Alternator failure modes

iamtheari

iamtheari

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This is in a Piper Arrow with a single master switch. My alternator quit working this weekend. I took off the cowl and measured voltages. I get full battery voltage with the master on, at two of the three terminals on the alternator (measured to the engine lift point as a ground reference). I also get full battery voltage across the terminals of the (what appears to be) big capacitor that the alternator is wired to.

I found a mouse carcass on the hangar floor near where the front of the plane had been parked but I can’t find any evidence of chewed wires. I had figured that the field wire had been chewed through but that doesn’t seem to be the problem.

What other diagnostics should I do before ordering a new alternator? Ideally without the engine running (I don’t like holding wires near a 180 horsepower, 6-foot diameter, unshielded fan.)
 
The Piper operator's manual likely has enough of a wiring diagram in it that should be sufficient to cover troubleshooting this.

The big capacitor is a noise suppressor. It will be connected across the alternator output and ground. The heavy gauge wire on the alternator is the for the output, which should have battery voltage on it when the master switch is on. There are also one or two other terminals depending on alternator type. One of them will be field excitation and the other one might be a ground (if the field isn't grounded through the case). With the engine off and master on I'd expect you to have the field fully energized.

From your description, it sounds like the regulator circuit is doing its job. You could pull the brushes out of the alternator to see how short they're getting before you order a rebuilt alternator.
 
mondtster said:
The Piper operator's manual likely has enough of a wiring diagram in it that should be sufficient to cover troubleshooting this.

The big capacitor is a noise suppressor. It will be connected across the alternator output and ground. The heavy gauge wire on the alternator is the for the output, which should have battery voltage on it when the master switch is on. There are also one or two other terminals depending on alternator type. One of them will be field excitation and the other one might be a ground (if the field isn't grounded through the case). With the engine off and master on I'd expect you to have the field fully energized.

From your description, it sounds like the regulator circuit is doing its job. You could pull the brushes out of the alternator to see how short they're getting before you order a rebuilt alternator.
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You are jumping directly to the charging circuit as the culprit or at least the contributing factor. Have you tested the battery?

-Skip
 
Skip Miller said:
You are jumping directly to the charging circuit as the culprit or at least the contributing factor. Have you tested the battery?
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I doubt it is the problem but it might be worth a little effort to check.
 
first check is a voltage check with the alternator off line and with it on line. with the alternator off line you should get battery voltage. with the alternator on line you should get something like 13.8 or so. if you are not getting that its something in the alternator or regulator circuit.
 
The battery seems to be fine. It kept my radios etc. going for a half hour, then sat for a few hours, then started the engine and ran the radios for almost an hour. Final voltage after all that drain is 11.8. The battery did not charge while I was running the engine for all that time. The ammeter registered 0 (or at least close enough to 0 to believe it's actually putting out 0 amps) except that, after I landed, it came back to life for a while on the ground. It was dead the rest of the day, though.

I haven't run my multimeter with the engine running. See above regarding wanting to avoid that if possible. I like the idea of inspecting the brushes, if I can figure out how to get at them.

idahoflier said:
I would start with referring to the appropriate Maintenance Manual for the procedures to troubleshoot the electrical system.
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This was too obvious for me to think of it on my own. I'm reading that section of the service manual now. Thank you for the hint.

mondtster said:
The big capacitor is a noise suppressor. It will be connected across the alternator output and ground. The heavy gauge wire on the alternator is the for the output, which should have battery voltage on it when the master switch is on. There are also one or two other terminals depending on alternator type. One of them will be field excitation and the other one might be a ground (if the field isn't grounded through the case). With the engine off and master on I'd expect you to have the field fully energized.
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That was about what I figured. I know I can find a small piece of steel to test the magnetic field being generated with the master on and the engine off, as well, but I didn't have anything handy with me at the hangar. I think on the alternator that I saw three terminals: output which also had a smaller gauge wire and had battery voltage with the master on and engine off, then two more that were connected across the terminals of the filter capacitor. I didn't see any other terminals and probably missed a wire or two since I am going from memory.
 
iamtheari said:
The battery seems to be fine. It kept my radios etc. going for a half hour, then sat for a few hours, then started the engine and ran the radios for almost an hour. Final voltage after all that drain is 11.8. The battery did not charge while I was running the engine for all that time. The ammeter registered 0 (or at least close enough to 0 to believe it's actually putting out 0 amps) except that, after I landed, it came back to life for a while on the ground. It was dead the rest of the day, though.
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Do yourself a favor and put a battery charger on that battery before you cost yourself a battery in addition to whatever else is wrong with the charging system. By battery industry standards, 11.8v is dead and my experience with aircraft batteries suggests that they are less tolerant to sitting dead for any length of time than an automotive battery.

I can't decide what to make of your description of the terminals on the back of the alternator. I'm either not picturing what you have going on right or the description is a bit off. I'd arm myself with a charging system schematic and a multi meter and revisit the plane when you have a little time to troubleshoot again.
 
mondtster said:
Do yourself a favor and put a battery charger on that battery before you cost yourself a battery in addition to whatever else is wrong with the charging system. By battery industry standards, 11.8v is dead and my experience with aircraft batteries suggests that they are less tolerant to sitting dead for any length of time than an automotive battery.

I can't decide what to make of your description of the terminals on the back of the alternator. I'm either not picturing what you have going on right or the description is a bit off. I'd arm myself with a charging system schematic and a multi meter and revisit the plane when you have a little time to troubleshoot again.
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Thanks for the tip. I'll put a trickle charger on it ASAP. The battery did hold 12.0V for a very long time so it seems pretty healthy and just got drawn down a bit thanks to not having a working alternator while I was flying home.

I'll definitely take a closer, more accurate look at the wiring when I get a chance. I'm sure I missed something in my first look as I was mostly trying to rule out the mouse having chewed through a wire before digging in any deeper.
 
I've been dealing with all sorts of alternator problems on my Archer this year. The full narrative of all of this is quite long and I've posted it before.

The latest(and hopefully last) discovery was the rear bearing in the alternator we got to replace the old bad one had failed and was allowing the shaft to wobble internally and damage the housing/brushes. It was still putting out power on the ground and was removed/bench tested. Every standard test one would normally do showed it to be working. Finally after hours of testing and head scratching they pulled the brush block off and found some uneven wear so they pulled it further apart and found what we hope has been the actual problem. Just waiting on the replacement to arrive and be installed.
 
iamtheari said:
What other diagnostics should I do before ordering a new alternator? Ideally without the engine running (I don’t like holding wires near a 180 horsepower, 6-foot diameter, unshielded fan.)
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You should take your alternator to the nearest "automotive" electrical shop and tell them it is from an airboat. Let them test it. Replace brushes if that is what they say..

If you don't have the guts to hold wires near a 180 horse fan, then pay somebody to fix your airplane. You are just wasting our time.

Jim
 
weirdjim said:
If you don't have the guts to hold wires near a 180 horse fan, then pay somebody to fix your airplane. You are just wasting our time.
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Guts to use a multimeter while the engine is running: Yes.

Desire to do that as the first step if there are other steps that can or should be done in the hangar without the big scary fan spinning: No. And that's why I posted here, to find out what other steps should be done prior to testing it with the engine running. Especially with the alternator clearly not delivering any current, at least not as far as the ammeter, I also think that there is less to learn with the engine running than with it stopped, but am open to input on that as well.

I like your suggestion of having my local automotive electric shop take a look at the alternator. Unfortunately, we don't have one of those locally so if it gets to that point I'll just have my A&P take a look at it, as he'll have to do eventually when it comes time to sign off on any repairs or replacements that are involved here. But I'm still going to try to save some time from the mechanic's backlog of planes by diagnosing things before bringing it over to his hangar.

cowman said:
I've been dealing with all sorts of alternator problems on my Archer this year. The full narrative of all of this is quite long and I've posted it before.

The latest(and hopefully last) discovery was the rear bearing in the alternator we got to replace the old bad one had failed and was allowing the shaft to wobble internally and damage the housing/brushes. It was still putting out power on the ground and was removed/bench tested. Every standard test one would normally do showed it to be working. Finally after hours of testing and head scratching they pulled the brush block off and found some uneven wear so they pulled it further apart and found what we hope has been the actual problem. Just waiting on the replacement to arrive and be installed.
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Thank you... I don't feel so bad about my vacuum pump anymore. It died just before the plane went in for annual. We got an overhauled vacuum pump to install during the annual inspection. It tested fine on the ground, albeit a bit weak at lower engine RPM compared to the old one. The test runs included a good bit of time running at higher RPM settings to balance the propeller. But then when I went for a test flight, the vacuum pump died just as I rotated. I had to buy a brand new one to replace it. And that one has been fine for 20 hours so far. Tip: New vacuum pumps cost such a small amount more than overhauled pumps that it's not even worth considering an overhauled one unless you can overhaul it at home.
 
Evey time there's a thread on alternator issues, I tell people to disconnect the field wire, take the ohmmeter and measure the resistance between the alternator's field terminal and ground. For a 12-volt system it should be around 4 ohms. Move the prop a bit and see if the needle jumps a lot (or the numbers are all over the place on a digital meter). That resistance should stay relatively steady with rotor movement.

On many airplanes you just pull the connector off the regulator and check the resistance at the F wire. Easy. The regulator is marked as to which one that is.

Worn-out or grease-contaminated field brushes are the biggest cause of alternator failure. The biggest. By a factor of ten, easily. Check that first and don't just start throwing money at the problem. If there is voltage at the alternator's output and field terminals, everything else is probably OK.
 
iamtheari said:
Thanks for the tip. I'll put a trickle charger on it ASAP. The battery did hold 12.0V for a very long time so it seems pretty healthy and just got drawn down a bit thanks to not having a working alternator while I was flying home.

I'll definitely take a closer, more accurate look at the wiring when I get a chance. I'm sure I missed something in my first look as I was mostly trying to rule out the mouse having chewed through a wire before digging in any deeper.
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FYI, 12.0 V is a pretty depleted battery. A fully charged lead acid battery should have a no-load voltage of about 12.6-12.7 V. With the engine running, you should see a bus voltage of something like 13.8-14.2 volts or so with a properly functioning alternator and voltage regulator in order to supply charge to the battery.
 
chemgeek said:
FYI, 12.0 V is a pretty depleted battery. A fully charged lead acid battery should have a no-load voltage of about 12.6-12.7 V. With the engine running, you should see a bus voltage of something like 13.8-14.2 volts or so with a properly functioning alternator and voltage regulator in order to supply charge to the battery.
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Those are the ranges I see on my engine monitor when the alternator is working.
 
To clear up future readers' confusion due to my past confusion in this thread:

Here are the wires on my alternator, which my inspection of the logbooks shows to be the original unit from 1968 other than a replaced voltage regulator and additional noise filter (Chrysler 2642997 according to the plane's equipment list):

The main terminal has three wires on it. One is the heavy gauge wire to the firewall. One is a smaller gauge wire that is connected to a filter capacitor that is mounted on the front of the alternator. The other is a smaller gauge wire that runs back to one terminal of the additional filter capacitor that is mounted to the engine mount using Adel clamps.

A ground terminal on the alternator is wired to the other terminal of the additional filter capacitor.

The field terminal is wired to a small gauge wire back to the firewall.

I had to go back to the office this afternoon, so I couldn't get my hands dirty disconnecting terminals and testing resistance. I am tempted to replace the alternator given that it is in all likelihood 50 years old, and given that I don't have local resources to overhaul or repair the unit.
 
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