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Over voltage

MarkCFI

Active Member
I purchased an already built rv-4. I had some electronic noise issues and replaced the alternator when it tested bad.

After start when I engage the alternator switch the voltage reads just about what it should. A little over 14v at 1000 rpm.

However at higher or cruise setting the voltage seems to fluctuate and at times bumps up to a little over 15 volts.

It’s an external regulator (C-8010-1) and not a very complex circuit. Would the fluctuating voltage that runs high typically be the regulator going bad? Seems like that would be the most likely culprit but I thought I would ask as it’s not in a very convenient place for removal

V/R
 
Any time I had high voltage it was a bad connection with the field wire.
When the external regulator does not sense good voltage it just increases the output voltage of the alternator.
 
I'd agree with Mark. If there is a bad connection the voltage regulator will sense a lower voltage. Then it will try to correct and raise the voltage, but if the connection is not good it won't be able to sense that the voltage is higher and it will continue to try to raise the voltage higher. Also at low RPM the alternator can only supply a portion of its full output so that would explain why the voltage is more reasonable at low RPM.
 
Good idea,

I will check that. The wiring is actually not a distinct plug at the alternator but the individual wires for field and ground with a spade connector on each.

Thanks
 
Just another data point, while I agree the most likely cause is the field wire, I had this exact same issue recently with my '96 vintage RV6A. Started out a few months ago having the voltage fluctuate during cruise and over the next few flights it got worse. I checked the wiring and there was no issue. So I replaced the Ford external voltage regulator from the aviation section of O'reilly's and that solved the problem.

$30ish and lifetime warranty.
 
Can even be a fuse

I had that problem, and it came down to a fuse in the field wire circuit that was acting as a variable resistor.
 
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The most common cause of over voltage with an externally regulated alternator is excess resistance on either the voltage sense feed (A terminal) or the ground. This causes the regulator to falsely think that bus voltage is low and up's the output with more current on the Field. Excess resistance on the field will often cause the breaker to trip, as the VR keeps increasing field current to keep up the voltage.

Larry
 
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The most common cause of over voltage with an externally regulated alternator is excess resistance on either the voltage sense feed (A terminal) or the ground. This causes the regulator to falsely think that bus voltage is low and up's the output with more current on the Field. Excess resistance on the field will often cause the breaker to trip, as the VR keeps increasing field current to keep up the voltage.

Larry

Bingo.

The trouble is not on the field wire (which sends current to the alternator feed to make power), it's on the voltage sense wire, which is what the regulator uses to determine HOW MUCH power to send to the field wire.

Field wire problems don't create an overvoltage scenario - sense wire problems do. Field wire problems show up as low output.
 
All good info.

I pulled the cowl and cleaned and tightened up the spade connectors on the alternator. There’s only a ground wire ( which seemed not real tight on the connector ) and the field wire connector. Also the output wire which is bolted on.

I did a quick test flight after runup and did not get the high voltage reading. The voltmeter does seem to jump a little between 13-14 volts. I’m not sure that’s abnormal but I actually haven’t ever had a voltmeter in a plane only ammeters (charge -discharge). Does a voltmeter read rock steady ?

I’m going to do a little longer flight weather permitting to see if the over voltage is really gone. I will also redo the connectors on the regulator as a final check and if it’s not stable will replace the regulator as a start.
 
All good info.

I pulled the cowl and cleaned and tightened up the spade connectors on the alternator. There’s only a ground wire ( which seemed not real tight on the connector ) and the field wire connector. Also the output wire which is bolted on.

I did a quick test flight after runup and did not get the high voltage reading. The voltmeter does seem to jump a little between 13-14 volts. I’m not sure that’s abnormal but I actually haven’t ever had a voltmeter in a plane only ammeters (charge -discharge). Does a voltmeter read rock steady ?

I’m going to do a little longer flight weather permitting to see if the over voltage is really gone. I will also redo the connectors on the regulator as a final check and if it’s not stable will replace the regulator as a start.

If you are using a ford type VR, the ground comes from the case, not a wire. You also need to trace the sense wire all the way back to the bus. I had this problem and it turned out to be a screw that loosened on the breaker that feeds the V/R A lead.

Larry
 
Just to add a couple of comments to previous items for clarification. The regulator uses the sense wire to sense the bus voltage. That could lead you to believe that the problem could only be with the sense wire. The thing to realize is the the voltage sensing is relative to the regulators ground. So if there is resistance in the ground path then the regulators ground could be above the actual frame ground. The result again could be a higher alternator voltage. The regulator may actually be regulating to the correct voltage relative to its local ground but there could be an additional voltage drop in the connection from the frame ground to the local regulator ground. This is all from a conceptual point of view mounting specifics and ground attachment methods could eliminate the likelihood of this being the case.
 
I very much appreciate everyone’s advice.

After insuring the alternator connectors were good and tight I noticed at higher power settings the voltmeter needle “wanders” a bit. Probably through a range of a couple of volts.

I purchased a cheap digital voltage display that plugs in to the 12v accessory port and it seems pretty much stable at 14.5 volts. I don’t know how sensitive it is or whether it can detect fluctuations in voltage as displayed by the analog gauge.

So as suggested I’m going to insure all connections to regulator and now also voltmeter to include grounds are secure. If that doesn’t fix analog voltmeter I’ll replace regulator and if then no improvement I’m going to be a little suspicious of the gauge itself.

I’ll post again as I work through those options.

Thanks again
 
An Analogy, knowing your known good values

Once had a boat that just could not produce spec RPM/thrust on its diesel package as seen on the engine tach.

I had the screw (prop) reshaped, fussed with the governor and injectors, you name it......

Finally got a strobe on the output shaft and realized that my issue was just a gauge problem.

Get a quorum of voltmeters riding the suspect circuit and vote out the misbehaver?
 
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