Alternator trouble/identification

[RideOrFly]

I'm chasing down an electrical problem and I'm hoping someone on here can offer some advice. Running the checklist in flight is about the extent of my expertise at this point. 

1979 M20J, roughly 5500 TTAF. Plane was down for a few months getting avionics. Annual was done immediately after. On my first flight after annual, my brand new AP disconnected and voltage light started flashing. Ammeter was reading steady at about -10. Everything had been normal during the run-up. Resetting breaker had no effect. Battery voltage dropped down to around 10. I took it back to the shop that did the annual. They tell me that the battery holds a charge just fine, but that my VR is fried. I ordered a new Zeftronics VR that was installed yesterday. Mechanic did ground run today, and problem persists. He checks the alternator and now tells me that the alternator brushes are bad, and that could have been what fried the VR. Unfortunately, the alternator doesn't have anything on it indicating what brand, part #, etc. it is. *edit* I finally found the part number for the alternator in my logbook, ELS-ALY-8520RS.

A few specific questions:

1) Does this sound like a normal approach to troubleshooting a problem? It seems like checking the alternator out would have been part of the initial assessment.

2) What are the odds that this is the original alternator?

3) Any idea how to figure out what brand/model it is? 

4) Should I just replace or overhaul the alternator at this point, or is replacing the brushes a reasonable thing to do?

Any input appreciated.

 

Edited February 22, 2023 by RideOrFly
answered one of my own questions

[Fly Boomer]

22 minutes ago, RideOrFly said:

I'm chasing down an electrical problem and I'm hoping someone on here can offer some advice. Running the checklist in flight is about the extent of my expertise at this point. 

1979 M20J, roughly 5500 TTAF. Plane was down for a few months getting avionics. Annual was done immediately after. On my first flight after annual, my brand new AP disconnected and voltage light started flashing. Ammeter was reading steady at about -10. Everything had been normal during the run-up. Resetting breaker had no effect. Battery voltage dropped down to around 10. I took it back to the shop that did the annual. They tell me that the battery holds a charge just fine, but that my VR is fried. I ordered a new Zeftronics VR that was installed yesterday. Mechanic did ground run today, and problem persists. He checks the alternator and now tells me that the alternator brushes are bad, and that could have been what fried the VR. Unfortunately, the alternator doesn't have anything on it indicating what brand, part #, etc. it is. So, can't order brushes for it. I've looked through the logs a few times now and I can't see where an alternator was replaced. Plenty of voltage regulators and alternator belts, but no alternator. 

A few specific questions:

1) Does this sound like a normal approach to troubleshooting a problem? It seems like checking the alternator out would have been part of the initial assessment.

2) What are the odds that this is the original alternator?

3) Any idea how to figure out what brand/model it is? 

4) Should I just replace the alternator at this point?

Any input appreciated.

 

Plane Power publishes this guide:

Troubleshooting Guide For Externally Regulated Alternators.pdf

[RideOrFly]

15 minutes ago, Fly Boomer said:

Plane Power publishes this guide:

Troubleshooting Guide For Externally Regulated Alternators.pdf 33.62 kB · 3 downloads

Thank you

[PT20J]

Unfortunately, that is the way some mechanics troubleshoot: replace a part. If that doesn’t fix it, replace something else. Repeat until problem goes away, or the owner runs put of money.

Bad brushes didn’t fry your VR. That’s just an excuse for replacing the wrong part. Think about it: If bad brushes fried you old VR, why didn’t they fry the new one? And, would engineers really design a VR that was susceptible to alternator brush wear since it is expected that alternator brushes eventually wear out?

So, you have a mechanic that is not good at troubleshooting electrical problems and also won’t fess up to a mistake. 

 

 

[A64Pilot]

It may not be the alternator still, it could be the Master switch or the field circuit breaker or the field wire connection on the alternator.

Easy to check the above before you buy an alternator, just as it takes about 30 sec to validate a voltage regulator, but to be fair it does require cowling removal to access the alternator to do so.

I would say it’s very unlikely an alternator lasted 5500 hours, possible I guess, but not likely.

On edit, these are easy checks any pilot can do, sure you can’t legally change the alt or VR, but I see no reason why you can’t fault isolate.

[Ragsf15e]

You’re learning about old aircraft electronics.  Some A&Ps are good at it, but many are not.  You need a digital voltmeter and one of the troubleshooting guides like posted above.  With the airplane off you can do most of the fault isolation yourself.  While you’re at it, check the connections on the alternator carefully for wear.  Those connections are notorious.  You can also follow the voltage from the vr to the alternator and from the alternator out.  Do this really carefully because the engine is running. You’ll likely need to follow the voltage through the system and find out where the problem is.

I went through this a couple years ago and it got pretty expensive before I troubleshot it myself, found a corroded connection and fixed it.  But at least I got an education and a new VR and alternator out of the deal…

[hammdo]

Like this one:

 

 

Happened the second flight after annual…

-Don

[RideOrFly]

1 hour ago, PT20J said:

So, you have a mechanic that is not good at troubleshooting electrical problems and also won’t fess up to a mistake. 

That was my suspicion, but didn't really have anything to back it up. I agree it doesn't make much sense for the alternator brushes to cook the VR. 

40 minutes ago, A64Pilot said:

It may not be the alternator still, it could be the Master switch or the field circuit breaker or the field wire connection on the alternator.

Easy to check the above before you buy an alternator, just as it takes about 30 sec to validate a voltage regulator, but to be fair it does require cowling removal to access the alternator to do so.

I would say it’s very unlikely an alternator lasted 5500 hours, possible I guess, but not likely.

On edit, these are easy checks any pilot can do, sure you can’t legally change the alt or VR, but I see no reason why you can’t fault isolate.

I'm going by the shop in the morning to take a look and talk with the mechanic some more. He did mention having checked the field wire with the initial "troubleshooting" but I'll have to check about the master switch, etc. I'm not the most mechanically inclined person, but if I can't trust the shop, I guess I don't have much choice other than to figure it out. 

 

25 minutes ago, Ragsf15e said:

I went through this a couple years ago and it got pretty expensive before I troubleshot it myself, found a corroded connection and fixed it.  But at least I got an education and a new VR and alternator out of the deal…

Glad you got it figured out...I wish I had more time and a better location to work through this kind of stuff. I'll get a look at the connections and order a voltmeter at least.

[RideOrFly]

21 minutes ago, hammdo said:

Like this one:

 

 

Happened the second flight after annual…

-Don

Geez. I really hope my mechanic could spot something like that. 

[hammdo]

I had to tug on it a bit to verify…

it was not obvious until it came out of the ring terminal…

-Don

[Ragsf15e]

44 minutes ago, RideOrFly said:

Geez. I really hope my mechanic could spot something like that. 

That’s why going through a step by step troubleshooting guide is pretty beneficial.  Some things are obvious but it’s harder when a connection looks ok but isn’t.  The voltmeter doesn’t lie (usually).

Im not the most mechanically inclined either, and I had to work on my electrical knowledge for sure, but I eventually got fed up with just throwing new parts at my problem.

@M20Doc has a simple way to check the VR Voltage is making it to the alternator.  Was that your paperclip check doc?  He usually doesn’t like these threads because we bash A&Ps on electrical knowledge, but he’s a rare breed and definitely knows what he’s doing.

[Guest]

Two paper clips or two cotter pins joined together in a daisy chain make a simple flux detector.  Turn on the master switch and alternator switch if they’re separate.  Hold the flux detector near the alternator pulley, the magnetism in the alternator should pull the paper clips toward the alternator, if not you need a voltmeter to figure out why.  There are two sides to the alternator, power control(the voltage regulator circuit) and power production(the rectifiers). It can be no field voltage from the power side or a bad ground wire or bad brushes causing lack of magnesium or a failed rectifier causing no output.

Broken wires sometimes hide under cable boots, tuck on the wires.

[hammdo]

I did the simple checks first:

• Belt - ck
• Alternator bolted and safety wire- ck
• Wires/ground tight, tug on wires bolted on alternator - ops…

I just did the simple items first - had my volt meter ready… like the paper clip idea…

-Don

[PT20J]

The system is pretty simple. DC current is supplied to the alternator rotor through the brushes creating a magnetic field rotated by the engine. The stator coils surrounding the rotor have an AC voltage induced in them by the rotating magnetic field. Diodes rectify the AC creating DC voltage at the alternator output. The output voltage varies with the time rate of change of the rotating field, so voltage would increase with rpm without a voltage regulator. The voltage regulator samples the output voltage and adjusts the field current which varies the magnetic field controlling the voltage.

Since the alternator is attached to a vibrating engine, loose or broken connections seem to be the most common failure. 

 

[Ragsf15e]

Despite Skip’s description, the system as a whole is pretty simple.

Juice goes into your VR from your bus (master switch and alt on, all breakers in).  It should be full bus voltage.  12/24 if on battery, 14/28 with the alternator on and working.  Juice comes out from the VR at a slightly lower voltage (~11.5v for a 14v system, but look at the troubleshooting guide).  Juice from the VR goes into the alternator (make sure you didn’t lose any in transit).  Skip’s magic happens.  Juice comes out of the alternator at 14/28v and goes back to the bus where it feeds everything, including the VR and the battery.  Follow the juice.  Make sure you don’t have a broken connection spilling it or a corroded connection that spills just some of it - but this doesn’t seem like your issue since your alternator is totally off line.

[81X]

One of the things I found when my brushes were wearing down is that it gradually took more RPM to make it to the regulator limit of 13.3v with a given load on taxi, so it was somewhat gradual.