Alternator field C/B

[Larry]

I have a 90J (24 volt) with a 400 hour Lycoming reman which included a new alternator.   Starting 25-30 hours ago, I will get a field C/B trip every 8-10 hours.  After 1 reset it will not trip for 4-10 hours.  At annual,  the field switch wires were checked and sprayed with contact cleaner, the field wire and T2 wires at the alternator were replaced.  They looked fine, but everything was opened.....  Yesterday, about 6-7 hours after annual, the field CB tripped again.  Tomorrow, I'm checking the regulator and master switch wire connections.

Any suggestions?

[EricJ]

Look for potential intermittent shorts between the field wire and its shield, or other local grounds.    Finding intermittent stuff is always hard, but sometimes you'll spot a wire hair from the shield somewhere it shouldn't be, like near the field wire conductor.   This usually happens near a connection, and there may be a cannon plug at the firewall where the shield and conductor are split on both sides of the connection.

Just a thought.

 

[Ragsf15e]

6 minutes ago, EricJ said:

Look for potential intermittent shorts between the field wire and its shield, or other local grounds.    Finding intermittent stuff is always hard, but sometimes you'll spot a wire hair from the shield somewhere it shouldn't be, like near the field wire conductor.   This usually happens near a connection, and there may be a cannon plug at the firewall where the shield and conductor are split on both sides of the connection.

Just a thought.

 

Maybe another thought is replacing the field cb?  It’s unlikely the cause but it’s cheap, easy, and you’ll make sure all the connections there are good.

do you by chance have an engine monitor?  You might set it to 1 second recording and look to see if it catches any electrical excursions at the time the cb pops?

[Vance Harral]

4 minutes ago, Ragsf15e said:

Maybe another thought is replacing the field cb?

Circuit breakers definitely wear out, especially if they are manually pulled from time to time.  Agree that it's not particularly likely to be the problem, but it's definitely in the the set of things to check and/or consider replacing.

[laytonl]

When does the cb trip?  Is it during flight or when you shut down?  
 

lee

[Stephan Kablitz]

HI Larry,

we had the exact same behaviour. Until this spring (fortunately 15 minutes out of the home airfield) it culminated in an alternator failure.

We identified the voltage regulator as the villain (black spots on the PCB), sent it to the Don Maxwell correction facility and now we have no more Field CB trips.

Brgds, Stephan

 

Edited September 28 by Stephan Kablitz

[Larry]

11 hours ago, laytonl said:

When does the cb trip?  Is it during flight or when you shut down?  
 

lee

Lee -

It has always tripped during level flight.  Last time it was 30 min after take-off.

Larry

 

[takair]

Generally speaking, alternator regulators have some form of “crowbar” over voltage protection.  When they detect over voltage, they intentionally cause the circuit breaker to trip.  You might tap in a recording volt meter to see if your voltage spikes.  This can also occur if the regulator is failing, as shown above.  

[laytonl]

This does seem to be pointing to the regulator.  Lee

[MikeOH]

WOW!  That's some pretty crappy solder workmanship on that CCA.  Looks like the transistor on the left was replaced at some point and they just tack soldered to the old leads!  Combined with the blown trace I sure hope you got a NEW CCA and not a repair of this one!

[Larry]

I called Don Maxwell to see if this sounded like a failure mode of the regulator and to have him repair it.  His recommendation was to replace the diode that goes from the F1 to ground as it's probably bad.  He said that it's probably getting an occasional spike which is tripping the breaker.  I've looked through the parts manual and don't see a callout for the diode.  I'll check the schematic for my plane, but I don't have the paper version, so it always takes twice as long with the digital version.

[PT20J]

18 minutes ago, Larry said:

I called Don Maxwell to see if this sounded like a failure mode of the regulator and to have him repair it.  His recommendation was to replace the diode that goes from the F1 to ground as it's probably bad.  He said that it's probably getting an occasional spike which is tripping the breaker.  I've looked through the parts manual and don't see a callout for the diode.  I'll check the schematic for my plane, but I don't have the paper version, so it always takes twice as long with the digital version.

The way to find electrical part numbers is to look on the schematic for your serial number and find the code for the part (on mine it is D1). Then look in the back of the service manual where the electrical components are listed and find the section for your serial number and look up the code and it will list the vendor and part number. For my airplane it is listed as a 1N2483 or 1N5060 or 3720GE.

[Larry]

Skip

Thanks for the info, I ordered it this afternoon.  As they say, start with the cheap things.

Larry