How to diagnose bad alternator vs voltage regulator?

[rbridges]

65 Mooney with alternator. It stopped working at very end of last flight. Checked wires, everything looks right including ground. IA won't get chance to look at it for a day or two. In the meantime just doing some research. How do you know which is bad?

I keep battery on float charger so it shows 12.2.-12.4v When I start plane, it drops to 11.9-12. Ammeter shows negative obviously.

 

[rbridges]

Pic of alternator

 

[rbridges]

Voltage regulator

 

[NoHeat]

Just curious: did you rule out the battery as the problem?

The voltage, while on the trickle charger, seems low, and that has to do with just the battery, not the alternator or voltage regulator.

 

[Dan Thomas]

With the master and alternator switches on, measure the voltage between the ground stud and power stud as identified. Should be around 11 volts. No voltage? Probably dead regulator or broken wiring or failed ALT switch or breaker. Voltage there, and it won't produce? Failed alternator brushes. See all that black dust around that area? That's from brushes wearing out. They do that, and it's the reason the airframe manufacturers recommend a 500-hour internal alternator inspection. Brushes, depending on installation, will often go 1000 hours or more, but if they fail they can do damage to the slip rings on the rotor, and now you're buying a new alternator instead of a set of brushes.

On that model, the brush holder is removable without even taking the alternator out. Once it's out you can see the slip rings. The two slotted-head screws hold it in there.

 

[Gary Ward]

65 Mooney with alternator. It stopped working at very end of last flight. Checked wires, everything looks right including ground. IA won't get chance to look at it for a day or two. In the meantime just doing some research. How do you know which is bad?

I keep battery on float charger so it shows 12.2.-12.4v When I start plane, it drops to 11.9-12. Ammeter shows negative obviously.

Depending on age or hours of use of course. I would suggest replacing both at the same time. Like said make sure the battery is good first.

The alt looks like a rebuilt unit, could send it back to the original re builder?

My alt quit working last winter. I called AeroTech of Louisville Ky to see how fast they could get me back in business. It was a Saturday at about 1PM. Somebody answered and took my order for Monday said they could rebuild one for me on Monday. Early Monday morning I call again to make sure they were for real and they were. They rebuilt one they had in stock for me by noontime in about 4 hours I am guessing?

My wife picked it up shortly after they finished the rebuild and dropped off our core/old one. I flew with it that night. Came with all paperwork, 337s I think and more. I highly recommend them. No shipping charges or possible damage thanks to my lovey wife.
Cost was the same as Aircraft Spruce and I got it faster since they were a 2 hour drive away.

 

[JAWS]

http://www.zeftronics.info/documents/troubleshooting/tshoot.html

 

[rbridges]

View attachment 98555

With the master and alternator switches on, measure the voltage between the ground stud and power stud as identified. Should be around 11 volts. No voltage? Probably dead regulator or broken wiring or failed ALT switch or breaker. Voltage there, and it won't produce? Failed alternator brushes. See all that black dust around that area? That's from brushes wearing out. They do that, and it's the reason the airframe manufacturers recommend a 500-hour internal alternator inspection. Brushes, depending on installation, will often go 1000 hours or more, but if they fail they can do damage to the slip rings on the rotor, and now you're buying a new alternator instead of a set of brushes.

On that model, the brush holder is removable without even taking the alternator out. Once it's out you can see the slip rings. The two slotted-head screws hold it in there.

Since I have no alternator switch, I assume just the master switch? I bought the plane 10 years ago, and never had anything done to the alternator. Prob put 600 hours on it during that time, so I know it may have some mileage on it.

I'm in middle Georgia, so any rebuild will have to be done via shipping. Do you think a new alternator at this point is wiser? Assuming it's the alternator, of course.

 

[rbridges]

Just curious: did you rule out the battery as the problem?

The voltage, while on the trickle charger, seems low, and that has to do with just the battery, not the alternator or voltage regulator.

I can't swear to the baseline voltage. I noticed the ammeter suddenly stopped showing positive at the end of my last flight and the battery power notifications showed up on my aspen panels. I did notice that I was only producing 13.3-13.4 instead of my usual 13.7-13.8 during part of the flight.

 

[NoHeat]

I can't swear to the baseline voltage.

Are you using a hand-held multimeter (that’s best, and they’re cheap), or are you relying on an analog gauge on the panel?

I think almost every airplane owner should have a multimeter, to assess battery health.

 

[rbridges]

Are you using a hand-held multimeter (that’s best, and they’re cheap), or are you relying on an analog gauge on the panel?

I think almost every airplane owner should have a multimeter, to assess battery health.

It was a digital gauge on the panel. Battery didn't even cross my mind, so I was just looking at the gauge to see if the alternator started chargine. I didn't pay close attention. I'm heading back out there this evening (98 heat index right now, ugh), and I'll put a multimeter on the battery. Thanks!

 

[Dan Thomas]

Since I have no alternator switch, I assume just the master switch? I bought the plane 10 years ago, and never had anything done to the alternator. Prob put 600 hours on it during that time, so I know it may have some mileage on it.

I'm in middle Georgia, so any rebuild will have to be done via shipping. Do you think a new alternator at this point is wiser? Assuming it's the alternator, of course.

I wouldn't assume it's the alternator without doing some troubleshooting---at a minimum, the checks I suggested. Yes, if no ALT switch, then just the master. Best is to get your mechanic on it, assuming he knows how to troubleshoot these systems. Unfortunately, there are some that just start throwing parts at the problem until it goes away. If I was there I'd do the voltage checks, then disconnect the field power line at the alternator and put my ohmmeter across the power stud and ground stud, and rotate the prop backward to see if the resistance was steady at around 3 to 5 ohms. If it's jumpy, or high, or open, the brushes are shot and there might be damage. Best is to disconnect every sparkplug before doing this, then you can rotate the prop any old way safely. An analog meter is best for this. Digital units just flash a bunch of numbers as they try to keep up.

I have mentioned before that alternators in airplanes don't last like the alternators in cars. Even if they're exactly the same thing, each is operated in a different regime. Both of them are geared (or belt driven) at a ratio that brings the alternator to redline RPM when the engine is at redline, to maximize alternator output at lower RPMs. The car almost never sees redline unless it's a teenage boy driving it. It will often run at a third of redline most of its life. The aircraft engine is near redline all the time except during approach and taxi. That high RPM wears out the carbon brushes, which are riding on copper slip rings on the alternator spinning rotor. More speed is more wear.

 

[rbridges]

I wouldn't assume it's the alternator without doing some troubleshooting---at a minimum, the checks I suggested. Yes, if no ALT switch, then just the master. Best is to get your mechanic on it, assuming he knows how to troubleshoot these systems. Unfortunately, there are some that just start throwing parts at the problem until it goes away.

I have mentioned before that alternators in airplanes don't last like the alternators in cars. Even if they're exactly the same thing, each is operated in a different regime. Both of them are geared (or belt driven) at a ratio that brings the alternator to redline RPM when the engine is at redline, to maximize alternator output at lower RPMs. The car almost never sees redline unless it's a teenage boy driving it. It will often run at a third of redline most of its life. The aircraft engine is near redline all the time except during approach and taxi. That high RPM wears out the carbon brushes, which are riding on copper slip rings on the alternator spinning rotor. More speed is more wear.

That's what I'm trying to avoid by doing a little troubleshooting ahead of time. He couldn't get to it Saturday, so it may be some time during the week. I'm going back to the hangar tonight when it cools down to do the test you showed me. Thanks for the info!

 

[Dan Thomas]

That's what I'm trying to avoid by doing a little troubleshooting ahead of time. He couldn't get to it Saturday, so it may be some time during the week. I'm going back to the hangar tonight when it cools down to do the test you showed me. Thanks for the info!

I edited my post just now. A bit more advice.

 

[rbridges]

View attachment 98555

With the master and alternator switches on, measure the voltage between the ground stud and power stud as identified. Should be around 11 volts. No voltage? Probably dead regulator or broken wiring or failed ALT switch or breaker. Voltage there, and it won't produce? Failed alternator brushes. See all that black dust around that area? That's from brushes wearing out. They do that, and it's the reason the airframe manufacturers recommend a 500-hour internal alternator inspection. Brushes, depending on installation, will often go 1000 hours or more, but if they fail they can do damage to the slip rings on the rotor, and now you're buying a new alternator instead of a set of brushes.

On that model, the brush holder is removable without even taking the alternator out. Once it's out you can see the slip rings. The two slotted-head screws hold it in there.

Okay, went to hangar. 12v between those two points with master switch on. So, it's most likely an alternator issue?

Battery was 13.1 volts measured at terminals. Maybe a little low because of discharge from Friday. I put a different trickle charger on it and it shows it's not fully charged, so I'll check it tomorrow.

 

[Dan Thomas]

Okay, went to hangar. 12v between those two points with master switch on. So, it's most likely an alternator issue?

Most likely. Make sure that the alternator's 60-amp breaker hasn't tripped.

The resistance measurement on the field would confirm the failure.

 

[rbridges]

Most likely. Make sure that the alternator's 60-amp breaker hasn't tripped.

The resistance measurement on the field would confirm the failure.

Thanks so much. I'll do the resistance checks when I get some time tomorrow. One last thing, what's your opinion on repaired/rebuilt alternators? Or just a pay more for new?

 

[Dan Thomas]

Thanks so much. I'll do the resistance checks when I get some time tomorrow. One last thing, what's your opinion on repaired/rebuilt alternators? Or just a pay more for new?

Depends on who does them. Some rebuilders do a good job, some don't. I never had any luck with Kelly stuff, which has become Hartzell. They would put way too much grease in the rear bearing and when the shaft went in the grease would extrude out and fly around and get all over the slip rings, contamination the carbon brushes and forming a resistive sludge that reduced the field current to almost nothing. I filed several ADRs on the problem, but it continued. Maybe it's still a problem, I don't know. I retired three years ago but the problem was bad enough 15 years ago. In the flight school I bought the service manuals and did the work myself.

Yours might just need new brushes. The brush holder comes out, and the mechanic could pull it and have a look at the brushes and slip rings. Takes five minutes to do that. Aviall has the stuff.

 

[rbridges]

Depends on who does them. Some rebuilders do a good job, some don't. I never had any luck with Kelly stuff, which has become Hartzell. They would put way too much grease in the rear bearing and when the shaft went in the grease would extrude out and fly around and get all over the slip rings, contamination the carbon brushes and forming a resistive sludge that reduced the field current to almost nothing. I filed several ADRs on the problem, but it continued. Maybe it's still a problem, I don't know. I retired three years ago but the problem was bad enough 15 years ago. In the flight school I bought the service manuals and did the work myself.

Yours might just need new brushes. The brush holder comes out, and the mechanic could pull it and have a look at the brushes and slip rings. Takes five minutes to do that. Aviall has the stuff.

Thanks. I really appreciate all of your time!