Please help me understand my alternator field switch circuit configuration

[Kooshball]

I am troubleshooting an issue preventing the correct voltage from reaching my B&C voltage regulator. In trouble shooting the alternator field switch in the panel, I found it wired as shown in the photo. The “M” is the master switch and the “F” is the field switch with is actually a 5a toggle breaker.

I am trying to understand why the field switch is connect to the bus and the master rather than being connected directly to the voltage regulator as shown in the recommended wiring diagram from B&C.

Any thoughts on this installation?

Thx!

 

[Bill.Peyton]

Without further information, which VR and Alternator, or your current schematic, its tough to tell. It does appear that the field is wired through the master to prevent the field from being energized without the master switch on. Where is your voltage sense wire connected? On Lr3b its pin 3 of the VR

 

[Kooshball]

Without further information, which VR and Alternator, or your current schematic, its tough to tell. It does appear that the field is wired through the master to prevent the field from being energized without the master switch on. Where is your voltage sense wire connected? On Lr3b its pin 3 of the VR

Bill this is on an RV4, with a new LR3B regulator and a freshly overhauled B&C SD-20 alternator. Pin 3, is the sense wire connection point.

 

[Avanza]

As I understand it the installation is correct and conforms to lr3b manual.

Good luck

 

[Bill.Peyton]

Where is pin three connected to your buss

 

[Mich48041]

Looking at your picture, it appears that your voltage regulator is wired exactly like B&C's DIAGRAM.
Power comes from the bus, through the 5A circuit breaker to the bottom left corner of the master
switch, then out of the bottom right corner of the master switch to the LR3B regulator.
To trouble shoot, connect the red probe of your volt meter to the bus and the black probe to the lower right corner of the master switch. With both the alternator breaker-switch and master switch turned on, the meter should read pretty close to zero volts.
If the meter reads more than a few millivolts, one of the switches is bad.

 

[Kooshball]

Looking at your picture, it appears that your voltage regulator is wired exactly like B&C's DIAGRAM.
Power comes from the bus, through the 5A circuit breaker to the bottom left corner of the master
switch, then out of the bottom right corner of the master switch to the LR3B regulator.
To trouble shoot, connect the red probe of your volt meter to the bus and the black probe to the lower right corner of the master switch. With both the alternator breaker-switch and master switch turned on, the meter should read pretty close to zero volts.
If the meter reads more than a few millivolts, one of the switches is bad.

Thanks folks! With the reg connected I’m only getting 2.4V at terminal 6, with that wire removed from the reg it reads bus voltage. This is something new that showed up after I installed the new regulator and also manifests itself with the loaner regulator that B&C sent me so two regulators are behaving the same way.

 

[Ruready]

Is the alternator not working? What is your bus voltage when the alternator is online?

You should have bus volt at the input to the regulator pin 6, the alternator rotor spins and gives love to the stator and than the diodes take the alternating love and give the direct to the bus, than the regulator checks the bus volts and modifies the field volts output pin 4, to maintain the correct bus volt.

 

[Kooshball]

Is the alternator not working? What is your bus voltage when the alternator is online?

You should have bus volt at the input to the regulator pin 6, the alternator rotor spins and gives love to the stator and than the diodes take the alternating love and give the direct to the bus, than the regulator checks the bus volts and modifies the field volts output pin 4, to maintain the correct bus volt.

I am not charging. 2-weeks ago I had bus voltage on terminal 6 with everything hooked up; now less than 3

 

[Ruready]

Ok so with the field wire from the master switch connected to pin 6 on the vr you have less than 3 volts on pin 6? And if you disconnect the wire from pin 6 you have 12 volts?

 

[Ruready]

If the wire has 12 volts on it before conn to the vr pin 6, and as soon as you touch it conn to pin 6 it goes down to 2 volts, you have a problem with the vr. To confirm the vr is bad, you simply connect the field wire directly to the wire that connects to pin 4, CAUTION TURN OFF AVIONICS AND EVERYTHING BEFORE DOING THAT. As long as you just do this momentarily it wont hurt anything, you will simply be full fielding the alternator, and the voltage will rise past 14 volts, my 24 volt system went to 36 volts when I bypassed my vr to confirm the vr was bad. So disconnect pins 6 and 4 and connect them together and than start the engine and see what your voltmeter shows

 

[Ruready]

You could run a new test wire from pin 4 to the alternator, or one from the bus to pin 6. If you have two VRs showing the same bad signal than maybe it is downline at the alternator that is causing the voltage drop… or could be that wire from the master switch to pin 6 .. as soon as it takes a load it drops voltage…. either way very qiuck and easy tests to figure it out.

 

[Ruready]

To trouble shoot, connect the red probe of your volt meter to the bus and the black probe to the lower right corner of the master switch. With both the alternator breaker-switch and master switch turned on, the meter should read pretty close to zero volts.
If the meter reads more than a few millivolts, one of the switches is bad.

I have not heard of this kind of test, both probes on hot points, is that correct or did you mean one probe on ground?

OK, I think I get what you are trying to test, if its to see if the switch shorted to ground… than there will be more problems with voltage going to ground though.

 

[Kooshball]

Ok so with the field wire from the master switch connected to pin 6 on the vr you have less than 3 volts on pin 6? And if you disconnect the wire from pin 6 you have 12 volts?

Correct. For the full picture here is the series of events (all ground based trouble shooting done with the engineer from B&C on the phone with me):

-Takeoff for a local flight and get a low voltage warning on climb out.
- trouble shoot by toggling the field switch and alternator breaker…still no output from the alternator.
-print out and follow the troubleshooting guides from B&C for the regulator and alternator.
-I found the voltage at the alternator field to be 10v when the alternator was plugged in (should be 12.3), then tested the resistance of the field terminal to the alternator case and found 160 ohm vs a spec of 7-10.
-sent the alternator to B&C to be rebuilt and tested it upon return and saw 6 ohms on the internal resistance but only 4-6v getting to it (B&C said the regulator is likely bad so they sent me a loaner and I wired it up and got correct readings).
-then I bought a new voltage regulator, installed it and everything tested good.
-I buttoned up the plane, stared the engine and even after runup, there was no output from the alternator.
-I went through the whole troubleshooting guide again and found the terminal 6 voltage issue in my original post.
-I then wired up the test regulator and it behaved the same way.

That is how I ended up where I am now.

Thanks for all the suggestions! Keep em coming since fly-in season is almost here and I’ve got to get this bird back in the air.

 

[Ruready]

There are only so many components, dont replace something unless you are sure its bad. Keep your avionics and everything else OFF and take your 6 and 4 wires off the VR and temporarily connect them together and full field the alternator and bypass the VR, either your alternator comes alive or it doesnt. Just do it for long enough to see the increase in voltage on your voltmeter and than kill the field switch, I guess its your DPST master, I would throw that in the garbage and get a single toggle for each one that way you can turn the master on and leave the alternator off. I like to start with the alternator off and than confirm the voltage increase after start, thats how I check each alternator on my twin. I like alternator problems, they are so simple, there is nothing to the whole system, you have your field wire and your fuse for that, the alternator and the VR, thats it, incredibly simple to troubleshoot. You should have this figured out in an hour tops, a roll of 18 gage wire to run new temporary wire from each component is a quick way to eliminate components

I would like to know what you mean by this though, everything tested ok.. was that before starting the engine and checking your alternator output? Or was that an ops check good with the engine running without the cowling on?

This is so simple though, you either have a problem with your field signal getting to the VR, or you have a problem with the output field signal to the alternator, both of those can be eliminated in 5 minutes by running new temporary wire. Assuming your grounds are good?

 

[Kooshball]

I would like to know what you mean by this though, everything tested ok.. was that before starting the engine and checking your alternator output? Or was that an ops check good with the engine running without the cowling on?

This was when I ran through the trouble shooting guide again and got the correct voltages in all the locations.

 

[Mich48041]

I’m only getting 2.4V at terminal 6, with that wire removed from the reg it reads bus voltage.

That tells me that either the master switch is bad or the circuit breaker is bad.
A bad circuit breaker is more likely. Why not just do the voltage test that I suggested in post #6
above and then you will know for sure. Whichever closed switch that has voltage across it is the bad one.

 

[Ruready]

That tells me that either the master switch is bad or the circuit breaker is bad.
A bad circuit breaker is more likely. Why not just do the voltage test that I suggested in post #6
above and then you will know for sure. Whichever closed switch that has voltage across it is the bad one.

Sounds like he is saying that he pulls the wire off pin 6 and the wire has the correct 12 volt signal. So the master is not the problem IF THATS WHAT HE MEANT?

 

[Ruready]

I am troubleshooting an issue preventing the correct voltage from reaching my B&C voltage regulator. In trouble shooting the alternator field switch in the panel, I found it wired as shown in the photo. The “M” is the master switch and the “F” is the field switch with is actually a 5a toggle breaker.

I am trying to understand why the field switch is connect to the bus and the master rather than being connected directly to the voltage regulator as shown in the recommended wiring diagram from B&C.

Any thoughts on this installation?

Thx!

Ok reread this and realized your 5amp field breaker is a TOGGLE switch, so I agree with you it should not be wired from your 5amp breaker through your master switch… thats CRAZY, take that wire that goes to terminal 6 off your master switch and connect it right to the 5 amp field toggle c/b.
.
Finding the problem here is very simple.. throw the troubleshooting guide out the window and get the fluke out and check

1. Check the field wire for 12 volts, I am not clear where you mean about having 2.4 volts, is the 2.4 on 6 with the wire connected to 6? And than you pulled that wire off 6 and the wire had 12 volts while not connected to 6? With the black lead to aircraft ground and red lead to wire that connects to term 6 on VR, it must be 12 volts, if not than like Mich says bad switch or cb or wire.

2. If check 1. above is good than check VR by disconnecting 6 and 4 wires from thr VR terminals and connecting the wires together and bypassing the VR and giving the alternator a full 12 volts to the field tab, start engine and if alternator comes online than BAD VR, if it doesnt come on than bad alternator. Done

This is simple stuff.
.

 

[Mich48041]

I forgot to mention that the voltage check across the switches is only valid when the circuit is loaded.
Thus, the voltage regulator should be connected. An even better load is an external load like a test light.
Voltage tests without a load are meaningless.
In case of smoke in the cockpit, most pilots would shut off the master switch. If the voltage regulator is not wired through the
master switch, then the alternator would keep supplying the aircraft with electrical power after the master switch is shut off.
The smoke in the cockpit could then turn into fire.

 

[Ruready]

I forgot to mention that the voltage check across the switches is only valid when the circuit is loaded.
Thus, the voltage regulator should be connected. An even better load is an external load like a test light.
Voltage tests without a load are meaningless.
In case of smoke in the cockpit, most pilots would shut off the master switch. If the voltage regulator is not wired through the
master switch, then the alternator would keep supplying the aircraft with electrical power after the master switch is shut off.
The smoke in the cockpit could then turn into fire.

Regarding the smoke fire issue, do you understand that with the battery master in the OFF position the field toggle/circuit breaker switch is not powered and the VR loses power

When I went through basic electronics school I cannot remember ever doing voltage checks without a lead on ground, I am really confused about your voltage check with a switch closed and powered and somehow showing a bad switch IF YOU HAVE VOLTAGE, and without a lead on ground, it hurt my brain trying to understand what you saying, please explain this voltage check.

This is your check from the way I read it, I have read it over and over and drew it out in the picture below, is that what you are asking him to check?
.

.

 

[Mich48041]

do you understand that with the battery master in the OFF position the field toggle/circuit breaker switch is not powered and the VR loses power

That is true in some aircraft. But in many aircraft, once the engine and alternator are running, the battery only serves to stabilize the system voltage. The alternator powers the electrical system, not the battery. If the battery is disconnected, the pilot might not even know it. In fact, it happened to me when my battery failed internally with a broken weld while flying. That had the same effect as shutting off the battery contactor. Everything was working fine until I closed the throttle to land. Then the EFIS shut down because the permanent magnet alternator in my RV-12 wasn't putting out enough power.
-
Yes, you correctly understand my suggestion to measure the voltage drop across the switches. Whenever current flows through a resistance, there will be a voltage drop. A bad switch has resistance which will drop voltage. Of course your method of comparing voltage to ground will work too. But it requires remembering the voltages and subtracting them. My method is easier for an old man like me. No remembering and subtracting required.

 

[Ruready]

Ok, I see how the alternator might still power slightly provide some power to the bus with the field still on and the battery OFF, because I had something strange happen to me once to switching from a genrator to an alternator system on an old Piper a few years ago…. cant remember for sure.. need to start taking notes at my age LOL .. but I believe I had the alt field connected to the bus, I just dont remember how I did it, but it was on the bus and did not go through the master switch and I did not have it going thru a switch, only thru a 5 amp fuse…. anyway I shut the battery switch off and expected to lose power…. BUT SOME THINGS WERE STILL POWERED…. that blew me away I did not understand it… all I could think was that with the alternator running it was putting 14 volts on the bus and also that field power back to the alternator and not letting it completely power off??? Anyone jump in please to comment, I am not sure but believe thats what happened. So I rerouted the field power through a separate toggle switch and when toggled off the problem went away.

So that makes sense that if you dont turn both the master AND THE FIELD toggle switches off that the bus might not power down. But if he wires it like I said all he has to do is make sure to select the alternator field off and no smoke or fire.

 

[Ruready]

Yes, you correctly understand my suggestion to measure the voltage drop across the switches. Whenever current flows through a resistance, there will be a voltage drop. A bad switch has resistance which will drop voltage. Of course your method of comparing voltage to ground will work too. But it requires remembering the voltages and subtracting them. My method is easier for an old man like me. No remembering and subtracting required.

Still think its much easier just to check for the proper voltage… no remembering any numbers except one, 12 volts, either have it or not.

 

[Mich48041]

Still think its much easier just to check for the proper voltage… no remembering any numbers except one, 12 volts, either have it or not.

Right, and the OP does not have 12 volts, only 2.4 volts. Now all he has to do is figure out where 10 volts is being dropped, either across the circuit breaker or master switch or some other bad connection. Thanks for drawing a picture of how to connect the voltmeter. Pictures are worth a thousand words. I suspect the meter will show 10 volts, thus indicating that the problem is one of those switches or terminals.

 

[Ruready]

Went through Navy avionics course back in 1988, never heard of checking for volt drop like that you always check with ground and power, made it simple. I agree on the signal loss.. possible bad ground or connection somewhere.

He has never answered my question whether the 12 volts was good at the wire that connects to term 6 when he pulls that wire off terminal 6? Sounds like he says yes signal is good until he connects the wire to the the VR.. than it drains down to 2.4 volts. that would mean something in the VR is bad. or possibly downrange of it.. That is my reason for bypassing the VR by disconnecting 6 and 4 and then connecting those 2 wires together and then starting it and see if the alternator comes online. This is 10 minute max checks... so I am done until he gets back with info

 

[Kooshball]

Went through Navy avionics course back in 1988, never heard of checking for volt drop like that you always check with ground and power, made it simple. I agree on the signal loss.. possible bad ground or connection somewhere.

He has never answered my question whether the 12 volts was good at the wire that connects to term 6 when he pulls that wire off terminal 6? Sounds like he says yes signal is good until he connects the wire to the the VR.. than it drains down to 2.4 volts. that would mean something in the VR is bad. or possibly downrange of it.. That is my reason for bypassing the VR by disconnecting 6 and 4 and then connecting those 2 wires together and then starting it and see if the alternator comes online. This is 10 minute max checks... so I am done until he gets back with info

Yes, “the 12 volts was good at the wire that connects to term 6 when he pulls that wire off terminal 6”. I hope to get to the hangar Tuesday night and I’ll have more information after getting a chance to implement all these suggestions. Thx!

 

[OKAV8r]

Note that B&C recommends AGAINST using a toggle- breaker. They are often flaky.
Use a Klixon pullable breaker and a good quality toggle switch. (BTW, I think B&C calls for a 7 amp breaker)

 

[Mich48041]

“the 12 volts was good at the wire that connects to term 6 when he pulls that wire off terminal 6”

Do NOT remove the wire from terminal 6. Doing that removes the load and voltage tests without a load are meaningless.
There should NOT be a voltage drop across the circuit breaker or across the master switch. If there is, replace the device.

 

[Ruready]

Do NOT remove the wire from terminal 6. Doing that removes the load and voltage tests without a load are meaningless.
There should NOT be a voltage drop across the circuit breaker or across the master switch. If there is, replace the device.

LOL... when you test your battery how do you do it then? You put a Volt meter on it, one lead on the negative and one on the positive and read your voltage. Where did you learn electronics? I will give you I am not the greatest but I did get paid to go through Naval aircraft electronics school, and than electronic countermeasures school and worked on some pretty high voltage radar jamming equipment for 5 years. One easy quick way to tell if its a problem in the circuit before terminal 6, is to take the battery and run a test wire from the pos term directly to terminal 6 on the VR. If the voltage stays good at term 6 than the problem is between the VR and the bus, that is probably where I would start, that test right there. If the voltage is still bad than look at the VR and downstream, also try a different battery and make sure the ground is good and battery terminals clean good contact

 

[Mich48041]

When I test my battery, I put a load on it and time how long it takes for the voltage to drop to a certain level.
Based on the info given by Kooshball above, I am confident that the problem is either the master switch or the alternator field breaker-switch or one of their terminals. If wrong, I will donate $50 to VAF. If Kooshball follows my voltmeter test advice in posts 6 and 17 and 29 above, then he will quickly determine which switch is bad without running any wires. The breaker is most likely the culprit and a meter will measure voltage across it provided that the load (regulator) is connected.

 

[Mich48041]

Ruready, Your background is impressive. Even though our troubleshooting methods differ, all that matters is that the electrical problem is solved. It is not a matter of right way versus wrong way, just different ways to accomplish the same goal: help Kooshball fix his charging problem.

 

[Mich48041]

Several years ago I purchased a circuit breaker from an online electronics store. Before installing it in my airplane, I tested it on the workbench by connecting it directly to a 12 volt battery. Instead of tripping off, it literally smoked. I installed a fuse instead.

 

[Mikeyb]

Like Joe says using a voltmeter to test voltage drop across individual components connected in series is well known and Important troubleshooting technique.

Diagnosing Voltage Drops Electrical Automotive Troubleshooting

Voltage drop is one of the most common electrical problems showing up in automotive shops today. This article features the symptoms of voltage drop, basic procedures and grounding tests to keep you safe.

www.fluke.com

 

[Ruready]

He will figure it out, its a simple system, with all the info in this thread he should have it solved within a couple hours and thats without coffee.. LOL
Back when I was working on electronics everyday the high tech Zenith 386 computers the Marine Corps used on our test bench .. guess it wasnt the computers fault but the program they used to troubleshoot that ECM gear… it was never right, would spit out a diagnosis but usually was not good, so my way of troubleshooting was grab a known good unit and swap parts til it worked. I suppose thats why I like the process of providing known good signals until it works, simplifies finding the fault…. going to autozone and buying a 50’ roll of test wire and sending a good signal to a problem component like that regulator is the way I would go instead of breaking my back trying to get a multimeter on the back side of my instrument panel. And my experience with charging issues has lead to batteries and volt regulators being the problem usually, so I like to make sure they are doing their job first, besides its a quick and easy check. Bypass components one at a time until you find the bad one LOL

 

[Ruready]

He has found the bad voltage signal at the input to the VR at terminal 6, the voltage is 12 volts right where it needs to be at the wire providing power to 6… but for some reason when the wire is connected to the terminal 6 the voltage is 2 volts on that terminal…. so either the signal is weak like Joe says it is maybe bad connection in switch and as soon as it connects it says WHOOOAA and quits, or something forward of terminal 6 is being greedy and draining it. Unless 2 volts is correct at that point… because I dont know that voltage regulator or how it works and what it does to that 12 volt input signal once it gets to term 6. But what I do know is how that Denso alternator that B and C uses works, bypassing the regulator by providing a known good 12 volts to wire 4 by disconnecting wires 6 and 4 from the regulator and connecting them together and full fielding the alternator will tell you if the regulator is the problem…. so this is just too easy of a problem to solve without even using a multimeter..

I had a problem a couple years ago where my battery shunt was blown… some low amp things were working but the high amp stuff was on vacation…. took me a couple days to figure out that the power to the bus was all going through the shunts ammeter bypass wire and could only power the low power items…. wonder why the high amp stuff didnt melt that little wire?? Anyway figured it out by connecting a battery up at the alternator connection and pushing power to the system that way and everything worked like it should.

 

[Kooshball]

Alright folks, here is what I have:

Both the master and alternator switches check out good per the instructions here, when closed there is essentially zero V showing on my DMM.

Bus voltage is: 12.13
Voltage on terminal 3 of the VR is: 12.05V which is less than 0.2v difference so according to B&C is good
Voltage on terminal 6 is: 2.1V

I have a new field switch that I temporarily wired (with completely different wires) from the bus to terminal 6 and I get the same 2V.

When I remove the wire from terminal 4 which is the output to the alternator, I still only see 2.1V on terminal 6. So even with the alternator out of the circuit I only get 2.1V.

All of this points to the voltage regulator, but the strange thing is that I still have the loaner voltage regulator from B&C that they send out as a “known good regulator” and it also shows 2.1V at terminal 6. What are the chances that both of these voltage regulators are bad in the exact same way?

 

[Kooshball]

Update. It was Bad wiring between the master and the VR. I ran a fresh one and everything checks out good so now I’m making it permanent clean and safe.

 

[Ruready]

Update. It was Bad wiring between the master and the VR. I ran a fresh one and everything checks out good so now I’m making it permanent clean and safe.

Glad you got it fixed, see how cool it is to run known good signals and make it work

 

[Mich48041]

Good going Kooshball.
I said above that if the problem was not the circuit breaker or master switch, then I would donate $50 to VAF. Well, I was close.
According to Koosball, The problem was the wire connected to the master switch. Wires themselves usually don't break in the middle, but rather at the end terminals. Anyway, below is a copy of my receipt paying $50 to VAF.

 

[Ruready]

Joe you were close enough.. it was a funky problem, I would have thought the switch or contacts to before the wire.

 

[Kooshball]

Im going to try and find the failure point. The original wire when ohmed end to end is showing 35K ohms and passes through two wiring harnesses

 

[Ruready]

Im going to try and find the failure point. The original wire when ohmed end to end is showing 35K ohms and passes through two wiring harnesses

Before you go rip in that wire out you might just want to double check it aint the master switch.. go test fly it with the temp wire and see if it stays working

 

[Ruready]

I have a new field switch that I temporarily wired (with completely different wires) from the bus to terminal 6 and I get the same 2V.

This should have worked... something is still wrong if this did not fix the problem

 

[Mich48041]

Maybe there is a splice in the failed wire.

 

[Ruready]

By running a new wire and a new field switch from the bus to terminal 6, that would be bypassing the wire from the master to term 6... so that doesnt make sense that it did not fix the problem... expect the problem to raise again. Unless the new switch or wire was bad to

 

[Kooshball]

to be clear, running a new wire fixed the problem, I just wanted to find the point where the original wire, connector, harness failed and to be sure that nothing is spliced into the original wire.

 

[nzrv8]

Went through Navy avionics course back in 1988, never heard of checking for volt drop like that you always check with ground and power, made it simple. I agree on the signal loss.. possible bad ground or connection somewhere.

He has never answered my question whether the 12 volts was good at the wire that connects to term 6 when he pulls that wire off terminal 6? Sounds like he says yes signal is good until he connects the wire to the the VR.. than it drains down to 2.4 volts. that would mean something in the VR is bad. or possibly downrange of it.. That is my reason for bypassing the VR by disconnecting 6 and 4 and then connecting those 2 wires together and then starting it and see if the alternator comes online. This is 10 minute max checks... so I am done until he gets back with info

Just for reference, Joes method of measuring voltage drop across a component or cable is standard, and correct.

 

[Ruready]

to be clear, running a new wire fixed the problem, I just wanted to find the point where the original wire, connector, harness failed and to be sure that nothing is spliced into the original wire.

It just doesn't make sense that if you bypassed the bad wire by going directly from the field switch to terminal 6 that it didn't work because you took the whole bad wire that you replaced out of the system.. That's all I'm trying to say

 

[Ruready]

Just for reference, Joes method of measuring voltage drop across a component or cable is standard, and correct.

Back when I learned electronics 35 years ago they taught us to look for resistance to look for an open to find a bad switch, not voltage drop