Prestolite 60Amp Alternator trouble.

Threedoor

New member
8
7
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Location
Lewiston, ID
I have an 83 M10a with the Prestolite alternator on it which o believe to be the same as the 900 series trucks.

I had a battery with two bad cells in it and lost the voltage regulator. It was overcharging. Chared parts in the regulator.
I replaced the batteries, fixed some connectivity issues. Checked ground to the alternator and it’s good. Brushes look great, no abnormal wear on the slip rings. Rectifier looks good. I Have 25V back to the field and nothing. Adjusting the pot does not bring the voltage up.

What I think I did was screwed up the position of the three wires between the rectifier and voltage regulator. I have TM 9-2920-225-34 and it does not help. As there is no diagram of the correct position for those three wires.
I took a picture of it on disassembly but think I still managed to get it wrong.
 

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Steelreaper80

Active member
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Location
Indianapolis IN
This is a common problem with military alternators. I have had 2 failures with the exact same problem. I will not run military alternators anymore. That is the absolute first thing that I replace on any truck. I went with an AC Delco 21SI clone at 24 volts/70 amps. It was about $110 at Auto Zone. You will either need the smaller mounting bracket for the side of the engine or I just used a cut-off wheel on an angle grinder and cut my existing bracket. I then used a large bolt and spacers to fit the alternator to the modified bracket. The only other thing you will need is the proper size belt for the new alternator. I have over 50K miles on the new alternator with ZERO issues. I also swapped out the military batteries for 2 group 31 batteries. The 2 group 31 batteries have more than enough power to start the 6CTA even in cold weather. If you have a different engine which I believe you do, the concept is still the same but just look at your bracket and change to the appropriate belt. I don't have any experience with the NHC-250 motor so I am not sure what modifications bracket-wise will be needed but I hope this at least points you in the right direction.

If anyone else has more knowledge than me on the NHC-250 please chime in.
 

ToddJK

Well-known member
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Location
Sparta, MI
I can second that on the prestolite alternator. They are very prone to failure. My first deuce, the alt went out twice, second time was literally 2 months after it was rebuilt at a local alternator shop. I got an AC remy and it was by far superior to the prestolite but it did need a new bracket in which my mechanic made to fit. It does seem though, at which point, if there's a bad battery or one with a bad cell and the alternator constantly over charges, it goes to crap rather quickly. I ran the military batteries which were junk and one went out within a few months. Once I got the AC alternator, I ran two bulldozer batteries, don't remember the brand, but those were 980CCA each. That truck never had an electrical charging or starting issue again. I would just look for an alternative alternator for that prestolite and fab up a new bracket to make it fit.
 

Threedoor

New member
8
7
3
Location
Lewiston, ID
Had o known these were failure prone I would have replaced it with a GM alt in the first place. As it is I believe I have the regulator wired incorrectly.

if anyone has a diagram of the internal wiring that’s better than the TM which does not include it that would be great. I do t really want to tear apart the one on my dumptruck to get a look at it.
 
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