Alternator failure, can’t find cause.

[cdeggz]

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
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

[rv8ch]

Originally Posted by cdeggz

...
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?

[Walt]

You do know that vacuum pad alt won’t produce anything below around 2k rpm right?

[N49ex]

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!

[Avanza]

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

[Carl Froehlich]

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

[johnbright]

I made a summary of vacuum pad alternator current output from the spec sheets, ref attached image.

[johnbright]

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:

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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.
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.

Bob . . .

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[cdeggz]

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!

[togaflyer]

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.