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Bus Voltage HIGH

ExtraKatana

Well Known Member
I have a call into B and C but I thought I would pull the immense knowledge as I’m sure several of you have the same set up. L-40 alternator and LR3D-14 external voltage regulator.

Bus voltage 13.0 terminal three voltage 13.0 terminal four voltage 11.57. Terminal six voltage 12.86…… That is before I start the engine and engage the alternator.

Within two minutes after starting the engine my voltage slowly creeps up to 16 at which time the panel circuit breaker attached to terminal six engages and takes the field out of the system. I have tried to adjust the very small screw driven voltage setting on the regular with no luck.

Thoughts?
 
Measure at the alt itself, should be within .5 of term 4.
Also suggest monitoring reg term 3 when it starts creeping up as that is the sense term.
 
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Measure at the alt itself, should be within .5 of term 4.
Also suggest monitoring reg term 3 when it starts creeping up as that is the sense term.

I did and it is. That test would reveal basically a bad wire or connection. Why would that all the sudden go bad?
I didn’t do all of the Ohm tests like between terminal 6 and the engine case but I did do this one. All of the connections are tight.
 
I did and it is. That test would reveal basically a bad wire or connection. Why would that all the sudden go bad?
I didn’t do all of the Ohm tests like between terminal 6 and the engine case but I did do this one. All of the connections are tight.

Did you do it with the alt plugged in?
I see bad wiring/switches all the time for multiple reasons (especially cessna style switches and cheap breakers).

And verify the reg case/term 7 has a good ground.
 
Yep

Alt was plugged in per instructions and ground is good. No Cessna switches here
Going to do an Ohm check this afternoon. Thats all thats left
 
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ALT PROBLEM

If the problem persist, I suggest removing the alternator and make a new
crimp on the F-connector. Put silicon on so the wire can´t vibrate.
I had this problem.:eek:
 
Tests

An individual at B and Chas been very helpful so far.

I have done the ohms test between pins seven and the negative contact on the battery and the result is .1 with a continuity buzzer. The same test was done between the negative post on the battery and the engine case. .1 again with a continuity buzzer.
 
Just me

If the guy at B&C doesnt recommend anything further, I would put a voltage meter across the regulator from the bus sense to the ground (on the reg) and see if the voltage changes. That should be rock solid.

If good than there is a wiring problem, most likely either ground or bus sense. By the way, what is on the bus sense circuit?

JMHO
 
Thanks Mike. That would be the input for terminal 3 on this regulator.

Since the problem manifests while the engine is running, should this test be done with engine running? Master on? .
 
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Ohm check

Thanks Mike. That would be the input for terminal 3 on this regulator.

Since the problem manifests while the engine is running, should this test be done with engine running? Master on? .

The first test is to make sure the bus sense on the regulator is hooked up to something. Then ohm it out and make sure it senses bus voltage (very low resistance to battery or main bus)
 
The job of the voltage regulator is to shove current into the field of an amount needed to make the regulation voltage (at terminal 3) come up to the set point. Presumably, the regulation voltage is very close to if not pretty much the bus voltage. (it should be provided that way,for example with a separate or low load breaker)

So, the first thing is to verify what in fact is showing up at terminal 3. You will need to either bring that out with a wire, or be able to access it with a reliable meter right at the terminal, with the engine running. The most likely situation will be that the regulation voltage at terminal three is not the bus/battery voltage. So if it is low or not connected at all, the regulator will interpret that as a command to jack up the field current and thus the bus/battery voltage, until an over voltage trip occurs. If it turns out that the regulation voltage is the same as the bus voltage, and tracks up with the bus voltage, then likely the regulator is defective. (like for example a broken circuit board trace from terminal 3...)
 
The voltage at terminal 3 is almost the same as the Bus voltage. It's terminal 6 that is going up outta sight and popping the CB on the panel. Resting voltage on terminal 6 is very close(.2 volt lower) to bus voltage also. When i start the engine it's another story however.
 
I chased a problem like this for a while. Turned out the screw holding the ring terminal on the back of the field breaker had become loose and was adding resistance to the sense circuit, thus driving up the regulated output voltage. I have a std ford style regulator, which takes the sense voltage from it's primary input, unlike the B&C. The screws has lock washers, but obviously not strong enough.

Best to confirm good continuity on the full sense input wiring.

Larry
 
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ALT PROBLEM

If the Field wire at the back of the L40 alternator is bad, the F-current
will be chopped (on/off/on/off) when engine is running due to vibration.
The voltage from the alternator will then be lower than required.
Bus and sense wire voltage will be lower than the regulator set point.
The regulator will try to increase voltage fed through the Field wire to the alternator. When the field wire at the back of the alternator makes contact,
the voltage at the bus will be so high that the regulator trips the circuit breaker.
Just a thought.
:eek:
 
I’ve done all the voltage and Ohms tests. Terminal 3 is hooked to its own fuse on a fuse block. Not jumpered. Will look closely this evening for loose connections.
 
Tim,

Where are you taking your voltage measurements? You want to see what the voltage regulator is seeing. So you should use the ground on the regulator when you measure the sense voltage on the regulator. When you run the engine if the voltage goes up to the 16V then the regulator isn’t regulating correctly. If that voltage is low for some reason then the regulator will try to push the voltage up.
 
I didn’t check voltages to ground in that manner but I will this evening. The ground wire showed continuity to the terminal and was .1 ohms
 
The voltage at terminal 3 is almost the same as the Bus voltage. It's terminal 6 that is going up outta sight and popping the CB on the panel. Resting voltage on terminal 6 is very close(.2 volt lower) to bus voltage also. When i start the engine it's another story however.

This doesn't make sense. Terminal 3, as the regulation input, should be the same as the bus voltage, which you say apparently it is. But terminal 6, as the bus field supply input, should also be solidly at bus voltage, being separated from the bus only by a field breaker, and perhaps a master switch contact. So, if you say it is going up, it shouldn't, can't if wired/measured right. So, time to verify all wiring, grounding, and measurement technique to get past what doesn't make sense.

Again, if the bus is in fact for sure going up, then the sense input should follow, so if it isn't, then there is a sensing continuity issue.
 
Yep. 6 is climbing to 16V and popping the CB. Sense voltage is steady and consistent with the engine not running.
Pulled the teg this evening. Going to BandC Monday.
 
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schematic

You know, a schematic of the electrical system around the reg and alternator would be helpful. We are all assuming it is what we think it is, but only a schematic would make all our visions correct. You know what they say about assuming....
 
I had this same issue last year when my Plane Power died and I replaced it with a B&C - my initial run tests and flight tests, and all the troubleshooting followed yours exactly. Eventually I ended up at the hangar with one of their guys on my phone, and I ran through all his troubleshooting requests with a multimeter with no joy. He said "Send back the regulator and we'll send you a warranty replacement."

I sent it in, they cross-shipped one to me, and I asked them to please let me know if the regulator bench-checked out OK - so that I would know for sure the problem was on my end. I never heard back from them, and the replacement regulator worked perfectly.
 
These regulators have a series power transistor from bus to field that is driven to provide the appropriate field current. That transistor is the one mounted on the case, and is a stressed device. When they short out (A common failure mode, which for example can occur by over heating of the transistor) the regulator shoves maximum current into the alternator, which is exactly how this kind of over voltage behavior can occur.
 
Bad regulator

sometimes you just have to bite the bullet and replace something. Installed a new LR3C Dash 14 and a five minute idle has voltage rocksolid at 14.3 V. I guess I’m within that 10% that experienced a problem but didn’t wire it incorrectly. I learned however and thank you for your help.
 
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