Anomalous Ammeter Readings + Loss of Alternator

[ArrowFlyer86]

Back again to learn from the PoA brain trust

First, here's my ammeter gauge:


Normally it's at 35A upon start and fades to ~10A after <1min. Yesterday it stayed at 35A after startup; first time I've seen that. I decided to stay in the pattern and keep a close eye on it. After retracting gear on initial takeoff it settled back to 35A. Sometime in the next 60 seconds it dropped to zero. No breakers blown. I verified it wasn't a low reading: it was 0. Adding load did nothing to the instrument. Landed, taxi'd it back, stayed at 0 the whole time.

This morning I start it up - surprisingly it reads 35A and then fades to about 25-30A on my taxi over to the A&P. Better than yday but still high. I add/subtract load to verify the gauge is actually responding to load.

I explain the issue to my A&P before heading back to work. They started it then sent me a picture showing it's at normal levels.


The current working thesis by the A&P is that a stuck starter could be the cause. My starter is a freshly overhauled unit purchased 2 months ago. I believe my starter solenoid is factory original from 52y ago. A grounding issue is also flagged a potential culprit. We're checking the wiring to rule that out.

My questions to the electro-mechanically inclined:
(1) A stuck starter explains the high current draw, but I don't understand how that would cause the alternator to stop generating any power in flight without blowing a breaker?
(2) If it's a grounding issue, same question. I can see how it'd have a high current draw but I don't see how that could cause the alternator to stop generating any power in flight without blowing any breakers?
(3) Since no breakers popped during y'days flight. What could cause the alternator to start generating power again today? I know "turning it off then back on again" is a common fix for electronics.. But given the wiring diagram I'm not sure what would have "reset" btw yday and today?

 

[ArrowFlyer86]

I should also add that the A&Ps measured voltages of the battery and alternator and found them at normal levels...

 

[Bell206]

What could cause the alternator to start generating power again today?

Really not enough specifics to add anything at this point, but one reason could be your voltage regulator sensed something during the high amp readings and killed the alternator field circuit. The next day no issue so it worked. Should be an easy find and fix....

 

[ArrowFlyer86]

Really not enough specifics to add anything at this point, but one reason could be your voltage regulator sensed something during the high amp readings and killed the alternator field circuit. The next day no issue so it worked. Should be an easy find and fix....

Interesting. I wasn't aware the voltage regulator could kill the current flow (well, unless you overloaded it with current and destroyed it). But I'd have thought the breaker would pop well before that.

The steps they took today aside from ground run and measuring voltages were inspecting the wiring (ground/field wire/..) and verifying its OK. As well as cleaning the terminals on the shunt (I think the shunt for the ammeter itself?). The plan is for a ground run in the morning and if all looks OK then I'm taking for a test flight to see what happens.

If everything works great tomorrow I really don't know where that'll leave me. Do I write-off the issue as a 1x anomalous event? Or do I proactively replace the starter solenoid? I guess that'll be a decision for tomorrow. But I do a lot of XC flyin' and I really don't want to be worried about having it happen again when I'm far from home. Blurg...

 

[Bell206]

. I wasn't aware the voltage regulator could kill the current flow

Its actually an over-voltage protection relay that will disconnect the field signal and drop off alternator, ie, the current flow. Its merely a guess but have seen during instances of high current flow an errant voltage spike occur.

Do I write-off the issue as a 1x anomalous event? Or do I proactively replace the starter solenoid?

The troubleshooting/systems checks should indicate whether the start relay has an issue or if there are any other potentional problems that should be addressed. See how tomorrow goes.

 

[Dan Thomas]

Any starter contactor that's 50+ years old should be replaced, along with the master contactor if it's also that old.

What serial number is your Arrow?

 

[ArrowFlyer86]

Any starter contactor that's 50+ years old should be replaced, along with the master contactor if it's also that old.

What serial number is your Arrow?

I share the same sentiment: it certainly can't hurt to replace things that are this old...
I can't say for certain they weren't replaced by a previous owner, but from my knowledge of the logs its been decades at least. If ever. I'll have to scour the logs to find that.
Serial# is 28R-7235182 for the aircraft.

 

[ArrowFlyer86]

Funny enough, @Dan Thomas - I just stumbled upon your post here from 2011.
Your post was in relation to a Cessna 150 but I'm wondering if the same dynamic could play out in my plane. Specifically this:

"The starter current doesn't pass through the ammeter, but it still runs the battery way down and the alternator ends up chasing it and pumping out more amps than it was designed for. Alternators don't have the current limiter in the regulator that the old generators did; the alternator is sized to take all the constant-current loads in the airplane, but the starter draws far more than that."

I'm wondering if a starter being stuck and operating as an unintentional generator could do that. Given that the ammeter was pegged at 35A the entire time, which is the exact same amperage it runs at when it's running power back to the battery after starting, makes me think it could be. But I may well be wrong in my understanding of the mechanics there.

 

[Dan Thomas]

Funny enough, @Dan Thomas - I just stumbled upon your post here from 2011.
Your post was in relation to a Cessna 150 but I'm wondering if the same dynamic could play out in my plane. Specifically this:

"The starter current doesn't pass through the ammeter, but it still runs the battery way down and the alternator ends up chasing it and pumping out more amps than it was designed for. Alternators don't have the current limiter in the regulator that the old generators did; the alternator is sized to take all the constant-current loads in the airplane, but the starter draws far more than that."

I'm wondering if a starter being stuck and operating as an unintentional generator could do that. Given that the ammeter was pegged at 35A the entire time, which is the exact same amperage it runs at when it's running power back to the battery after starting, makes me think it could be. But I may well be wrong in my understanding of the mechanics there.

The alternator can easily handle the 35 amps. I am looking for the wiring diagram for your airplane. Might take awhile.

 

[Dan Thomas]

I have looked at the wiring diagram. I see nothing unusual about it. I think it was likely a stuck starter contactor, and if the ammeter shunt connections were old and oxidized, the 35-amp current might have heated it up until contact was lost, and the next day it was cool and working again. Just a guess. Bell's suggestion that a voltage spike might have occurred, tripping the overvolt sensor and shutting down the regulator, is valid also.

Oh, one more: Bad grounds at the overvolt sensor and regulator can confuse them and make stupid problems. Without the ground reference, they don't know what the voltage really is.

 

[ArrowFlyer86]

I have looked at the wiring diagram. I see nothing unusual about it. I think it was likely a stuck starter contactor, and if the ammeter shunt connections were old and oxidized, the 35-amp current might have heated it up until contact was lost, and the next day it was cool and working again. Just a guess. Bell's suggestion that a voltage spike might have occurred, tripping the overvolt sensor and shutting down the regulator, is valid also.

Oh, one more: Bad grounds at the overvolt sensor and regulator can confuse them and make stupid problems. Without the ground reference, they don't know what the voltage really is.

Thanks Dan - I certainly appreciate the second pair of eyes on it!
I'm going to raise these possibilities tomorrow when we test out the airplane just to make sure the highest likelihood culprits are reviewed. I don't recall the shop mentioning checking the voltage regulator/overvoltage protector to ensure those GND connections are OK so I'll have to ask them about that. I'll find out how old the starter solenoid really is and if there's doubt about its condition I'll probably end up replacing it. It seems cheap enough.

Should have more tomorrow

Thanks again guys.

 

[Daleandee]

Any starter contactor that's 50+ years old should be replaced, along with the master contactor if it's also that old.

What serial number is your Arrow?

Just today I had to replace the starter contactor/relay/solenoid on my motorcycle. It was being slow to start intermittently. Charged the battery and did a load test and it was OK. Next test was on the relay and watching the continuity vary with a VOM while switching proved that at times it was working correctly and other times there was high resistance.

I said all of that to emphasize Dan's point. Connections are always suspect with intermittent power issues so just as a poor wiring connection can cause such behaviour so can old, worn out relays. My contactor was 18 years old so I feel I got my dollars worth out of it ...

 

[Jim Carpenter]

Along the lines of a grounding issue, have a look at your battery box/cables. Piper's earlier "pass-through" battery cables and the flat braided ground cable can get flakey after years of use. (just a total WAG).

 

[ArrowFlyer86]

Follow-up from my shop today was that they cleaned the terminals for the ammeter shunt, added more slack in the line because it was exceptionally taught (caught on a bracket). They tracked all the lines running to/from alternator to verify they're cleaned and fastened securely.
We recently readjusted the alternator during annual (2 weeks ago) since the bracket on the bottom was loose and needed replacement, and IDK if during that process the line got overly-tightened. Now there's a bit more slack. They did not address connections at the solenoid during their work.

Their working hypothesis is that maybe I have a bad ammeter gauge that was giving faulty readings or the tight wiring was causing an intermittent connection. The A&P suggested it's quite possible I didn't lose my alternator power even though the ammeter dropped to zero. One recommendation was getting a voltage meter (my plane doesn't have a built-in one) that I plug into my cigar lighter so if it happens again I can determine if I'm actually drawing off battery or if its a faulty gauge. That's a cheap and easy piece of diagnostic equipment so I'm getting it. Ultimately they conceded that they're not positive about the diagnosis but given that they can't replicate it's the best working thesis. I went for a flight and everything looked normal.

I still think it'd be prudent to replace the old master/starter solenoids. I think this is them? Right next to the battery in back? They're pretty cheap from aircraft spruce and what I've read online is that swapping them is a quick procedure.
They look a little weathered for my liking. Shop still has the logs so I haven't verified the age, but I suspect they haven't been replaced in decades.

 

[Dan Thomas]

One of those will be the master contactor. The other is likely the external-power contactor. The starter contactor is usually near the starter, probably on the firewall. This is from the airplane's wiring diagram, and you could identify the contactors (solenoids) by the numbers on their wires:

 

[ArrowFlyer86]

One of those will be the master contactor. The other is likely the external-power contactor. The starter contactor is usually near the starter, probably on the firewall. This is from the airplane's wiring diagram, and you could identify the contactors (solenoids) by the numbers on their wires:

View attachment 129973

Thanks. I thought they were grouped together. I hadn't heard of an external power contractor!

They look old...

 

[Dave Palacios]

Back again to learn from the PoA brain trust

First, here's my ammeter gauge:
View attachment 129936

Normally it's at 35A upon start and fades to ~10A after <1min. Yesterday it stayed at 35A after startup; first time I've seen that. I decided to stay in the pattern and keep a close eye on it. After retracting gear on initial takeoff it settled back to 35A. Sometime in the next 60 seconds it dropped to zero. No breakers blown. I verified it wasn't a low reading: it was 0. Adding load did nothing to the instrument. Landed, taxi'd it back, stayed at 0 the whole time.

This morning I start it up - surprisingly it reads 35A and then fades to about 25-30A on my taxi over to the A&P. Better than yday but still high. I add/subtract load to verify the gauge is actually responding to load.

I explain the issue to my A&P before heading back to work. They started it then sent me a picture showing it's at normal levels.
View attachment 129937

The current working thesis by the A&P is that a stuck starter could be the cause. My starter is a freshly overhauled unit purchased 2 months ago. I believe my starter solenoid is factory original from 52y ago. A grounding issue is also flagged a potential culprit. We're checking the wiring to rule that out.

My questions to the electro-mechanically inclined:
(1) A stuck starter explains the high current draw, but I don't understand how that would cause the alternator to stop generating any power in flight without blowing a breaker?
(2) If it's a grounding issue, same question. I can see how it'd have a high current draw but I don't see how that could cause the alternator to stop generating any power in flight without blowing any breakers?
(3) Since no breakers popped during y'days flight. What could cause the alternator to start generating power again today? I know "turning it off then back on again" is a common fix for electronics.. But given the wiring diagram I'm not sure what would have "reset" btw yday and today?

Replace 57 yer old starter solenoid. common for them to stick after years, if it is going bad it could get stuck and stay suck in which case with heat it can weld closed then the battery solenoid could also follow and a possible fire could occur.

 

[mondtster]

Since everyone is piling on about the starter solenoid, it is worth mentioning that it is common for them to get welded contacts when you try to start an engine with a dead or failing battery. I'd expect it to be more probable in aircraft equipped with antique starters.

 

[Datadriver]

Back again to learn from the PoA brain trust

First, here's my ammeter gauge:
View attachment 129936

Normally it's at 35A upon start and fades to ~10A after <1min. Yesterday it stayed at 35A after startup; first time I've seen that. I decided to stay in the pattern and keep a close eye on it. After retracting gear on initial takeoff it settled back to 35A. Sometime in the next 60 seconds it dropped to zero. No breakers blown. I verified it wasn't a low reading: it was 0. Adding load did nothing to the instrument. Landed, taxi'd it back, stayed at 0 the whole time.

This morning I start it up - surprisingly it reads 35A and then fades to about 25-30A on my taxi over to the A&P. Better than yday but still high. I add/subtract load to verify the gauge is actually responding to load.

I explain the issue to my A&P before heading back to work. They started it then sent me a picture showing it's at normal levels.
View attachment 129937

The current working thesis by the A&P is that a stuck starter could be the cause. My starter is a freshly overhauled unit purchased 2 months ago. I believe my starter solenoid is factory original from 52y ago. A grounding issue is also flagged a potential culprit. We're checking the wiring to rule that out.

My questions to the electro-mechanically inclined:
(1) A stuck starter explains the high current draw, but I don't understand how that would cause the alternator to stop generating any power in flight without blowing a breaker?
(2) If it's a grounding issue, same question. I can see how it'd have a high current draw but I don't see how that could cause the alternator to stop generating any power in flight without blowing any breakers?
(3) Since no breakers popped during y'days flight. What could cause the alternator to start generating power again today? I know "turning it off then back on again" is a common fix for electronics.. But given the wiring diagram I'm not sure what would have "reset" btw yday and today?

1) The spike in amps once the engine is running is to be expected as your battery charges back up quickly. After a few minutes it should drop down to what you are used to seeing in flight

2) Check the field wire on the alternator. My Arrow IV had a loose one and amps would go to zero as the alternator stopped generating current. The field wire is one of the little wires, not the big wire.

3) Understand how your ammeter is wired. Mine only shows current load if the alternator is running.

 

[Dan Thomas]

Since everyone is piling on about the starter solenoid, it is worth mentioning that it is common for them to get welded contacts when you try to start an engine with a dead or failing battery. I'd expect it to be more probable in aircraft equipped with antique starters.

Yup, and the contact disc also tends to erode its way down into the square-head contacts, where it then gets hot and expands and locks itself there.

 

[ArrowFlyer86]

Unfortunately our steps to rectify did not fix.

It failed on me again today as I did a touch and go at a local airport. Jumped to 35A then fell to 0.

I cycled the alternator switch next to master. Doing this brought the alternator back each time. After flipping it back ON the ammeter would register a current at about 35A and then fall to 0 about 2-5 seconds later. I repeated this 3x in the air.

Once I landed at home base and was taxiing back, I cycled the alternator switch once more. Off then on again. Then it seemed to work without failing. No breakers tripped at any point.

I don't have many data points but it seems so far that the failures may be correlated with higher power settings/vibration.

Time to do some research and see what could behave like this.

 

[ArrowFlyer86]

Isn't the starter gear supposed to retract when its not being used? (like the blue arrow shows)
Pictured here with the engine off...

 

[Daleandee]

Isn't the starter gear supposed to retract when its not being used? (like the blue arrow shows)
Pictured here with the engine off...
View attachment 130075

It does on mine ...

 

[Bell206]

Isn't the starter gear supposed to retract when its not being used?

Depends. Sometimes it takes a successful start to trip the bendix gear back with some starters.
But if you think you just spent that last few laps flying with the starter engaged, I would expect to see more indications of that other than the 35A to zero indications.
However, I'm just guessing here based on what you've posted.

 

[ArrowFlyer86]

I have a feeling this is the issue. I only recently got the overhauled starter, and the lines to/from alternator and other components were just checked and cleaned during several hours of troubleshooting.

Not much changed during annual and post annual is when this all popped up. So anything that looks out of place is suspect at this point.

 

[Bell206]

I have a feeling this is the issue.

And you've verified the altenator physically drops off line at the field and output circuits and the load is 35A?

 

[Domenick]

If that is an OEM starter, when the bendix gear remains in the ring gear while the engine is running, it makes an ungodly racket. If the engine is NOT running and you bump the starter for just an instant, the starter will turn, throw the bendix gear into the ring gear, and the bendix gear will stay engaged. This is temporary hack for those sticky bendix gears. Could that have happened when the above photo was taken?

Investigate the bendix engagement, but I doubt it.

 

[ArrowFlyer86]

Appreciate the feedback.

I'll do more troubleshooting if you're suggesting this is unlikely to be it. I was really hoping the starter would be the answer

@Bell206 in air the the only piece of diagnostic equipment I currently have is an ammeter. I have verified the alternator is dropping off, to 0A, but only after rising to 35A first. I reproduced that in flight 3x yesterday. I don't have any other equipment on board during flight to verify.

When I cycled the switch for the alternator (during flight) after it fell to 0, it will then temporarily bring the alternator/ammeter readings back on line for a few seconds (at 35A) and then abruptly fail and drop back to 0.

Sidebar: The only other symptom, which may or may not be related, is that post annual I have gotten some crackling in the headset at takeoff and cruise power (both pilot/copilot jacks). By accident I discovered that if I increase the load on the electrical system (specifically turning on the landing light or another decent amp draw device) the crackling disappears. Might have nothing to do with it but electrical issues often don't exist in a vacuum so thought I'd mention it.

As always, all recommendations are appreciated. Going to head to the hangar now and see what else I can find.

 

[Dan Thomas]

If the ammeter falls to zero in flight, but not on the ground while taxiing, I suspect an overvolt sensor problem. When the alternator speeds up, its output voltage rises and the regulator starts limiting field current to keep it within spec. If the voltage rises to 18 volts, the overvolt sensor shuts the regulator off and the ammeter will go to zero. Recycling the ALT switch will release the sensor so it turns the regulator on again.

So if the voltage is spiking, you might have a bad regulator. If the voltage isn't spiking but merely going to max charging voltage, around 14 volts, and the ammeter goes to zero, the overvolt sensor might be shot or its ground is compromised.

It looks like this:



Put a voltmeter on the system and run the engine at various RPMs and see what the voltage does and when the ammeter drops off.

 

[Dan Thomas]

If that is an OEM starter, when the bendix gear remains in the ring gear while the engine is running, it makes an ungodly racket. If the engine is NOT running and you bump the starter for just an instant, the starter will turn, throw the bendix gear into the ring gear, and the bendix gear will stay engaged. This is temporary hack for those sticky bendix gears. Could that have happened when the above photo was taken?

Investigate the bendix engagement, but I doubt it.

If the starter stays energized. that drive gear will disengage and retract as the engine RPM comes up, and the starter will keep running rather quietly. That drive relies on the Bendix's inertia when the starter suddenly starts rotating, but one the starter is running, the inertia is gone and that drive will retract with the starter running. The drive's engagement-locking feature was to keep it engaged until the engine started. Non-locking drives will disengage if one cylinder fires but the engine doesn't catch and run. Frustrating.

 

[Bell206]

I don't have any other equipment on board during flight to verify.

Do you have an AUX power port in the cockpit. Either rig up a VOM or get you a cheap voltage indicator and plug it in to the port. But better to use a VOM. As mentioned in one of Dan's posts a bad ground can cause the regulator/over volt relay not to function properly. While knowing what the actually amp and voltage values are in flight will take this to the next level. Could be what you see in flight on the amp indicator is not accurate? However just seeing what the volts are will get you half way there. What I find interesting is if it is the starter why doesn't it stay running after the start on the ground?

 

[ArrowFlyer86]

Do you have an AUX power port in the cockpit. Either rig up a VOM or get you a cheap voltage indicator and plug it in to the port. But better to use a VOM. As mentioned in one of Dan's posts a bad ground can cause the regulator/over volt relay not to function properly. While knowing what the actually amp and voltage values are in flight will take this to the next level. Could be what you see in flight on the amp indicator is not accurate? However just seeing what the volts are will get you half way there. What I find interesting is if it is the starter why doesn't it stay running after the start on the ground?

Grabbed my VOM, and on my way to the hangar I just picked up a voltage indicator to plug into the cigar lighter port as another diagnostic tool.

It's possible that what I'm seeing on the ammeter is not accurate, but it seems too consistent to be a failing gauge or something. And we just cleaned the terminals on the shunt. And generally the ammeter is just a really simple device.

As for the starter... That I do not know. What made me inclined to pick that as the culprit is that it's a device we just recently changed. And a starter messing up could have a notable impact on the electrical system. And given that we spent hours verifying alternator wiring and grounds a couple days ago - it seems increasingly likely that those aren't the cause. But ya never know.

I'm going to start testing and logging my results shortly. Thanks again for the recommendations. Will update with results.

 

[Dan Thomas]

It isn't likely the alternator itself, but for the last15 years or so there have been rebuilders messing up the Ford alternators way too often. The rear bearing is a Torrington needle bearing, and has a seal on it. Those clumsy rebuilders put way too much grease in that bearing, and when the alternator is assembled and the rotor goes into the bearing, that excess grease gets extruded and gets on the slip ring insulator assembly, and then the alternator spins and the grease is flung all around the brush cavity and gets on the brushes and slip rings. It dissolves a bit of the brushes' graphite and makes a resistive sludge on the slip rings, limiting the field current and therefore the output of the alternator. One can detect this by disconnecting the field wire at the regulator and measuring the resistance between ground and that wire. For a 12-volt alternator it should be around 3 o 5 ohms, and that should stay steady as you move the prop some. Disconnect all the plug leads before you do this, please! In affected alternators, and I have encountered way too many of them, I have seen resistances as high as 50 and 60 ohms, and the resistance bounces all over the place as the alternator is rotated.

I don't remember how many Service Difficulty Reports I filed on these things. Many. They had no effect on the chief offender, Kelly (now Hartzell, I think, or has that changed, too?). Every Kelly alternator failed.

 

[ArrowFlyer86]

OK some results from my troubleshooting today. Essentially I went out there with VOM to measure battery terminal voltage and a voltage sensor to plug into the cigar lighter port. I went for a 30 minute cruise followed by a landing at a local airport then another 10 minute cruise.
Since I wasn't able to get the alternator to fail like it did before, I did 2 additional restarts on the ground to see if I could get it to occur. All observations here are in the order they occurred.

It was about 2-3 minutes in between restarts after the flight. I shut down, got out, checked to make sure the starter retracted properly (paranoid from yday), and then restarted.



FYI: this is where the ammeter settles after the plane has been running for a while. I call this ~5.


No breakers blew except one (because I suck). I hooked an alligator clip from the VOM into the cigar lighter port and in those tight confines it shorted +/-. After I reset the breaker everything was fine. That was my fault not the plane. It was easier to just use the voltage sensor built for measuring that

I spent a couple hours looking at various lines and didn't find any loose connections or things that seemed like they'd be malfunctioning, with one exception.
I just had both mags overhauled and sparkplugs gapped/cleaned at annual.
These leads going into the plugs should not be frayed right? I thought if that wire came in contact with the metal then you wouldn't get a spark, b/c there'd be a short to ground?



EDIT: Made edit in screenshot for clarity.

 

[ArrowFlyer86]

^Just to note... I know the mags don't draw power from the alternator so I don't think it has explanatory power for the electrical issue I've been experiencing.
But nonetheless, it seems like the shielding on the wires should be addressed?

 

[Bell206]

OK some results from my troubleshooting today.

Now you have a good baseline of values to work from. Unfortunately don't see any issues. But since it appears the problem is intermittent may want to perform a very detailed visual inspection of the complete battery and alternator system components/wires from one end to another. You may simply have a chafed wire somewhere that has been "repositioned" and is no longer affecting the system. Pull the cowling and front seats and look at all items with a bright light and mirror for any defects. There are other tests that may show something like a wiggle test of the wires but at this point a good visual/tactile check of the complete systems vs only certain areas may reveal an issue.

These leads going into the plugs should not be frayed right?

What you are seeing is the weave of the shielding not "fraying". The blue material is a chafe preventative that has torn. Could you repair that, sure, but I'd let your mechanic make that call. And as a FYI, the shielding is connected to ground.

 

[ArrowFlyer86]

Now you have a good baseline of values to work from. Unfortunately don't see any issues. But since it appears the problem is intermittent may want to perform a very detailed visual inspection of the complete battery and alternator system components/wires from one end to another. You may simply have a chafed wire somewhere that has been "repositioned" and is no longer affecting the system. Pull the cowling and front seats and look at all items with a bright light and mirror for any defects. There are other tests that may show something like a wiggle test of the wires but at this point a good visual/tactile check of the complete systems vs only certain areas may reveal an issue.

What you are seeing is the weave of the shielding not "fraying". The blue material is a chafe preventative that has torn. Could you repair that, sure, but I'd let your mechanic make that call. And as a FYI, the shielding is connected to ground.

Thanks @Bell206

I'm going to go do another test flight and see if I can get it to occur.

After that I'll plan to follow the above steps to pull cowling/seats and get in there to visually check.

 

[Domenick]

If the starter stays energized. that drive gear will disengage and retract as the engine RPM comes up, and the starter will keep running rather quietly. That drive relies on the Bendix's inertia when the starter suddenly starts rotating, but one the starter is running, the inertia is gone and that drive will retract with the starter running. The drive's engagement-locking feature was to keep it engaged until the engine started. Non-locking drives will disengage if one cylinder fires but the engine doesn't catch and run. Frustrating.

Good point. I had not considered the bendix gear retracting and the starter remaining energized. That may not be particularly noticeable over engine noise.

 

[Pinecone]

If the ammeter falls to zero in flight, but not on the ground while taxiing, I suspect an overvolt sensor problem. When the alternator speeds up, its output voltage rises and the regulator starts limiting field current to keep it within spec. If the voltage rises to 18 volts, the overvolt sensor shuts the regulator off and the ammeter will go to zero. Recycling the ALT switch will release the sensor so it turns the regulator on again.

So if the voltage is spiking, you might have a bad regulator. If the voltage isn't spiking but merely going to max charging voltage, around 14 volts, and the ammeter goes to zero, the overvolt sensor might be shot or its ground is compromised.

Put a voltmeter on the system and run the engine at various RPMs and see what the voltage does and when the ammeter drops off.

It could also be a broken/disconnected sense wire. So the voltage regulator is seeing 0 volts, so raises field voltage/current to try to get the proper voltage.

I just had this with one of my alternators.

 

[ArrowFlyer86]

That's intriguing.

When it's charging the battery after starting it's at 35A, and on the few occasions where it's messed up it jumps to 35A before failing. The only thing that's unique to me about 35A specifically is that it's what it normally reads when the battery is being replenished. Could that be a clue supporting this?

Other than that I did a test flight today. All normal readings, exact same as yesterday. Still sitting in the plane ATM with the seats out, a bright light, telescoping mirror and verifying every connection under the panel is not loose.