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Alternator failure, can’t find cause.

cdeggz

Well Known Member
It seems my backup alternator has (or wiring or regulator) has failed. It’s a 40amp B&C with LR3C regulator.

Alternator one shows 14 V and current flow. Alt two instead makes the bus voltage drop to 12V and no current.

I have gone through the entire B&C troubleshooting guide and find no issue.
Oh
I callled B&C tech support and they are stumped. Alternator has been on the engine for 200 hours and used hardly ever (checks mostly).

Here’s what I have checked so far:
Battery to pin 7: .2 ohms
Battery to engine case : .4 ohms
Master on (primary battery) bus voltage: 12V
Pin3 voltage: 11.93 volts:D
Pin 6: 11.76V
Pin 4: 10.53V
Alternator field resistance: 7 ohms
Field voltage 10.5V
B-lead: 11.98 volts

Yesterday I did an engine run while checking voltage at B lead and around the fuse and shut, all showed battery voltage (12V)

I made sure the alternator turns inside when I move the prop.

Any other ideas?

-Craig
 
...
Any other ideas?
...

Hi Craig, between this and your issue with starting, I think you need to get a second pair of eyes on your wiring. Is there a nearby EAA chapter that might have a Bob Nuckolls disciple that might be able to come over and have a look?
 
You do know that vacuum pad alt won’t produce anything below around 2k rpm right?
 
If you do really see field voltage, AND you are spinning it at a reasonable RPM (not sure 2000 is actually required, mine puts out well below that), then it's time to suspect an alternator internal problem. At that point I would take it off the engine (yes, that lower left nut is no fun, but if you have torched a wrench into the right shape to reach it, it's not too bad...). On the bench, adapt a 12 point socket or a short piece of stiff hose with the right inner diameter and reducers to be able to drive he spline (with care, not to damage it) with an electric drill, feed the field from a DC power supply and vary it from 0 to something less than 12V with a load, say with some 12V light bulbs and see what you get. If the way you measured it in place is correct, then it probably won't put out on the bench either.

You could do the same test on the plane, disconnecting the output and field, essentially taking the regulator out of the picture and forcing the field with a power supply and putting on your own non-battery load. Since you do measure a field resistance, it seems ok, so the only other possibility is a broken internal connection to the output or from the stator windings to the rectifiers. Rectifier failure is unlikely, as multiple diodes would need to be open...

Good luck!
 
Regulator LR3C

Please note that the LR3C regulator is intended for Primary Alternator, not for the Standby Alternator. The regulator for the standby should be SB1B.
The voltage setting on Primary Regulator is 14,4 V and 13,0 V for Standby.
If the system Voltage reads below 13,0 V the Standby Alternator is activated.
You can find B/C documents here.
https://bandc.com/reference-library/

Good Luck
 
I assume this is a new issue, as this alternator did work at some point. Did you look to see if the drive gear is still intact?

Carl
 
LR3 as backup regulator.

I know it's thread drift but some clarification.

Be aware the setpoints of both the LR3 and SB1 regulators are field adjustable.

Bob Nuckolls does not recommend SB1 "for simple dual alternator installations". There's nothing wrong with using it but voltage sag is a better indicator of overload than the hall sensor and lamp.

A Bob Nuckolls quote:

=====================================

Posted on Aeroelectric List Mon Sep 24, 2018
.
.

The stand-by versions of the alternator controller are not recommended (by Bob) for simple dual alternator installations where the auto-switch feature is not necessary... this was a 'bell-n-whistle' aimed primarily at the STC market for TC aircraft that were receiving the pad driven, aux alternators.

Yeah, 13.0V is a bit too low . . . 13.5 is better. Will talk with B&C about this.
.
.

Bob . . .

======================================
 
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Mickey, that will be the next step post-omicron if none of the ideas below work out for me.

Walt, I did not know 2k was the lower limit, but I have been doing my testing at cruise.

N49ex, yeah I was hoping to avoid pulling the alternator but it sounds like reality is setting in. Thanks for all the tips. Very helpful!

Avanza, thank you. I'll double check and make sure I got the right number. It's a simple dual alternator setup. Both connected to the main bus and individually activated using switches on the panel.

Carl, yes it's a new issue. I can see the alternator innards spinning when I turn the prop. Looks like I'll be pulling the alternator next for a better look.

John, thanks for the data and insight. This could help with troubleshooting.

You all are wonderful, thank you!
 
Did you pull the plug from your alternator and check to verify your getting 11.5 is volts at the plug. Just insure power is getting from the switch and/or breaker to the alternator.
 
Have you tried bypassing the regulator (short, temp test) like we use to do on our old cars?
 
Have you tried bypassing the regulator (short, temp test) like we use to do on our old cars?

Ed I like this idea. Are you thinking 12V straight to the field? I assume I would need to hook the output (b) up to something that is not the plane?
 
Hey Mich. O was seeing a cure t on the field that matched the other alternator. I want to say it was a round 4 amps but I can’t remember exactly.

I guess that is the quick summary:
-Field current seen
-B-lead voltage only at 12 V
-no current from alternator
Those make it look like a faulty alternator, but:
-Alternator shows 7 ohms
-alternator turns with the engine
Which makes the alternator sound like it should be working.

Seems pulling the alternator for further inspection is warranted.
 
Craig------as you are also having ignition issues as posted in another thread, I would look for any common link between the two.

Start with the grounds, my $0.02.

It might be time to bring in someone experienced in electrical systems to take a look at your plane.

Good luck, let us know what you end up finding.
 
Hey Craig,

My 40 amp breaker on the power supply from the alternator had popped and I reset it but had the same type of issue and could not track it down.

Found that there was no continuity across the 40 amp breaker !
Breaker replaced and all was good
 
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Hey Mich. O was seeing a cure t on the field that matched the other alternator. I want to say it was a round 4 amps but I can’t remember exactly.

I guess that is the quick summary:
-Field current seen
-B-lead voltage only at 12 V
-no current from alternator
Those make it look like a faulty alternator, but:
-Alternator shows 7 ohms
-alternator turns with the engine
Which makes the alternator sound like it should be working.

Seems pulling the alternator for further inspection is warranted.

If you are seeing 12V at the output, that means the breaker is good and you are seeing the backfeed from the battery, and it is not rising above the static battery voltage because the alternator it in fact not putting out current. Before pulling it out, as others have said, you could force it in place- it's ok to put 12V on the field, disconnect the output from the plane, and put a decent load on the output, something like a heater or a pair of 12V light bulbs in series (with 12V on the field it would put out more than 14V!), and see if it puts out anything. That is the dirt-simplest configuration which should ferret out for sure if the alternator is bad. B&C would probably fix it readily at that short age....
 
Recheck the "B" lead voltage at the alternator. But this time connect a high
wattage lamp in parallel with the voltmeter. Expect to see battery voltage.
A load such as a lamp will make problems show up better.
-
The alternator might NOT be turning fast enough if the drive coupling is slipping.
Field current of 3 or 4 amps is reasonable.
 
Grounds

As was alluded to earlier, there may be common issues that point to something other than alternator.
I would check all the grounds from engine to firewall to battery to alternator's regulator.

Just my $.02
 
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