ALT LOW VOLTS bus breaker keeps popping

[warrenehc]

Hello,

Over my last flight I had my ALT LOW VOLTS

Fuse that is in my battery bus section. I pushed it back in and all was good. It did it another time close my home airport, pushed it in again and all was good.

I am at the airplane to see if I can figure it out and am having no luck. 
 

When all power is off I can push it in and it stays, but if I turn the battery master it pops and will not go back in. It’s a 2 breaker.

I have a 2 year old new alt. When in flight it produces 14.2 volts regularly.

Anyone have any suggestions? 
 

Thanks in advance.

Edited August 25, 2024 by warrenehc

[Vance Harral]

That's not the original panel in from a 67F, so it seems this airplane has custom avionics.  I have factory schematics for various vintages of the M20F, but I'm not seeing any circuit breakers in them labeled ALT LOW VOLTS.  Avionics shops are supposed to provide the customer with new schematics when a major change like this is made, but the shops almost never do that.

So... just a guess... ALT LOW VOLTS doesn't strike me as having anything to do with the alternator itself.  Rather, I'm guessing that breaker (it's a circuit breaker, not a fuse) powers a system that lights a warning light and/or sounds a buzzer, when bus voltage falls below what the voltage regulator is trying to hold.  Do you have a "LOW VOLTAGE" light in your panel?  If so, it's probably powered by that breaker, and won't work if the breaker is tripped.  The light may be failing in a way that is causing high current, and therefore causing the breaker to trip.  It's also possible the breaker itself is just worn out and is tripping even though the current it's carrying is normal - this does happen on occasion.

Suggest you check the bulbs and/or wiring associated with your LOW VOLTAGE light/buzzer, if you have one.  But again, this is all just a guess.

[EricJ]

+1 that you need a schematic or need to trace out what that breaker is protecting.   I've no idea what an ALT LOW VOLTS breaker might be protecting.   

Does anything stop working when that breaker pops?

 

[warrenehc]

Yes, volts go from 14.2ish to 12.2ish while in cruise.

[EricJ]

22 minutes ago, warrenehc said:

Yes, volts go from 14.2ish to 12.2ish while in cruise.

Your airplane appears to have been re-wired in an unusual way.   Is there a supplement in your POH or in your ICA paperwork or anything that suggests what that breaker does?   Whoever did that should have provided documentation reflecting the change, and if there isn't any documentation, somebody will likely have to trace that out and see what it is doing.   It sounds like maybe it is turning off the regulator, but the ALT FIELD breaker is usually what does that, and you have one of those, too.

[dzeleski]

This might have something like a B&C LR3D-14 Linear Regulator installed. The 2amp breaker is a sense line.

 

https://www.aircraftspruce.com/catalog/eppages/lr3reg14.php

https://www.aircraftspruce.com/catalog/pdf/07-06742-Wiring.pdf

https://www.aircraftspruce.com/catalog/pdf/07-06742-Manual.pdf
 

 

Wiring schematic shows it as a bus voltage sense. The troubleshooting doc says it’s used for voltage regulation and a low voltage alarm. It specifically says this circuit is required for the alternator to function.

Youll need to check the installed parts to confirm my wild ass guess however

Was any new equipment recently installed? If this guess is correct it seems there is a specific voltage drop minimum from the bus to the pin that is the voltage sense. You may also just have a bad connection or chafed wire somewhere.

Hope that helps!

Edited August 25, 2024 by dzeleski

[Yetti]

I would guess a short.   Could be a short at the panel, but probably a short at the alternator.   Did they do a new wiring harness with the new alternator?  Probably not.   Do all new wires to the alternator.    Could be simple like a wire fell off.   Use a phone to snap picture behind the panel.     right side engine cowl to check the wires to the alternator.    Could be a short in the alternator.

I spent 4 hours diagnosing a battery issue on a fire truck yesterday.    Then started the truck and found that voltage on the firewall would not get above 12.1 volts.    We are replacing the alternator today.

[shawnd]

You could use a continuity tester to see if the wire goes to the typically suspect locations if you don't have the schematics. Of course, with an A&P and follow all the safety precautions first! 

Typical suspect locations:

1. One of the terminals on the back of the alternator
2. Voltage Regulator
3. ALT field switch
4. Back of the annunciator panel (guess)

Could use the Wire Tone Tracer to trace it too. Will take time but doable.

[Aerodon]

A PlanePower alternator control unit (ACU) has a 'fault lamp' that operates when there is a fault by provide a ground for a lamp and 1A CB.  I think my 252 has something similar for 'low volts'.

There is not much that can go wrong with this circuit other than a short in the wiring.  And if you are having problems with this annunciator while the voltages appear fine, more than likely to be a wiring short.  Not difficult to trouble shoot either, apart from the intermittent nature.

 

Aerodon

 

 

[warrenehc]

New wrinkle.

I can get the breaker to reset. But when I add any throttle it pops. 
 

Any specific ideas. Different than the above?

[RoundTwo]

Do you have an engine monitor you can download data from?

[warrenehc]

On 8/25/2024 at 1:36 PM, dzeleski said:

This might have something like a B&C LR3D-14 Linear Regulator installed. The 2amp breaker is a sense line.

 

https://www.aircraftspruce.com/catalog/eppages/lr3reg14.php

https://www.aircraftspruce.com/catalog/pdf/07-06742-Wiring.pdf

https://www.aircraftspruce.com/catalog/pdf/07-06742-Manual.pdf
 

 

Wiring schematic shows it as a bus voltage sense. The troubleshooting doc says it’s used for voltage regulation and a low voltage alarm. It specifically says this circuit is required for the alternator to function.

Youll need to check the installed parts to confirm my wild ass guess however

Was any new equipment recently installed? If this guess is correct it seems there is a specific voltage drop minimum from the bus to the pin that is the voltage sense. You may also just have a bad connection or chafed wire somewhere.

Hope that helps!

Yes, you are correct. It’s a B&C.

we checked the alternator and it was fine. Believe we violated it to somewhere between the battery and the breaker. With everything off, and when I turn on the master that is when it pops..

We removed all wires from that regulator one at a time and it kept popping we removed them all and isolated the regulator and it still popped.

No new equipment installed.

replaced breaker still pops.

we disconnected everything around the battery and cleaned all of the terminals and reconnected everything. Still no luck.

prior to it completely failing it would pop and I would be able to push it back in and it would all go back to normal. I did this for two flights of three hours total time now, when it pops, I can push it back in, but it will not stay engaged.

Edited October 2, 2024 by warrenehc

[MikeOH]

Sure sounds like you need to carefully trace the full length of the wire off that CB to find the short.