M1078 Alternator/ 12vdc side of 24+12vdc Alternator/ Please educate me

[Lugnuts]

Gentlemen; I have a charging problem and it goes like this/ The Battery configuration is 2 group 31 batteries/ The system has been very functional for over a year/ Now I have the 12vdc side draining while it's running and my charge rate on the 24vdc side is acting like the parasitic draws we use to have on the M35's/ The one battery gets drained while the other one cooks/ So on this on I checked if the 12vdc side was charging and I found it is not. According to schematics the one Regulator controls both the 12vdc side and the 24vdc side. I'm not really familiar with how/ But the 12vdc side of the Alternator is reading 12.4 volt and the 24vdc side is reading 28vdc. That is where I draw my conclusion that the 12vdc side is not working. Can it be the regulator since the 24vdc side is working or is it the Alternator???
How do I go about to test this? Is there a test spot to check the Alternator for full load output???
Thank you and I look forward to your responses.
Lugnuts

 

[Ronmar]

The regulator monitors 24v and regulates the field to maintain that output. the 12and 24v windings(two 12v windings) are in series, and the 24V output from the stator passes thru diodes like any other alternator,

The 12v output from the first stator windings pass thru SCRs. These SCRs are controlled/fired by the regulator which turns them on and off like a switching power supply to regulate the 12v output to be 1/2 of the 24 output.

what you are describing is exactly what happens when you pull 12v loads out of the middle of a 24v only powered battery bank.

alt or reg, good question you will need to swap one or the other to be sure.

I would swap the batteries with each other to insure you do not have a bad 12v batt overloading the alt, and I would also insure you have good 12v connection from battery all the way to the alt.

Do you see the same voltages at alt and battery while running? Different voltages while running/loaded across a circuit can indicate bad connections. The polarity box being in the weather is one place where this can happen.

what year truck?

 

[Lugnuts]

The regulator monitors 24v and regulates the field to maintain that output. the 12and 24v windings(two 12v windings) are in series, and the 24V output from the stator passes thru diodes like any other alternator,

The 12v output from the first stator windings pass thru SCRs. These SCRs are controlled/fired by the regulator which turns them on and off like a switching power supply to regulate the 12v output to be 1/2 of the 24 output.

what you are describing is exactly what happens when you pull 12v loads out of the middle of a 24v only powered battery bank.

alt or reg, good question you will need to swap one or the other to be sure.

I would swap the batteries with each other to insure you do not have a bad 12v batt overloading the alt, and I would also insure you have good 12v connection from battery all the way to the alt.

Do you see the same voltages at alt and battery while running? Different voltages while running/loaded across a circuit can indicate bad connections. The polarity box being in the weather is one place where this can happen.

what year truck?

Thank you for the response and more diagnostic clues. The voltage at the battery of 12.4vdc was the same at the alternator/ The SCRs are probably a rectifier. Please clarify/ I will check the other things and get the year of the truck./ I was wondering about the polarity box and now I know I should probably as a matter of preventive maintenance/ Thank you

 

[coachgeo]

.../ I was wondering about the polarity box and now I know I should probably as a matter of preventive maintenance/...

if you had 12.4v at both places then the 12v side of the Polarity box IMHO likely fine cause it had to go thru that to get to the battery where you measured it to be equal output of alternator

 

[Ronmar]

Thank you for the response and more diagnostic clues. The voltage at the battery of 12.4vdc was the same at the alternator/ The SCRs are probably a rectifier. Please clarify/ I will check the other things and get the year of the truck./ I was wondering about the polarity box and now I know I should probably as a matter of preventive maintenance/ Thank you

An SCR is a switchable Diode. No trigger, nothing flows, trigger applied, current flows in one direction like a diode, so they rectify the 12v output, just like the diodes on the 24V output.

The alt and batteries both connect to the battery terminal on the polarity box it doesn’t pass thru it, but bad connections there are common. if you are seeing the same voltage at battery and alt the connections are probably OK, Another thing to do is check the regulator connector, an issue there could cause this as well as either part having issues…

 

[Lugnuts]

In an overall view of what you have posted is it possible to check the alternator by removing the 12 volt leg from the alternator, starting the truck, and measuring the voltage at the 12 post of the Alternator? That is, if is 28.2 vdc at the 24 volt side it should be by theory 14.1volts at the 12 volt leg. If it is 14.1 volts at that leg then when reconnected to the battery the output should by theory be the same at the battery. If not then look for where it went.
The SCR's, are they in the Alternator or elsewhere?

 

[Ronmar]

If your batteries are OK it shouldn't make a difference. I also cannot say for certain wether it would even work. I did something similar at one time to attempt to measure individual 12and24 loads. The alt would not stay online, and would only pulse, like it was looking for/needed the 12-24v relationship. When it tried to come online and didn't see it, or perhaps saw a 12v over voltage, it would drop back offline and try again in a few seconds.
Not sure if that is normal, it was just my experience.

the diodes and SCRs are located inside the alternator casing…

 

[GeneralDisorder]

First things first - disconnect each battery and test it's voltage. If one is not 12.6v+ then charge them and test again. Then test again after they sit disconnected for a day.

A weak/dying/dead battery will cause all kinds of havoc with the system.

Also what are your LED's on your regulator doing? I would assume you have a flashing green on the 28v side. What color is the 14v side?

Florida..... you have the humidities..... my truck came from Texas and I can relate. Disassemble and clean EVERY SINGLE ground or power connection you can find. And there's a LOT of them. It will take you days to find and clean the majority. I'm still discovering grounds missing star washers, mud wasp nests, and general PVT Snuffy type issues and my truck is a 2008 that the military only put about 2000 miles on.

 

[Lugnuts]

If your batteries are OK it shouldn't make a difference. I also cannot say for certain wether it would even work. I did something similar at one time to attempt to measure individual 12and24 loads. The alt would not stay online, and would only pulse, like it was looking for/needed the 12-24v relationship. When it tried to come online and didn't see it, or perhaps saw a 12v over voltage, it would drop back offline and try again in a few seconds.
Not sure if that is normal, it was just my experience.

the diodes and SCRs are located inside the alternator casing…

Thank you.
I am going the same route of diagnostics, Kinda sorta. I got to thinking about load and diodes in the box. I am tomorrow going to look at the schematic to scrutinize where the connection is for the power from the alternator to go back to the batteries. One side or the other of the polarity box would make a difference.
When you say that the alternator went off-line are you saying that both 12 and 24 quit outputting or just one side of the charge? and how does it quit charging, by shutting a signal off to the SCR's?
I'm not scrutinizing you but interested in you research.
I have come to a conclusion that you can't have the 24vdc without having the leg of 12vdc (duh) but the load factor of even a bad battery would it take out the whole 24vdc system from charging? Logic says it should but I see that it continues to regulate the 24vdc voltage so the 12vdc leg can't be gone.
Yesterday I took the load off the 12 volt side of the alternator. The output was 26.9 on the 24 volt side and 14.0 on the 12 volt side. When I connected the load back it was battery voltage of 12.4 at the alternator and the battery.
Did you ever check to see what type of load you had at the 24 volt side and the 12 volt side while the truck was running? I haven't looked at the schematic but it seems that the main load is on the 12 volt side for running and the 24 volt is for starting.
I expose my ignorance for the moment and ask is the Transmission and ECM set up on 12 volt?
The batteries are good/ Date on batteries are also last year/ But I haven't closed my thinking to that somehow there isn't a problem there.

Thanks a bunch for the input

 

[Ronmar]

When I say off line mine stopped generating any 12 or 24v power…

yes the 12v is part of the overall 24v. The 12 is tapped off to the output thru the SCRs. Since you have the overall 24, the 12 windings are obviously OK. But the SCRs could be bad or the reg isn’t firing them. The fact that you saw 14 with the 12v battery feed disconnected is interesting. thats coming from somewhere, so it could be that some of the SCRs are dead, and not enough remain to pass enough current to meet the basic 12v load. Thats where alternators become tricky. Are they not making enough or are they being overloaded to the point that they cant maintain rated voltage, or are they partly damaged so they cannot even meet normal load.

Yes most of the load is on the 12v side when the lights are on, the trans, heater blower, fuel and air dryer heaters are all 24.

when you say batteries are good, you mean TESTED good just now, right? If y0u disconnect their leads what do they measure right now? Did you swap 12 and 24 batts like I initially suggested? If you have a internal short that absorbed a bunch of charging current on the 0-12v battery that could cause an issue. If this was the case and you swapped them the charging symptoms would change and you would know you are messing with a problem then…

 

[Lugnuts]

When I say off line mine stopped generating any 12 or 24v power…

yes the 12v is part of the overall 24v. The 12 is tapped off to the output thru the SCRs. Since you have the overall 24, the 12 windings are obviously OK. But the SCRs could be bad or the reg isn’t firing them. The fact that you saw 14 with the 12v battery feed disconnected is interesting. thats coming from somewhere, so it could be that some of the SCRs are dead, and not enough remain to pass enough current to meet the basic 12v load. Thats where alternators become tricky. Are they not making enough or are they being overloaded to the point that they cant maintain rated voltage, or are they partly damaged so they cannot even meet normal load.

Yes most of the load is on the 12v side when the lights are on, the trans, heater blower, fuel and air dryer heaters are all 24.

when you say batteries are good, you mean TESTED good just now, right? If y0u disconnect their leads what do they measure right now? Did you swap 12 and 24 batts like I initially suggested? If you have a internal short that absorbed a bunch of charging current on the 0-12v battery that could cause an issue. If this was the case and you swapped them the charging symptoms would change and you would know you are messing with a problem then…

Thank you. I did load test them but tomorrow I will get to work on this seriously, I will swap the batteries and I checked the schematics and as I figured the Alternator is connected on the battery side of the polarity box. Going to definitely check connections.

 

[Lugnuts]

So in the end I have a happy one.
After checking battery condition, after cleaning connections from alternator to batteries, I still had not my repair.
So I checked Voltage Drop going from Alternator to Polarity Box. The numbers were bouncing all over.
Some believe in Divine Intervention and I do and was given recall that when usually you have this phenomena that it is power looking for a ground as was the case.
The Alternator housing is aluminum and underneath the ground strap was corrosion. Not bad but enough that I hadn't a good ground.
Thank you all for the comments and instruction and also for knowing how they rid multi battery configurations from parasitic draw. I know I'm probably behind the times but better late than never.
Lugnuts

 

[profo]

its probably the regulator its turns on/off the 12 and 24 charging intermitingly thats the lil lights that blink back and forth basically cycles 12 then 24 and the 12 side is not working which is most common problem with the regulator fail, I change them on my lmtv camper almost every other year and when I run out of spare partsI am going to put a 12 volt alternator with a battery equilizer, cause the most load on the lmtv when runing is the 12 volt side. Your 12 volt side of alt should be between 13.8 and 14.4 when charging and 24 should be between 26-28 vdc meassure with a meter at the alt terminal, the case or frame is ground or use the neg terminal on it also, . Also another note at least on both of my lmtv's A1's if you leave the light switch (stop light first click to right)on it will kill the 12 volt battery over a few weeks cause if you look at the fuse panel the led on the brake circuit is lit up drawing from the battery all the time.

 

[Lugnuts]

its probably the regulator its turns on/off the 12 and 24 charging intermitingly thats the lil lights that blink back and forth basically cycles 12 then 24 and the 12 side is not working which is most common problem with the regulator fail, I change them on my lmtv camper almost every other year and when I run out of spare partsI am going to put a 12 volt alternator with a battery equilizer, cause the most load on the lmtv when runing is the 12 volt side. Your 12 volt side of alt should be between 13.8 and 14.4 when charging and 24 should be between 26-28 vdc meassure with a meter at the alt terminal, the case or frame is ground or use the neg terminal on it also, . Also another note at least on both of my lmtv's A1's if you leave the light switch (stop light first click to right)on it will kill the 12 volt battery over a few weeks cause if you look at the fuse panel the led on the brake circuit is lit up drawing from the battery all the time.

How does the battery equalizer system work? I'll claim ignorance.

 

[Ronmar]

The neihoff regulator regulates the field for a 28.2v output. The 12v tapped out of the middle of the windings is fed thru SCRs(switchable diodes). The regulator controls/fires the gates of the SCRs like a switching power supply and regulates that output to half the main output or 14.1v…. If you are not getting those two voltages then something is up such as bad components(alt or reg), bad batts causing overload, or bad connections preventing proper current flow…

the standalone equalizers do the same thing. They provide a center tap output that is exactly 1/2 of the total output and will flow current from one bat to the other to get the individual battery voltages within .1-.2v of each other.

thats how I have my maint charge hooked up. A solar powered 13.2v solar powered supply on the 0-12v batt as that is where the vampire load is, and a small equalizer that passes current to the 12-24batt to keep it floated nearly to the same level.
They work best when the power input is to the overall series voltage then they can float the center voltage to exactly one half. When the power is fed from one end or the other(12v applied to either battery), the equalizer can only get the second battery up to within .1 pr .2 v of the battery with the power applied. But .1-.2 isn't too bad…