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M916 Alternator Trouble

Boosted_Nelson_Inc.

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On thursday my brother and I ran up Sparta, WI to pick up our newest acquisition, a 1980 (GL said it was a '77) AM General M916. All the fluid were full, including a nearly full tank of diesel:p. But the truck had no power, and when the GL rep tried to jump it from a HMMWV, it still didn't have enough juice. So the GL rep turned off the fuel control selenoid, and jumped the starter selenoid and it fired right up. At first the voltmeter read no voltage, but by the time we left the lot it read 13 volts at idle. Cruising down the highway it read just over 16 volts, but everything was working perfectly.

The truck made the 350 mile drive back to Indiana without any problems. So we decided it should be able to handle a 12 mile drive into town for the Great Lakes Museum of Military History's vehicle show. The first indication something was wrong was that the jake brake stopped working, and then about a block from the museum all the lights went out. So after poking around a bit we discovered that some of the wiring on the alternator decided to fry itself. All of the batteries had read around 12 volts in the morning, but when we checked again the front 2 were around 12 and the rear 2 were closer to 6 volts.

So I guess the question is, should I assume that since the voltmeter had been reading high, that the alternator probably needs to be rebuilt, or hopefully it was just a wiring issue that can be fixed?

The first 3 pics are the fried wiring, the next 2 show the suprisingly clean alternator in my 915, and the last is just a pic of the truck.
 

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NDT

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You are not the first Steel Soldiers member that this has happend to. 16 volts on the 12 volt battery pair is quite high. Should be 14 or so and 28 across both pairs with the engine running. The alternator is a commercial model and can be rebuilt by a good alternator shop. The "transformer rectifier" device takes the 3 phase AC output from the 12 volt alternator before it is rectified into DC, (see the green wires on the un-toasted picture), then increases the voltage through a transformer, and then converts it to DC through a rectifier, and charges the second battery bank with 28 volts. When you are facing the battery box, the LH battery pair are in parallel for 12 volts, and the RH battery pair are also in parallell, and both battery banks are in series for 24 volts. Check the wiring over carefully for shorts, especially in the battery box, before you fry a second alternator.
 

R Racing

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St. Leonard, MD
Did you replace the batteries at all ? its a comon problem that the original batteries short out internally. and can cook your alternator. My M920 had 2 shorted batteries in it and with a m816 m35a3 and a m813 trying to jumper at the GL lot all at the same time it wouldn't even turn over . replaced all 4 batteries and it started all on its own in 17 degrees temp. Replace yor batteries and have your alternator tested.
 

eldgenb

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Spokane WA
Did you replace the batteries at all ? its a comon problem that the original batteries short out internally. and can cook your alternator. My M920 had 2 shorted batteries in it and with a m816 m35a3 and a m813 trying to jumper at the GL lot all at the same time it wouldn't even turn over . replaced all 4 batteries and it started all on its own in 17 degrees temp. Replace yor batteries and have your alternator tested.

Mine was exactly the same as his, replace the battery's and then test the alternator.
 

Barneyszoo

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When I recovered my 813 I didn't look to see that the batt were not securd, on the way home one slid over and shorted on the cover (step). I chasted gremlins for months, one thing I found was the inside of the alt. was all burned, but it still worked and put out the correct voltage. My prob turned out to be the black control box on the firewall. Don't know but this maybe another place to look for burned wires.
 

simp5782

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My 28v output has failed. 14v side is just fine. Has anyone ever attempted rebuilding the transformer/rectifier?
Refurbished aftermarket alternator is around $330

You can use just an equalizer to go from 12v to 24v to keep the battery up on the 24v side.

Or use a step up converter to charge.

I use a straight 24v alternator and a 100amp equalizer to get my 12v. More reliable. 60k miles this year alone.
 

NDT

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The transformer rectifier device is potted in, meaning the components are surrounded by solid potting material. It's not fixable. Disconnect the wires going to it before it catches fire.
 

Bballer182

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Refurbished aftermarket alternator is around $330

You can use just an equalizer to go from 12v to 24v to keep the battery up on the 24v side.

Or use a step up converter to charge.

I use a straight 24v alternator and a 100amp equalizer to get my 12v. More reliable. 60k miles this year alone.
I considered doing a 12 -> 24 booster to charge the starting batteries. Also considered a new different alternator setup.
You have any part #'s to recommend?
 
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