Voltage charging help

[ZZZZZZCOBRA1]

Question looking for some help. I was having alternator issues so replaced with a after market alternator. This is where you hook up just the positive cable to the battery and the ground cable. Saved me $600-$700 on the rebuild quote.




- When I check voltage across the two battery's after I put each of them on a charger I have 24.8 volts.




- I then measure the positive cable where it attaches to the back of the alternator and this reading matches the battery's. I have 24.8 volts.




- When I start it without the positive cable hooked up the alternator the alternator puts out like 27.5 volts.




- Problem, when I hook up the positive cable to the alternator the voltage on the back of the alternator now drops from the 27.5 volts without the cable and only shows say 23.5- 24 Volts.




I looked over the positive cable running back to the batteries and see no issues. Brand new alternator ground wire to engine. Any feedback besides look in your TM would be greatly appreciated.

 

[Valley Rock]

Did you load tested your batteries individually after charging them to make 100% certain that they are tip top ?

 

[ZZZZZZCOBRA1]

Did you load tested your batteries individually after charging them to make 100% certain that they are tip top ?

No I did not. I was having troubles with my previous alternator so I pulled that off charged the batteries and replaced with this new alternator.

 

[sue]

Should not spin the alt. At speed (run it) real
bad for alt diodes.

 

[sue]

Should have added, do not run the alt with out
batts hooked up.

 

[ZZZZZZCOBRA1]

Ok thank you.

 

[TexMark]

The problem you describe is exactly the issue I am having as well. I have a new 60 amp alternator, and new batteries that are fully charged to 100%, and the output from the alt is 24.6 volts. When I bought my '87 M998, it h

ad an aftermarket alt that charged the batteries fine. I'm not sure what voltage it was putting out, but the voltage gauge was in the green. I purchased a rebuilt alt to get my truck back to mil-spec parts. T

hat alt would only put out the same 24.6 volts, and I would have to charge the batteries every couple of weeks. So I broke down and purchased a new alt and new batteries, but as stated before have the same results now. J

ust curious, what size alternator do you have?​

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[simp5782]

What gauge wire are you running? Is your block ground good enough? 4 gauge should be enough for both. Or double run your ground. Ground on the alternator to the block and to the frame.

I have identical sister 33si alternators on 2 different 5 tons. Same pulleys. Same mounts. One puts out 26.8 at idle. One puts out 24.9. Their output is the same at 1500rpms though. The voltage you see with it not hooked up that is higher is without a load on it.

Some of the brushless alternators do not spin fast enough to produce voltage at idle beyond what it takes to maintain the batteries. You would need to install a smaller pulley to get the voltage up higher at idle
.

 

[ZZZZZZCOBRA1]

The problem you describe is exactly the issue I am having as well. I have a new 60 amp alternator, and new batteries that are fully charged to 100%, and the output from the alt is 24.6 volts. When I bought my '87 M998, it h

ad an aftermarket alt that charged the batteries fine. I'm not sure what voltage it was putting out, but the voltage gauge was in the green. I purchased a rebuilt alt to get my truck back to mil-spec parts. T

hat alt would only put out the same 24.6 volts, and I would have to charge the batteries every couple of weeks. So I broke down and purchased a new alt and new batteries, but as stated before have the same results now. J

ust curious, what size alternator do you have?​

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Here is the link to what I bought and installed.

https://www.amazon.com/gp/product/B00OIAB3B8/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1

I have not checked the voltage with the engine sped up. I drove it today and when moving it was in the low green. Someone on another forum said that I need to be using wire 568 also besides just the positive to the starter and the ground. They said the 568 should run to the #2 on the back of the new alternator and that would bump up the voltage as this is a keyed power and does something. I went to connect it but when I tried the key was forward and would spark bad when trying to connect it to test temporarily and was scared to permanently connect it with a clip and try. How is yours connected?

 

[ZZZZZZCOBRA1]

I am just using that huge factory power wire that was on there that runs to the starter. This was used on the 200 amp I took off. I put a new ground on that is pretty thick wire as the factory ground was two short. Are you thinking that a larger ground wire or more than one ground wire could increase voltage?

As far as the pulley I just matched up to what someone else was using. So do you think if I went to a smaller pulley I should get the extra volts that I need. I have a 4.5" that I am currently using.

 

[TexMark]

When I first got my truck, the aftermarket alt was installed. The #2 wire had the connector cut off, so I soldered a connector of the same type back on it. The 60 amp alt that is spec for the HMMWV has a wire w/ connector attached that connects to the #2 wire. From what I could find, the #2 wire sends a signal to the alt that it needs to charge more.

 

[ZZZZZZCOBRA1]

Do you know if that was wire 268? Did you notice more voltage once you did this?

 

[simp5782]

I am just using that huge factory power wire that was on there that runs to the starter. This was used on the 200 amp I took off. I put a new ground on that is pretty thick wire as the factory ground was two short. Are you thinking that a larger ground wire or more than one ground wire could increase voltage?

As far as the pulley I just matched up to what someone else was using. So do you think if I went to a smaller pulley I should get the extra volts that I need. I have a 4.5" that I am currently using.

Run a wire directly to the battery or starter. If it is like the other military trucks that power wire goes thru the control box for the exciter on the stock alternator

 

[papakb]

The #2A wire is the AC output from the alternator that tells the control box that the engine is running. That in turn keeps you from accidentally actuating the starter on an already running engine.