002 and bridge it across the alternator field terminal to ground (anode to plus, cathode to ground).  Something about transient voltage spikes.  The problem never reoccured and the plane was sold that way ten years later.  It's unclear to me if they actually installed diodes like that in the factory because I haven't seen it documented.

Poking around Mooney Space I found something very similar from a June 1, 2017 post titled Electrical Issue Help Please.  Not sure the best way to link to it so I just did a cut and paste:

_____________________________

I had the exact same problem in 1992 when I bought my first Mooney, an M20C with electric gear and an alternator conversion.  Every so often, lowering the landing gear would knock the electrical system offline by tripping the overvoltage relay.  Never on gear retraction or any other scenario, only gear extension.  It was easily reset, but annoying.

I was stationed in Texas at the time so on a day off I flew down to Kerrville and parked at the factory.  A nice older guy came and escorted me around.  He said he liked my airplane, the M20C was always one of his favorites.  He asked how I liked it and I said great, except I have this weird electrical problem and described it to him.

He asked if I had an alternator conversion, and when I said yes, he said that they had a similar problem in the mid-70s when they were creating the M20J 201.  (I was talking to Bill Wheat, which I didn't realize was a very big deal until much later.)

He said the problem was that when the landing gear drops, sometimes gravity (or turbulence) will push the gear down which actually drives the landing gear motor, which then acts as a generator and can throw some crazy voltage into the system.  This isn't a problem until you have an alternator which includes an overvoltage relay.

He reached into his desk drawer and pulled out a couple of small diodes and explained where to install them into the electrical relay junctions of the landing gear system.

I updated my maintenance manual with a note saying it was modified IAW factory representative Bill Wheat and the problem went away.

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Interesting idea about the diode. My schematic shows a diode on the alternator, but it doesn’t make sense to me since it is on the grounded field terminal. It might be a mistake and should be on the other terminal. I’ll have to look next time I have the cowling off. 
 

Screenshot 2023-01-24 at 9.51.43 PM.png

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Wow… I don’t want to get too excited at the coincidences here, but… here goes: I’ve only had 4 electrical ‘moments’ with this plane:

1. ALT FIELD CB popped on a long flight from TX to DC. Reset. No issue for years after. 

2. Ever since buying plane, the volts were low: 12.8-13.2. Finally at annual had the VR adjusted up. Produced 14-14.4 thereafter. 

3. VOLTAGE light after lower landing gear on approach to KHHR. Local A&Ps couldn’t find any issue. Home mechanics said it was the field wire faulty. (40 hours ago)

4. The present issue in thread above. Interesting that on my first flight, the plane was damp after two weeks of heavy rain on the ramp (albeit covered but still…). On the second flight, the issue was gone: until I lowered landing gear on a bumpy approach back home at KSBP. 
 

the parallels are striking: wet/moisture and/or landing gear lowering, seems to be happening around the same time as this issue appears. I’d be curious to check out the diode resolution. 

does anyone have a photo of their wiring with a diode installed?

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I looked at a couple of schematics for later models and the diode goes from the terminal with the field wire to ground, so I think my schematic is wrong. Diodes are cheap and it couldn’t hurt to give it a try.

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Thinking some more about the diode I may have specified a reverse polarity, specifically after looking at the schematic provided by @PT20J.   Looking at some later year electrical diagrams I see a diode installation between the Field terminal and ground.  However, the Cathode is connected to the Field terminal and Anode to ground.  Make sense otherwise the Field would be continuously tied to ground.  Can't find any specs on the diode Mooney used.  If someone has a late model parts manual perhaps there is a reference to D1.

image.png.0e0aaf59495c1e61e37709c4f3c677c8.png

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There are reasonably good troubleshooting guides on both Zeftronics and Plane Power websites.  Throwing parts at the problem seems how most of these issues are solved but troubleshooting them fully is usually possible (intermittent problems are tough).  The thing is that our airplanes are old.  There are almost invariably noisy, corroded, or loose connections (or CBs) somewhere.  If a new VR or Alt doesn’t fix the problem, you’ll still be back to a voltmeter and troubleshooting guide or finding a shop competent in electrical issues (sometimes difficult).

At the very least, I’d suggest checking the connections at the alternator carefully.  You might also check the voltage on the field wire and alternator output (if you have low voltage), this can help narrow it down.  

Or just do what I did and replace the VR and Alternator.  When that didn’t fix the problem, I learned how to use a voltmeter and went through the system more carefully to isolate the problem.  It cost a lot more, but I got some education.

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Too bad we can’t call Bill!

:)

I spoke to him twice…

Then came across his signature on the AW line in my 1965 M20C’s original airframe logbook…

He knew every detail of every model for 35+ years… what drawing to look things up in… prior to cad drawings… some things weren’t always where you expected them to be…

I said 35 years because my plane was built in 1965, my first phone call to him was  in Y2k… :)  looking for an owners manual…

A few years later I was looking for the diameter of the MP line’s weep hole….

Best regards,

-a-

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Update on this - I did a few small checks based on my very novice (but learning ;) ) electrical skills.

Checked the voltage on the field wire with battery/alt switch on. Was showing about 11V.

Checked the resistance on the field wire (removed one of two wires; batt/alt off; resistance checked at terminals). Showed between 0-1 ohms. (Mostly 0 but occasionally bounced up a bit toward 1 ohm, then steady at zero)

Its been suggested that this indicates issues with the brushes on the alternator that need to be replaced.

What do others think? I have photos of the alternator area if that matters.

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51 minutes ago, FlyingCanuck said:

Update on this - I did a few small checks based on my very novice (but learning ;) ) electrical skills.

Checked the voltage on the field wire with battery/alt switch on. Was showing about 11V.

Checked the resistance on the field wire (removed one of two wires; batt/alt off; resistance checked at terminals). Showed between 0-1 ohms. (Mostly 0 but occasionally bounced up a bit toward 1 ohm, then steady at zero)

Its been suggested that this indicates issues with the brushes on the alternator that need to be replaced.

What do others think? I have photos of the alternator area if that matters.

I recommend this:

https://www.zeftronics.com/wp-content/uploads/2014/06/Z14PS.pdf
 

I think you’ll be a page 2 type of system.

11v at the field seems reasonable, but the troubleshooting will lead you through systematically.  If you get frisky, run it and check the alternator output at the alternator terminals.  Use long wires with clips - be safe!  I found measuring actual voltages with battery or engine on was working better for me that resistance with power off.

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1 hour ago, Yetti said:

so in my 75F I replaced the brushes in the alternator.  $160 from Spruce.   Still had problems.  Replaced VR with Zefttronics.   Problem solved.

Thanks - do you remember what your symptoms were going into the process?

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On 1/29/2023 at 3:33 PM, FlyingCanuck said:

Thanks - do you remember what your symptoms were going into the process?

Flying along and noticed voltage low.  Returned to base.  Checked belt and wires.    Did not know when brushes were done last.  So did them first.  Can do them on the plane.   Did not fix it so did the VR.   Can rationalize it all under just Preventive Maintenance.   As in don't have to worry about those two things.

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10 hours ago, jetdriven said:

Under what section of FAR 43 are  you justifying It a preventative maintenance? Just curious

In my mind and the rationalization of spending dollars without doing the diagnostic work and just throwing parts at the problem and getting lucky.   As far as the actual work for the STC for the VR and the updated W&B that was signed off by the grumpy IA.   Sorry to use the PM term and get your regulatory dander up.   The good news is Zefttronics is  a Texas company and the VR was here lickety split. 

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10 hours ago, Justin Schmidt said:

That is almost exactly what happened to me. For my issue it ended up being a broken wire at the crimp at the connector of field wire and alternator. I would check all connectors.

I had already rebuilt the wiring harness a couple of years ago, And it has been a long time since I messed up a crimp.   but yes I did check that first.  If you are using the crappy thin crimpers that hurt your hands you are going to have problems.    These are the ones to get.   https://www.amazon.com/Insulated-Non-Insulated-Klein-Tools-1005/dp/B0006M6Y5M/ref=asc_df_B0006M6Y5M/?tag=hyprod-20&linkCode=df0&hvadid=309802506143&hvpos=&hvnetw=g&hvrand=9315089380813873912&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9027898&hvtargid=pla-433451501200&region_id=674469&th=1

The nice thing about them is that the cutters actually work and you can use the cutters as a stripper if you get good.

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