Alternator circuit breaker tripping

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RickC

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RIckC
I've got a 1964 172E, and recently the 5A circuit breaker for the alternator (Plane Power conversion) has been tripping. It has happened twice, both times after a flight when I'm on the ground at a very low power setting. After the first occurrence I charged the battery, then pushed the breaker back in, and everything seemed fine. Then after an hour of flying, the same thing happened. While flying, my voltmeter seemed to indicate typical bus voltage (13.4-13.5). The ammeter needle, however seemed to be a hair further on the "charging" side than usual, at least that was my perception at the time. Any ideas? Thanks.
 
An indication of an increased charge could indicate a loose battery plate, shorting out. Have the battery load tested. How old/what type of battery is it?

-Skip
 
The battery is a Concorde AGM, only 2-1/2 years old. I pull it and charge it on the Concorde specific battery minder several times a year. Thanks.
 
Any battery problem will show on the ammeter. Everything except starter current goes through that ammeter.
Either that breaker is bad or there's an intermittent short in the line to the regulator. Field current does not pass through it. It just turns the regulator on and off. The load on the alternator does not affect it, unless Plane Power's regulator is internally very much different from other regulators. Consult Plane Power.
 
Thanks everyone for the great info. I had the battery load tested and it's fine. I have a new breaker coming, so I'll replace that and see if it fixes it. If not, at least I can rule those things out and continue the troubleshooting.
 
The easiest and potentially least expensive option is to check the alternator field wire for fraying or other potential shorting.

Do the easy stuff first.
 
Thanks for the input. It seems fine at the breaker attachment point, and also where it connects to the voltage regulator. The rest is in a bundle from hell behind the panel. Any tips on getting a good look at that part of it?
 
RickC said:
Thanks for the input. It seems fine at the breaker attachment point, and also where it connects to the voltage regulator. The rest is in a bundle from hell behind the panel. Any tips on getting a good look at that part of it?
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Cutting ties and opening the bundle is about the only way.
 
LOL, that's what I was afraid of....I always assumed there was some sort of magic involved
 
Dan Thomas said:
Cutting ties and opening the bundle is about the only way.
Click to expand...

The other thing you could try is to disconnect both ends of the wire and measure continuity from either end to ground. Reach up and move the bundle around while you do this. If there is a short, this may disclose it.
 
NavAir said:
The other thing you could try is to disconnect both ends of the wire and measure continuity from either end to ground. Reach up and move the bundle around while you do this. If there is a short, this may disclose it.
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great idea, thanks.
 
rotormatic1 said:
On a 172, the ammeter only shows if the battery is charging or discharging

https://planepower.aero/aircraft-alternator-support/troubleshooting/

http://ronkilber.tripod.com/alternat/alternat.htm
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do you know how I would go about testing voltage at these various points, as shown on the PP troubleshooting doc? I'm familiar with touching the leads to positive and negative terminals, but never had to test voltage at a single point of contact. Yes, I'm stupid, but I can follow directions.
 
Check the alternator field wire at the alternator and firewall forward. Chaffing and terminal connector issues are most likely there.
 
RickC said:
I'm familiar with touching the leads to positive and negative terminals, but never had to test voltage at a single point of contact.
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FWIW: Perhaps get your mechanic to show you and provide a 2nd sets of hands. He might also have an idea where to start the "wiggle test" first to see if you can narrow down the location of the offending wire.;)
 
RickC said:
do you know how I would go about testing voltage at these various points, as shown on the PP troubleshooting doc? I'm familiar with touching the leads to positive and negative terminals, but never had to test voltage at a single point of contact. Yes, I'm stupid, but I can follow directions.
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The negative lead goes to ground. The firewall or some other good clean airframe point.
 
Update on this...had a shop check all of the wiring and connections, did all of the troubleshooting using the Plane Power troubleshooting doc, all checked out ok. But when I went to do a test flight the breaker popped at run-up. So this weekend I disconnected the battery, unscrewed the nut holding the master switch in place (the old-school push/pull type), sprayed some ACF-50 in there and worked the switch in and out several times, and gave it a few minutes to dry. Hooked the battery back up, turned on the master, started engine, flew for an hour, zero problems. Same thing this morning. Closely watched the ammeter and voltmeter on both flights and they acted normal thoughout. Not ready to call it "solved", but 2 times with no problems hopefully is the start of a pattern. Fingers crossed!
 
RickC said:
Update on this...had a shop check all of the wiring and connections, did all of the troubleshooting using the Plane Power troubleshooting doc, all checked out ok. But when I went to do a test flight the breaker popped at run-up. So this weekend I disconnected the battery, unscrewed the nut holding the master switch in place (the old-school push/pull type), sprayed some ACF-50 in there and worked the switch in and out several times, and gave it a few minutes to dry. Hooked the battery back up, turned on the master, started engine, flew for an hour, zero problems. Same thing this morning. Closely watched the ammeter and voltmeter on both flights and they acted normal thoughout. Not ready to call it "solved", but 2 times with no problems hopefully is the start of a pattern. Fingers crossed!
Click to expand...

I would closely check the wires to that switch which were disturbed when you removed the switch. Look at possible chafe areas and check the insulation for wear.
 
PaulS said:
I would closely check the wires to that switch which were disturbed when you removed the switch. Look at possible chafe areas and check the insulation for wear.
Click to expand...

actually my intention was to remove the master switch and check for exactly that, but it's hidden behind/above the ignition switch and the associated bundle of wires, so I was a little paranoid about messing with that. All I did was loosen the nut holding it in the panel, which only allowed some very slight movement. But that theory does make a lot of sense. Any tips for getting a look back there without disturbing the ignition switch wires?
 
RickC said:
actually my intention was to remove the master switch and check for exactly that, but it's hidden behind/above the ignition switch and the associated bundle of wires, so I was a little paranoid about messing with that. All I did was loosen the nut holding it in the panel, which only allowed some very slight movement. But that theory does make a lot of sense. Any tips for getting a look back there without disturbing the ignition switch wires?
Click to expand...

It would help if you had some pics. I have no idea what it looks like back there. But in general you want to look at spots where the bundle contacts other surfaces and check for chafing there. It also helps to know which wires are the suspect ones via a wiring diagram. What I suspect happened is when you worked on the switch, you tugged the damaged wire moving it away from it's contact point, so you will probably be good until it wears through again or the wire migrates back to where it was.

There's a guy on youtube who is a car mechanic that videos his troubleshooting techniques. He is really good. While he is not an A and P, the techniques are still good for a lot of problems airplanes experience. He just did a fuse blowing episode, one of the things he said was that he doesn't touch anything until he understands what he is hunting for. He doesn't undo wire harnesses, he just looks after figuring out which wire is probably having a problem. If you are doing this yourself, it might not hurt watch the video.

 
Dan Thomas said:
Any battery problem will show on the ammeter. Everything except starter current goes through that ammeter.
Either that breaker is bad or there's an intermittent short in the line to the regulator. Field current does not pass through it. It just turns the regulator on and off. The load on the alternator does not affect it, unless Plane Power's regulator is internally very much different from other regulators. Consult Plane Power.
Click to expand...

Actually, the field current does pass through the field breaker... unless you install an automotive regulator. Then the field current comes via the A wire. Then you have lost control of the alternator.
 
PaulMillner said:
Actually, the field current does pass through the field breaker... unless you install an automotive regulator. Then the field current comes via the A wire. Then you have lost control of the alternator.
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The major airframe manufacturers all used automotive regulators for many years until the ACU came along. The field current comes from the A circuit in those older units. The alt switch turns the regulator on and off.
 
Dan Thomas said:
The major airframe manufacturers all used automotive regulators for many years until the ACU came along. The field current comes from the A circuit in those older units. The alt switch turns the regulator on and off.
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I'm intimately familiar with the Ford regulators.

through version D, the field power flowed through the field breaker.

around version L, the automotive version powered from A+ which was unacceptable for over voltage action... opening the field breaker circuit led to alternator runaway.

so actually OLDER worked the way I described, not as you described. That drove Cessna to the ACU to avoid these disasters from folks putting auto regulators in airplanes... and saved $$ by combining the OV function.

Paul
 
PaulMillner said:
around version L, the automotive version powered from A+ which was unacceptable for over voltage action... opening the field breaker circuit led to alternator runaway.
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Was that the reason for the "Do not turn alternator off in flight" placard?
 
If equipped with a (correctly wired) dual Master switch there should be no problem turning the Alternator part Off with Battery part On. Do not turn alternator off in flight may be related to the original alternator circuit wherein pulling the 60 Amp alternator Circuit Breaker would cause the output of the alternator to go to full output. This was caused because the power and Voltage sense circuit to the regulator was connected to the Bus. The consequence of pulling the 60 Amp breaker with that circuit was that the Bus sense saw the Bus Voltage fall and called for full Alternator output which will blow the diodes. There was a service bulletin regarding this issue on the 28 Volt aircraft.
 
Semiflex said:
If equipped with a (correctly wired) dual Master switch there should be no problem turning the Alternator part Off with Battery part On. Do not turn alternator off in flight may be related to the original alternator circuit wherein pulling the 60 Amp alternator Circuit Breaker would cause the output of the alternator to go to full output. This was caused because the power and Voltage sense circuit to the regulator was connected to the Bus. The consequence of pulling the 60 Amp breaker with that circuit was that the Bus sense saw the Bus Voltage fall and called for full Alternator output which will blow the diodes. There was a service bulletin regarding this issue on the 28 Volt aircraft.
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This is how the typical Cessna is wired:

1716577856554.png

The ALT switch turns the regulator on and allows current from the regulators A terminal to flow to the field That current passes through the alternator's field relay, and shutting off the ALT switch opens that relay. The only current the field can then get is the tiny current that fires the low-voltage light.

In the Cessna, that regulator's A terminal is connected to the alternator output terminal, electrically the same as being connected to the bus except for the alternator's output breaker.

I once had a kit on one of my trucks that switched the alternator and regulator in such a way that the alternator field got full battery voltage, and the alternator output was disconnected from the battery and sent to a 120-volt receptacle. That alternator, once the engine was run up to an RPM high enough that a calibrated neon light came on, was generating 120 volts DC and the diodes did not care. It was DC, but it powered lights and series-wound motors just fine.

I would have trouble imagining that pulling the 60-amp breaker would fry the diodes. Where is the current going to make its way through the diodes to fry them? Even the reverse voltage against the diodes should have no effect if there is no path on the other side. Do you have a Cessna SB number for that problem? In all my time as a mechanic I never came across it.

Edit: I can see the reverse-current path through the diodes across to other diodes biased forward to ground or to any of the three stator windings.
 
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