Alternator Field Fuse Tripping Resolved

[boom3]

Long?

I have about 375 hours now on my RV-7A since new. My alternator is a B & C 60amp with the LR3C-14 voltage regulator. To date it has never missed a beat. Recently after a local flight while on downwind to land at my airport I notice my low voltage light flashing. (Something I?d never seen in flight) I checked my 5amp circuit breaker, which wasn?t tripped, and tried cycling my alternator field switch. No Change. After landing I pulled out my POH so I could identify and check the fuse. (My field circuit starts out with and automotive style ATC fuse then is wired through a Potter and Brumfield 5A circuit breaker, then through the master switch, alternator switch, then is connected to the LR3C-14.) I found that my 5A fuse was blown. I went ahead and replaced the fuse and started the engine again. It blew right away. OK well it?s trying to tell me something so I pulled it the hanger and went home for the evening.

On my next trip to the airport I started digging into it. Everything looked good and I couldn?t find any reason for it to be tripping. I followed B & C?s troubleshooting guide and verified all the voltages and resistances were correct. I also followed all the wires and checked for chaffing, grounding, or poor connections. Finding nothing I put in another fuse and pulled it outside and started it up. Everything was good and I was indicating a steady 14.4 volts. I turned it off, preflighted, and started it again with no issues. No issues during the runup either so I decided to take it up for another short local flight. Stronger winds aloft over the Cascades provided some light turbulence here and there but the voltage remained rock steady. I flew it around the airport environment for about an hour with no issues. However upon reducing power on downwind it popped again. OK something is really up. Out of time I put it away again.

I did some research on VAF and read about some possible culprits. I read about problems with the field connections on the alternator itself not making good contact, bad master switches, bad alternator switches, and bad circuit breakers. I also called B & C who were great and very willing to help. I emailed them my troubleshooting checklist and we talked about everything. They were pretty convinced it was something with the wiring and mentioned most of the same things I?d read about on VAF. They really stand behind their products and said they should not have a problem making TBO. They said that most, like 98 percent, of all problems end up being a bad connection somewhere in the wiring.

My next opportunity several days later I decided to really tear into it. I replaced the master switch, alternator switch, bypassed the circuit breaker (still kept the fuse), and followed every wire. I disconnected every single connection, checked the crimps and re-crimped for good measure. Everything looked great. I even removed the plastic alternator connector from the plane and took to it to the bench where I could give each conductor a good tug and meter for any changes in resistance. Although not avionics, my whole career has been in the electronics / low voltage field so I was pretty confident with my wiring. I?m definitely not perfect though so it was nice to physically verify everything again. 100% confident in the wiring I put it back together again and pulled it outside to start it. The fuse blew again right away. Dang! Well that leaves the alternator or the regulator itself, but which is it? I drove home wondering what to do next.

I called B & C again the next day and they really couldn?t offer much more help. If all the voltages and resistances are reading correctly it should work fine. The Odyssey battery crossed my mind as I thought I read about that being a possibility.

On the weekend I got the bright idea to run up to the hangar and remove both the alternator and voltage regulator and bring them home. At home I mounted everything to a board and rewired everything from scratch. I have an old drill press with a 1/3hp motor with a pulley that would provide the alternator adequate RPM. Upon powering it up the battery would start to charge for about 5 seconds and then the fuse would blow. Watching my digital VOM meter closely, it would settle down about 14.4V then suddenly climb to above 15V before blowing the fuse. It appears the LR3C-14?s crowbar overvoltage circuit was doing its job and blowing the fuse.

Things setup at home. (field fuse not installed in this picture)


I made another call to B & C and they agreed it was voltage regulator causing the problem and not the alternator. Although not happy about buying another voltage regulator, I was ecstatic about finally finding the problem. I ordered a new one from Aircraft Spruce West and was able to get everything re-installed Thursday evening. Although dark, I needed fuel so I fired it up and taxied to the pumps to try it out. I loaded it up pretty good with pretty much every light on and the voltage stayed nice and steady. Another start and the taxi back to the hangar was the same, everything was great.

On Friday, during daylight, I was able to fly it again after work. I flew around the airport awhile, turning on and off landing lights, varying RPM, etc and everything was perfect. I ended up flying it for a couple hours with several landings. I?m confident it?s fixed!!! Ok now where was I. Oh ya, waiting for decent flying weather again.

Some things I learned...

B & C is a great company to deal with.

Even a 7.5A ATC style fuse will still blow before the 5A circuit breaker. When I find one, I will replace it with a 7.5A slow blow.

Although 98 percent of the problems are caused by wiring or connections, that isn?t always the case.

Part of me wishes I?d just pulled everything out of the plane at the beginning, but it would suck to do all that and then find a simple bad connection somewhere.

[BillL]

Jeff,
Excellent troubleshooting. I like your test bench! I will have to build one. (sooner or later)

Question - what value does a fuse and CB have for the field line. Because even the CB will pop if the crowbar is tripped, and it can be reset just before landing in an emergency. I only have the CB (I think).

Also, I see you were beyond the warranty, so do you have some understanding of the contributing factors to the failure, temp maybe? Steady high power demand? Cost per hour seems a little high for this component alone.

I am installing the Plane Power 60A, but still watching all alternator issues with interest as one should easily last 1000 hrs if operated within it's envelope. If we know what that is.

Last, being an electronics type, will you attempt to repair the unit removed?

[boom3]

Hi Bill,

Question - what value does a fuse and CB have for the field line. Because even the CB will pop if the crowbar is tripped, and it can be reset just before landing in an emergency. I only have the CB (I think).

Ya I don't think it has any value and the fuse is more difficult to replace in flight then simply resetting the circuit breaker. I wired it that way because there really isn't a great place to pick up power other than the master buss fuse panel. I will however find a 7.5A slow blow so hopefully the circuit breaker will trip before the fuse.

Also, I see you were beyond the warranty, so do you have some understanding of the contributing factors to the failure, temp maybe? Steady high power demand? Cost per hour seems a little high for this component alone.

I don't have a good answer for that. It is mounted on the inside of the firewall where it doesn't get warm. I doubt it's high power demand because I never come close to the 60A the system is capable of. Vibration? Maybe, but everything vibrates on an airplane. I do agree about the cost per hour and hope I never need to replace it again. Sometimes things do go bad so I'm gonna hope I just got unlucky.

Last, being an electronics type, will you attempt to repair the unit removed?

Without the schematic I won't likely try and repair the unit. I did take it apart and was surprised how much was really inside it. It is pretty old school as far circuit boards go. Nice big components that could pretty easily be replaced. I'd rather not always be thinking about it on long trips though.

[vlittle]

I had a similar problem, but with a pp internally regulated alternator. I made a similar bench test setup, but used a half inch electric drill and a socket to drive the alternator pulley nut.

Turns out the brand new pp had a bad ovp circuit. Replacing the regulator module fixed the problem.

Next time, it's an automotive alternator for me. A 5 minute test at the local shop can save hours of debugging specialty 'aircraft' devices.

[Zuldarin]

Woo hoo, glad you found it Jeff. Although hearing that a PP is just as susceptible doesn't make me any happier. :-)

[alpinelakespilot2000]

Jeff,
I had a similar problem with my PP alt blowing the field fuse and/or tripping the breaker at about 50 hours. After talking to PP they suggested the fuse might be blowing too quick. I replaced it with a fuse link constructed per Bob Nuckolls and have had no problems since, with either the fuse link or the breaker. Perhaps a slow blow fuse will be your solution.

[Rupester]

... a very interesting problem, troubleshooting, and resolution. All these experiences are helpful and illustrative for the rest of us.

[vlittle]

Originally Posted by alpinelakespilot2000

Jeff,
I had a similar problem with my PP alt blowing the field fuse and/or tripping the breaker at about 50 hours. After talking to PP they suggested the fuse might be blowing too quick. I replaced it with a fuse link constructed per Bob Nuckolls and have had no problems since, with either the fuse link or the breaker. Perhaps a slow blow fuse will be your solution.

Just so there is no confusion here... My pp alternator ovp was defective, it was not the fuse or breaker in my case. The ovp was tripping around 14 volts. The repaired pp tripped at about 17.5 volts. I went through the regimine recommended by pp. they even wanted me to replace my brand new odyssey battery.

Once i built a similar bench set to what jeff did, the results were obvious. I used a voltmeter and an oscilloscope to verify everything. It was a bad alternator... More specifically the internal ovp/regulator module. It would have been a 30 second test at the factory before it was shipped.

Big thanks to jeff for publishing his setup. As i mentioned earlier, you can use a half-inch electric drill to drive the alternator if you don't have a spare electric motor sitting in the hangar. Btw, this would be a good project for a local eaa chapter to build up... I think alternator failures are approaching 100% over the last 5 years for the rv folk i fly with, including my two.

[Darrel Starr]

Jeff, I just ran across your post searching for a similar experience to mine. My 5 amp Potter& Brumfield field CB tripped as I was reducing power on downwind in my Super Cub. I have a 40 amp B&C alternator and the same B&C voltage regulator as you have. Hours on the Super Cub are 324 vs 375 for your RV.
Greg Jones, General Manager at B&C, has been great emailing back and forth. They now have my regulator to be checked out next week. I ran through the 2 page B&C trouble shooting guide and all voltage & resistance readings were in the acceptable range. I have had the Odyssey SBS J16 (PC680) battery checked out by "Batteries +" for cranking amps (228) and holding a 100 amp load for 10 seconds -- OK. It holds a charge just fine.
Before landing, I tried to reset the circuit breaker a couple of times -- I now understand that that is a no-no risking more harm.
The only thing I have found "wrong" so far is a "weld berry" on the triangular piece inside the circuit breaker but that might be a result not a cause.
My question, Jeff, did your problem stay solved when you installed the new regulator?
Thanks, Darrel Starr