Alternator issues

[Grant_Waite]

Went to fly last Thursday and couldn’t get the plane to stop showing a noticeable discharge. Didn’t matter if the avionics were on or off or at higher power settings. I did one lap in the pattern and watched my Jpi show a dip from 11.8V to 11.5V after I put gear up. I decided at that point something wasn’t right and came right back around to land. Fast forward today I installed my new Concorde xc battery, thinking it was the 3yr old Gill battery and to no avail the same issue. Fast forward to me taking the cowling off and looking at the alternator and touching the wires to check if they were loose and it works again after a few starts. I held a razor blade to the face of the alternator and it has magnetism and there’s voltage going to field wire with the master on and engine off. None of the wires looked bad, or were noticeably loose. I moved and touched everyone… they were all tight. I couldn’t find in the logbooks when that alternator was last overhauled or repaired. I plan to fly Saturday and do a few touch and goes to see if the shock from landing moves something loose or whatever could have been causing the issue. My mechanic will look at it Monday but I was honestly ready to send the alternator off for OH. Aero Accessories said it’s 630bucks which isn’t bad but I don’t want to spend it if I don’t have to. Should I still send it off to be OH if my mechanic can’t find anything? I obviously did something to make it work so it has to be at the wires going to the alternator or the plastic pieces that holds the field wires inside the alternator. 

[hammdo]

In both my cases, the connector was loose on the wire or broke off. I’d check those first…

Very common on our Mooneys…

 

-Don

[Grant_Waite]

Just now, hammdo said:

In both my cases, the connector was loose on the wire or broke off. I’d check those first…

-Don

That’s the weird thing nothing was broke off and noticeable loose. I could move one of the wires but it would rotate the whole ring connector with the wire. The nut on it was quite snug. 

[hammdo]

On the back of mine where the white wire is attached on top, that ‘black’ section should be removable (diodes and brushes I think). There is a kit for that to replace. 

May be what needs changing…

-Don

[Andy95W]

Those crimp connectors lead a hard life in a hot, difficult environment.  They don’t last forever.  Connector failure is far more common than alternator failure.

[PT20J]

[hammdo]

Forgot about that chart! I have that too…

-Don

[Grant_Waite]

36 minutes ago, PT20J said:

I just looked at that chart seems intimidating to me but I’m sure any competent mechanic is capable. The time and money to troubleshoot though could be close to 630 in labor right? 130hr for my mechanic. To get to all these spots and test them, or is it not that long of a job. I searched the logs and found that in 05 the I’m assuming current alternator was installed. That would mean it has roughly 1764hrs on it which mostly has been in the horrid environment that Florida is. 

[Jsno]

I had that issue on my M20F three months ago.  Found that the 70 amp circuit breaker broken off where the wire connects to it.  

[N201MKTurbo]

If it started working when you wiggled the wires, it is probably the brush holder. They are kind of weak where the studs connect to the brush wires. You can fix it if you remove the brush holder and solder the ring terminals (internal) to the studs. Or you can buy a new brush holder. There are about three different brush holders and the brushes don't always fit a different brush holder, so if you buy a new brush holder, you should probably get new brushes too.

[Fly Boomer]

7 hours ago, Grant_Waite said:

The time and money to troubleshoot though could be close to 630 in labor right?

The problem with loading up the parts cannon is that the alternator might be okay.  If the new alternator doesn't fix the problem, then you still have to do the troubleshooting.  Besides, your mechanic may have a boat payment coming up. 

[N201MKTurbo]

9 hours ago, hammdo said:

In both my cases, the connector was loose on the wire or broke off. I’d check those first…

Very common on our Mooneys…

 

-Don

You seem to be missing your alternator cooling duct. It is part of the alternator conversion to add the cooling duct fitting to the front of the baffling. If you don't use the cooling duct, your alternator will run cooler if you remove the shroud from the back. 

[Grant_Waite]

The engine was overhauled in 2019 by Zephyr so I’m not sure if the price of the overhaul included an overhaul exchange alternator or not. I called the previous owner and he and I both agree that nothing was changed out when the oh was done. He wasn’t the type to cheap out on things like an alternator. I spoke with 2 shops they said it could be possible that the brushes could be going bad and sometimes they make contact and sometimes they don’t. 
Luckily I have a personal relationship with my mechanic so I don’t have to worry about certain things like most. 

[N201MKTurbo]

2 hours ago, Grant_Waite said:

The engine was overhauled in 2019 by Zephyr so I’m not sure if the price of the overhaul included an overhaul exchange alternator or not. I called the previous owner and he and I both agree that nothing was changed out when the oh was done. He wasn’t the type to cheap out on things like an alternator. I spoke with 2 shops they said it could be possible that the brushes could be going bad and sometimes they make contact and sometimes they don’t. 
Luckily I have a personal relationship with my mechanic so I don’t have to worry about certain things like most. 

I wasn’t saying there is a problem with the brushes. The problem would be with the brush holder. The brush holder connects the field wires with the brushes and supports the brushes. You can easily remove it to inspect it. Just remove the field wires, then remove the two bolts outside of the field wires and pull it out. Inspect the connections on the back side of the field wire studs. If they are loose, you have found your intermittent connection. To replace the brush holder, push the brushes down and stick a wire through the little hole to hold the brushes down, slide it back into the alternator, put the bolts back in and pull the wire out which lets the brushes contact the slip rings. This can all be done easily without removing the alternator. 

[PT20J]

1 hour ago, N201MKTurbo said:

I wasn’t saying there is a problem with the brushes. The problem would be with the brush holder. The brush holder connects the field wires with the brushes and supports the brushes. You can easily remove it to inspect it. Just remove the field wires, then remove the two bolts outside of the field wires and pull it out. Inspect the connections on the back side of the field wire studs. If they are loose, you have found your intermittent connection. To replace the brush holder, push the brushes down and stick a wire through the little hole to hold the brushes down, slide it back into the alternator, put the bolts back in and pull the wire out which lets the brushes contact the slip rings. This can all be done easily without removing the alternator. 

Makes sense. A bad field connection (at the alternator terminal, the brushes, or -less likely - at the regulator) will disable the alternator. Easy (inexpensive) to check.

[Grant_Waite]

I’ll report back when I have time to take a look and how I go about fixing it 

[201Steve]

I’ve had 4 “alternator failures”. First thing I’ve done is reset the alt field circuit breaker. It will reset the VR, either causing it to come back online (as indicated by voltage) or not. That gives you an idea. 
 

3 of the 4 events was broken connectors on the back of the alternator. One the field wire and twice the field wire jumper. 
 

the fourth event, still broken wires, just way more tricky. I was convinced it was the VR going bad so I bought a Zefftronics VR (which by the way, their install instructions tell you exactly where to read resistance and voltage and what value it should be at each point if you want me to track it down for u). While installing, I had a sneaking suspicion about the canon plug that plugs into the VR. I disassembled the cannon plug on my back from underneath the panel (pain in ass) and voila- smoking gun. The field wire had chafed off insulation and was grounding against the wire shielding. I went ahead and used the new VR since I had it already, but I could have sent it back and re-used my original VR.  Had I not been up there replacing the VR and taking the additional time to inspect the guts of the cannon plug, it would have been a nightmare never ending diagnostic. 
I was proud of my curiosity that day  

 

[Grant_Waite]

So my frustration continues with a never ending aog issue. Got my old alternator overhauled back this week. Brushes were shot in the old one. Went to the airport  today since the mechanic was installing it. Went to start up the plane and it didn’t work, the alt circuit breaker popped and a burning smell came. No smoke just the smell. The smell was coming from the newly overhauled alternator. I believe my mechanic made an honest mistake. My old alternator didn’t have a post for aux and this one does. It also doesn’t have any clear label of a ground. Just a positive one and aux with F1 and F2 obviously. You can see where I’m going with this maybe. In looking at the pictures of the old wiring, he mistakenly put all 3 of the grounding wires or whatever 3 wires on the aux pole. I believe my newly overhauled alternator got fried in the process you can visibly see and smell it around the aux pole. I don’t understand alternator wiring or any of that but has anyone else had this happen? I’ll attach a picture of the old alternator and how it was wired… it was grounded using a screw, I don’t believe it was engraved - at all. The old ground was just to the right of where the aux pole is. THE ALTERNATOR I GOT BACK IS MY OLD ONE, NOT AN EXCHANGE. Kinda pissed a bit but what can I do. Alternator is covered under a 1yr warranty but I’m not going to tell the shop… yeah I think my mechanic fucked it up. Let me know what y’all think or what I should do. Hopefully the shop will cover it under warranty and pay the shipping there and back. I’ve circled where the old ground was and where I think he should have grounded them but didn’t. 

[N201MKTurbo]

I would get a new mechanic.

[N201MKTurbo]

If you would have done as I suggested above, you would be out a couple hundred bucks and flying a week ago.

[PT20J]

Hard lesson. Next time, refer to the schematic in the service manual for the correct connections.

[Grant_Waite]

Yeah I should have just bought some brushes used in the car world and called it good. I’ve never heard of a car having issues but planes seem to have issues all the time. 

[Grant_Waite]

It only shows 2 wires off the alternator but mine has like 5 different ones albeit one is a jumper. One says plus and looks like a 2 maybe for F2 idk.With everything else I’ve always been able to find an install manual but I couldn’t find anything for the alternator. Every mechanic makes mistakes eventually. At least this one was on the smaller scale of them. Good mechanics are hard to find unless you are your own. 

[1980Mooney]

On 4/4/2025 at 3:55 PM, Grant_Waite said:

Luckily I have a personal relationship with my mechanic so I don’t have to worry about certain things like most. 

 

13 hours ago, Grant_Waite said:

So my frustration continues with a never ending aog issue. Got my old alternator overhauled back this week. Brushes were shot in the old one. Went to the airport  today since the mechanic was installing it. Went to start up the plane and it didn’t work, the alt circuit breaker popped and a burning smell came. No smoke just the smell. The smell was coming from the newly overhauled alternator. I believe my mechanic made an honest mistake. My old alternator didn’t have a post for aux and this one does. It also doesn’t have any clear label of a ground. Just a positive one and aux with F1 and F2 obviously. You can see where I’m going with this maybe. In looking at the pictures of the old wiring, he mistakenly put all 3 of the grounding wires or whatever 3 wires on the aux pole. I believe my newly overhauled alternator got fried in the process you can visibly see and smell it around the aux pole. I don’t understand alternator wiring or any of that but has anyone else had this happen? I’ll attach a picture of the old alternator and how it was wired… it was grounded using a screw, I don’t believe it was engraved - at all. The old ground was just to the right of where the aux pole is. THE ALTERNATOR I GOT BACK IS MY OLD ONE, NOT AN EXCHANGE. Kinda pissed a bit but what can I do. Alternator is covered under a 1yr warranty but I’m not going to tell the shop… yeah I think my mechanic fucked it up. Let me know what y’all think or what I should do. Hopefully the shop will cover it under warranty and pay the shipping there and back. I’ve circled where the old ground was and where I think he should have grounded them but didn’t. 

 

11 hours ago, Grant_Waite said:

It only shows 2 wires off the alternator but mine has like 5 different ones albeit one is a jumper. One says plus and looks like a 2 maybe for F2 idk.With everything else I’ve always been able to find an install manual but I couldn’t find anything for the alternator. Every mechanic makes mistakes eventually. At least this one was on the smaller scale of them. Good mechanics are hard to find unless you are your own. 

Putting "everything else" on the Aux terminal is not just a mistake.  It is a fundamental lack of understanding on a basic aircraft system.  @PT20J provided a basic schematic in the "troubleshoot" chart above although it does not provide much input on the AUX terminal.  Here are 2 topics that also cover this.  Ther first is for a 1976 M20F with exactly the same Hartzel alternator as you.  

In the second one @N201MKTurbo points out "That is your AUX terminal. Was anything connected to it? The AUX terminal is a separate set of + diodes that is used for either regulator voltage sense or for the idiot light or both."

The Aux terminal is internally tied to common connection of the stator (output) windings and will normally provide about 85% of the main output voltage.  Your A&P just wired 85% (about 50 Amps) directly to ground on that braided wire sheath.  You are lucky that it didn't set the plane on fire.

You are right "Good mechanics are hard to find".   

 

[PT20J]

There are no electrical installation instructions because the use of the B+, F1, F2 and case ground are standard. The Aux terminal provides a low current output and it's use varies with the installation. Per the Mooney schematic above, it is not used on this model. Some later M20Js used it for driving the Hobbs meter. Sometimes it is used for powering an idiot light or the voltage regulator. But it is NEVER connected to a ground. Older alternators had a single field terminal and the other end of the field winding was grounded internally. When there are two field terminals, one is connected to the field wire and the other is jumpered to ground (it doesn't matter which is power and which is ground.) The schematic in the troubleshooting guide above shows a generic connection diagram.