Alternator over voltage

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dcowan

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Rogersville, MO
I have an M936A2 that when I start it pegs the voltmeter for exactly 5 minutes then drops down to normal. I have measured with a voltmeter to confirm and it goes to ~34 volts for the 5 minutes then drops back to ~25. When I got the truck the wire to the alternator was disconnected and now I know why. I know there is an adjustment but before I try that I wanted to get peoples thoughts on why it would be so high. It does have two 12v batteries that are undersized in my opinion and I plan on upgrading to group 31's.
 

simp5782

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typical military worn out alternator. regulator just goes thru the roof.. Usually ends up with a blown Power control box when it fritz' out. It should be fine even with 24Fs in it. or 65 series batteries which are smaller than 31s.

I sell replacement brushless 24v alternators. and Clint (74M35A2) sells the engine bracket for a direct bolt on to the 8.3. I also have rebuilt control boxes.
 

simp5782

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What is up with the 5 minute time though. Its exact. Has to be some kind of control circuit?
Just the regulator inside. Depending on how much current draw you have while starting it shouldn't take it more than a few seconds to read the battery voltage and get them back up to power.

The stock alternator has an exciter wire from the PCB box to get told when to turn on and to energize.
 

simp5782

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Brutacus

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Marion, AR.
making a spacer to space it to the front more. Different pulley. Here is swamp's writeup on the 939 but the 809 is the same thing. Member Brutucus is doing the alternator install on his 818 when he gets around to it.

https://www.steelsoldiers.com/showthread.php?155692-M939-NHC250-Delco-Remy-33si-Alternator-Upgrade&highlight=

I have gotten around to it, I just haven't finished it yet. I haven't made the modification to the adjustment arm yet.


HPIM2492.jpgHPIM2493.jpgHPIM2494.jpg

Depending on the bracket you'll have to slot the bolt holes about 1/2" forward. You might have to slot it a little more. As for the spacer, get some D.O.M. tubing, 1" O.D., .5" I.D. I just needed my cut at 2 1/8". The mounting holes on the Delco alternators are 1/2", so you'll have to re-drill the mounting bracket for the larger bolt. The adjustment arm slot will have to be enlarged for the 1/2" bolt as well. You can use spacers for the adjustment arm, or put a dog leg bend in it so you don't have to use spacers.
 

77 AMG

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Once again, this place comes to the rescue on time. My alternator has decided that it wants to do the "overcharge dance" recently. Thanks everyone!
 

TexAndy

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When I got my m923a2 out of ft hood a few years ago, it was missing the alternator. I bought one of the alternators out of one of the other trucks in that same auction from the scrap yard that won them. And that one had a bad voltage regulator.

I wound up buying the wrong voltage regulator too. (hellas) Had to get a local machinist buddy to turn down the lip slightly on his lathe so it would fit in the alternator body.

My anecdotal impression is that these military alternators are kinda garbage. When mine breaks down again, I'm going civilian conversion.
 

74M35A2

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Livonia, MI
The stock military alternators use a remote sense wire as the primary method of setting regulation (system) voltage. If this circuit is broken (internally, or externally [wrong shut down switch order]), there is no fail-safe. It then assumes system voltage is zero, so it drives the alternator to max output to try and get the voltage up. This is the reason so many of them over-volt. When it does, it can damage the ABS module, PCB module, and CTIS module. This is also way too much voltage for recharging the batteries, and will damage them as well. Not sure why yours times out after 5 minutes.

Other failure modes of these antiquated alternators are bad connections in the connection under the cover (like 1800's phone systems, just friction fit), regulator failure, and pulley/shaft interface failure. About 4 different yet common failure modes for these units, of which a modern compact civilian replacement resolves all of. Even the Chinese Delco clones work well. The stock ones do look cool, I wanted to keep mine just for that reason, but as soon as I had another person shut off my truck, and they used the wrong switch order, and wiped out my ABS module, I said that won't happen again, and swapped out to civilian.
 

Brutacus

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Location
Marion, AR.
here is what I used to for my adjustment arm. I found 2 flanged bushings at Fastenal. They have an O.D. of 1/2", and an I.D. of 3/8". The rest is grade 8 hardware.

s-l1600.jpg

Here is the hardware I had to use to bolt up the arm.


HPIM2495.jpgHPIM2496.jpg


I had to modify the block by cutting out the center, and welding ears on the bottom drilling them out for the 1/2" bolt.

HPIM2497.jpgHPIM2498.jpgHPIM2499.jpg


I will have to find the correct pulley, but that's not that big a deal. Those things are all over ebay and other places. I just need to find out the part number that has the V grooves close together.
Also, with this setup, I didn't have to swap out my belts due to the smaller pulley on the new alternator.
 
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Brutacus

Member
139
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Location
Marion, AR.
I just mounted it before dark. I haven't ran it yet. I don't have my batteries on the truck right now. I'm going to sandblast, and paint the plate the batteries set on. So all that stuff is off the truck right now.
 
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