Airplane eating alternators

[flyingcheesehead]

Gaah.

We have an Archer - The one Pete and I brought to Gaston's last weekend - That seems to like eating alternators for lunch. In fact, it's been at Gaston's the last 3 years in a row, and the first 2 out of those 3 it's paid a visit to the shop at Mountain Home for electrical issues. Right when I was sighing with relief because we didn't have to take it in this year, David (dparter) emailed me tonight saying that it's failing - It'll cut out and after he resets it it'll work for 20-30 minutes and cut out again. :-(

It's also possible to hear a whine in the headsets that varies with engine speed and goes away when the alternator is shut off.

What can we do to solve this problem? I know electrical issues are difficult to diagnose, but where should we start given these symptoms?

 

[twdeckard]

Greetings Kent!

I enjoyed meeting you this last weekend.

Diagnosing an electrical problem like this should probably be framed in the form of a contest! My entry is diode at the rectifier.

Good luck, these things can be infuriating. Here are some pictures of 83F on its trip to the Bahama's.

 

[ScottM]

Friend of mine just had that issue. Turned out to be a bad wire. He had replaced the alternator twice, voltage regulator and a myriad of other things chasing the ghost. The wire that was broken was intermittent and caused a current surge that tripped the alternator off.

 

[RotorAndWing]

Friend of mine jsut had that issue. Turned out to be a bad wire. He had replaced the alternator twice, voltage regulator and a myriad of other things chasing the ghost. The wire that was broken was intermittent and caused a current surge that tripped the alternator off.

The wiring for the alternator takes quite a beating over time. Go through and replace all the wires and you'll probably have the problem disappear along with the noise.

 

[Let'sgoflying!]

-where are they being overhauled? Not K.A., right?
-is the cooling ducting being replaced properly each time?

 

[wabower]

Roger that, also the various connectors, especially the snap/plug-in type can be the source of intermittant problems.

Friend of mine jsut had that issue. Turned out to be a bad wire. He had replaced the alternator twice, voltage regulator and a myriad of other things chasing the ghost. The wire that was broken was intermittent and caused a current surge that tripped the alternator off.

 

[gismo]

First off what you really need to do is determine if it's really alternators going bad. I've seen too many cases where parts like alternators and regulators were replaced willy nilly because the mechanic didn't have a clue what else to do.

Second thing if you really do have a bad alternator is to find out what specifically went wrong with it. Usually excessive whine is a bad diode but diodes generally fail as a result of excess voltage, reversed polarity, or heat. Voltage issues can come from poor/loose connections. Reversed polarity is almost always the result of improper "jumping". Heat issues are usually a combination of poor cooling (bad fan, no source of cool air, too close to hot exhaust or cylinder etc) and high loads. Alternators are supposed to be capable of delivering their full output continuously but anything higher than about 75% for a long time is asking for overheating problems.

 

[Dan Thomas]

First off what you really need to do is determine if it's really alternators going bad. I've seen too many cases where parts like alternators and regulators were replaced willy nilly because the mechanic didn't have a clue what else to do.

Second thing if you really do have a bad alternator is to find out what specifically went wrong with it. Usually excessive whine is a bad diode but diodes generally fail as a result of excess voltage, reversed polarity, or heat. Voltage issues can come from poor/loose connections. Reversed polarity is almost always the result of improper "jumping". Heat issues are usually a combination of poor cooling (bad fan, no source of cool air, too close to hot exhaust or cylinder etc) and high loads. Alternators are supposed to be capable of delivering their full output continuously but anything higher than about 75% for a long time is asking for overheating problems.

We seldom replace an alternator in our fleet. If we do, it's usually because the slip rings have finally worn to the point that there's no sense putting in new brushes. More recently, we've had to replace some bearings, probably because they're being outsourced. The seals come loose and the grease escapes and the bearing dries up and fails. And we've had three brand-new Electrosystems (Kelly) alternators fail after 50 hours or so because the factory put way too much grease in the rear bearing, and that grease flew out and got onto the slip rings where it contaminates the brushes and makes a sticky black coating that doesn't conduct the field current too well. I would bet a doughnut that this is the OP's problem.

Old regulators will stick and overwork an alternator. If there's no overvoltage detector, or the pilot doesn't notice the constantly high charging rate, the alternator will heat up and burn out. Boils the battery dry, too.

Dan

 

[dparter]

The current symptoms are that after 20-30 minutes of flight time, the aternator (amp meter) needle goes to 0. Cycling the alternator off and on, the needle jumps to slightly to the right of center, moves to slightly to the left of center, stays there, and then 20-30 minutes later, does it again.

--david

 

[gismo]

The current symptoms are that after 20-30 minutes of flight time, the aternator (amp meter) needle goes to 0. Cycling the alternator off and on, the needle jumps to slightly to the right of center, moves to slightly to the left of center, stays there, and then 20-30 minutes later, does it again.

--david

Assuming the ammeter is connected between the battery and the main bus (as is the case with most Cessna singles) the behavior you describe is pretty close to normal and could be nothing more than a gauge with a slight offset in the zero point. A voltage reading or two would provide significantly more information.

 

[RotorAndWing]

Every aircraft I've owned that didn't come with a voltmeter I installed one. Digital voltmeters are inexpensive and easy to install. Even the automotive cheap ones that plug into a cigarette lighter are better than nothing.

 

[vontresc]

Lance this is the Piper charging amps indicator, and not the charge discharge type in Cessnas.

 

[Dan Thomas]

Lance this is the Piper charging amps indicator, and not the charge discharge type in Cessnas.

The Piper loadmeter is between the alternator and the bus. It measures alternator output. It should never read less than zero. If it is, a bad diode is allowing backflow. Might be doing it only after it heats up. Bad diodes make a whine in the headset, too.

Dan

 

[flyingcheesehead]

Lance this is the Piper charging amps indicator, and not the charge discharge type in Cessnas.

The Piper loadmeter is between the alternator and the bus. It measures alternator output. It should never read less than zero.

It's not reading less than zero. On this gauge, zero is all the way to the left, 70 is all the way to the right, 35 is in the middle and the normal electrical load, depending on lighting and such, is around the midpoint. I think what's happening when David turns it back on is that it's momentarily recharging the battery a little bit from when it was drained slightly after the failure, and then going back to the normal load.

The question is, what's causing it to stop charging after 20-30 min??? I'm pretty sure this alternator is less than 2 months old.

 

[weirdjim]

In a city as large as Madison, I'd be willing to bet that there are half a dozen "auto electric service" (that's how the yellow pages list them) companies. Have a "qualified aircraft maintenance person" pull the alternator, take it and put it on the "qualified aircraft" counter of the alternator fixit shop (that's roughly equivalent to the "qualified aircraft" tool section of Harbor Freight and the "qualified aircraft" parts section of Pep Boys), tell the person behind the counter that this part from your airboat does this and that and that you are willing to pay to have it tested. My best bet is that the alternator will test just fine. Take it back to your "qualified aircraft maintenance person" and ask them to find out what is wrong with the aircraft, that the alternator tests just fine, and that there is something wrong with the rest of the electrical system.

If that "qualified aircraft maintenance person" insists on replacing the alternator, replace the "qualified aircraft maintenance person" instead.

My 8% of two bits.


Jim

 

[Cervelorod]

If you take it to the aviation section of Pep Boys, make sure they know how to test it. Better yet understand the proper connections for testing and double check them. My Cessna alternator was replaced because the aviation section of NAPA auto parts didn't test it correctly. My A&P tested it after it had been replaced, and said it tested fine, but the bearings were worn. He probably said that to make me feel like less of an idiot for replacing a good alt.. My problem was the regulator, and once replaced with a Zeftronics, all my problems went away.

My 6% of $0.02....

Rod

 

[ScottM]

If you take it to the aviation section of Pep Boys, make sure they know how to test it. Better yet understand the proper connections for testing and double check them. My Cessna alternator was replaced because the aviation section of NAPA auto parts didn't test it correctly. My A&P tested it after it had been replaced, and said it tested fine, but the bearings were worn. He probably said that to make me feel like less of an idiot for replacing a good alt.. My problem was the regulator, and once replaced with a Zeftronics, all my problems went away.

My 6% of $0.02....

Rod

What was different about your alternator? The one in my Cherokee is a Chrysler one.

 

[Cervelorod]

They forgot to hook a wire up. The alt I have requires one additional connection, that IIRC the auto version doesn't use. I have notes in my logbook, but they are not accessible to me right now.

 

[RotorAndWing]

When I ran my business I took all my alternators to the local automotive shop. They were Delco alternators and the only difference between them and automotive were a few internal parts were different. I would have the shop test them, disassemble (while I was there) and give me a list of parts needed for the repair. I would order the parts from my supplier and have the shop reassemble and function test the units under my supervision.

 

[Ted]

Every aircraft I've owned that didn't come with a voltmeter I installed one. Digital voltmeters are inexpensive and easy to install. Even the automotive cheap ones that plug into a cigarette lighter are better than nothing.

I just bought one of those out for my Aztec. After the charging system failure I had last month, I figure it's a worthwhile investment. I bought one from Summit Racing. Cheap and works. The only problem I have right now is lack of abundance of cigarette lighters (my truck has 5, my plane has 1). Seeing as I doubt adding one would be well received, I've just been using a splitter and some electrical tape.

The Piper loadmeter is between the alternator and the bus. It measures alternator output. It should never read less than zero. If it is, a bad diode is allowing backflow. Might be doing it only after it heats up. Bad diodes make a whine in the headset, too.

I've observed that on the Archer, however on my Aztec the ammeter has a 3-position switch. Left Alternator, Battery, Right Alternator. The problem is the thing has +/- 100 amps with a 90 degree sweep, which makes the thing thoroughly useless. If there was a +/- 30 amp gauge, that would be far more useful.

 

[dparter]

An update: currently at youngstown, after the alternator stopped resetting after goign to zero (I'll post somethign elsewhere about this trip).

On the ground, fully loaded, the ammeter was reading 0, but the ALT light was not on. Mechanic was hoping that meant just a meter failure. He checked the battery -- 12v, but the alternator was putting out 11.5 (I think that was what he said, maybe he said between 11 and 11.5) -- just enough to not light the ALT light most of the time (it did flicker on). He is now doing more diagnostic work...

--david

 

[gismo]

An update: currently at youngstown, after the alternator stopped resetting after goign to zero (I'll post somethign elsewhere about this trip).

On the ground, fully loaded, the ammeter was reading 0, but the ALT light was not on. Mechanic was hoping that meant just a meter failure. He checked the battery -- 12v, but the alternator was putting out 11.5 (I think that was what he said, maybe he said between 11 and 11.5) -- just enough to not light the ALT light most of the time (it did flicker on). He is now doing more diagnostic work...

--david

Now that it's quit solidly, the problem should be easy to find. The terminal on the alternator that feeds the voltage regulator and monitor on some setups is separate from the main output and uses some separate diodes so it's possible to have a different voltage there than on the battery (which is fed by the main output terminal). But 12v at the main bus and/or battery clearly indicates a lack of output from the alternator which jibes with the 0 loadmeter reading (BTW the "ammeter" in your plane should really be called a loadmeter since it's measuring the load on the alternator output). What you should be looking for is full voltage (approx 11-12v) on the field winding and 1-2 amps of field current with the engine stopped and the alternator on. If you have the voltage but no current the slip rings are likely not making contact, but the rotor (field) winding could be open. No field voltage usually means a regultor problem but could also be a bad connection in the field/regulator wiring. If you have field voltage and current, you need to check for voltage on the alternator main output terminal with the engine running. If the voltage is less than 13 volts the diodes are probably bad but there could be a problem with the stator windings.

 

[weirdjim]

If you take it to the aviation section of Pep Boys, make sure they know how to test it.
Rod

No, never, not , no way. Nobody said anything about taking it to Pep Boys (or Auto Zone, or Kragen, or NAPA or ...). Take it to somebody that does NOTHING but auto'lectrics. That's what I said to do. I wouldn't trust a general parts store with a headlight bulb from my motor scooter, much less an alternator that they probably have never seen that model in their life.

No, use the right folks for the right job.

Jim

 

[nyoung]

When I first purchased my Cherokee 180... My alternator tripped offline on several occasions. I replaced the alternator (twice) the voltage regulator (once), and the over-voltage regulator (once).

It ended up being a bad field circuit breaker... which was presenting variable resistance depending on pressure on the case.

The field CB taps off the main bus, and delivers current to the VR/OVR. When there is a resistance in this path, there is a voltage drop... which means the VR turns up the current headed out to the alternator field... which means more voltage is output from the alternator. Then, if a bit of turbulence causes the variable/random resistance to go away... voila, the full bus voltage (which is higher than it should be) is presented to the OVR, and the OVR circuit trips offline, which takes the alternator offline. In my case, cycling the field current switch would usually bring the alternator back.

Obviously - this situation is not good for the electronic system in the plane. The battery ends up sinking a ton of extra current, and the bus voltage is excessively high, which can kill avionics.

After talking to other Cherokee owners... they have experienced similar failures with a root cause of a faulty field current switch introducing a random/variable resistance into the path.

In short, it is absolutely imperative that the path from the main bus to the VR/OVR combo have no resistance. That means replacing the cabling or at least cleaning up the connections to the main bus, field switch, breaker, VR, and OVR is a good idea.

 

[Dan Thomas]

When I first purchased my Cherokee 180... My alternator tripped offline on several occasions. I replaced the alternator (twice) the voltage regulator (once), and the over-voltage regulator (once).

It ended up being a bad field circuit breaker... which was presenting variable resistance depending on pressure on the case.

The field CB taps off the main bus, and delivers current to the VR/OVR. When there is a resistance in this path, there is a voltage drop... which means the VR turns up the current headed out to the alternator field... which means more voltage is output from the alternator. Then, if a bit of turbulence causes the variable/random resistance to go away... voila, the full bus voltage (which is higher than it should be) is presented to the OVR, and the OVR circuit trips offline, which takes the alternator offline. In my case, cycling the field current switch would usually bring the alternator back.

Must be different than the Cessna setup. The field breaker does nothing more than feed the field switch, which turns the regulator's internal shutoff relay on. The field current comes from the alternator's "A" terminal (which is also connected to the bus via the alternator output breaker) to the reg's "A" terminal and feeds the voltage control (field current) relay. Or the electronic circuitry now found in new regulators. So the field CB has pretty much no say over field current flow except to shut the regulator off altogether.
It's been a long time since I fixed a Cherokee, but I remember them having the same sort of regulator in them. The "certified" aircraft-quality regulator that comes off the same line as the auto parts.

Dan

 

[nyoung]

Yep. Attempt an embedded image/diagram of the Cherokee electrical system follows.

Must be different than the Cessna setup.

Dan

 

[Dan Thomas]

Yep. Attempt an embedded image/diagram of the Cherokee electrical system follows.

There's no "A" connection to the regulator, so the field current has to be suppled via the "IGN" terminal that's fed by the field breaker. A different regulator internally, then.

Dan