FMTV - 24/12 Converter instead of the Niehoff dual-voltage alternator?

Wile E. Coyote

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Bought some lightly cannibalized FMTV A0 trucks a couple of years ago and finally getting around to putting them back together. Two have the alternators robbed off of them (Niehoff N1506-1), and since trying to buy a replacement from our supplier friends is almost what was paid for the trucks - I was thinking of going with the straight 24V alternator which seem to be around more reasonably priced - and wiring a 24V to 12V converter to run the 12V end. Anyone try that yet? Does anyone know the all-up current draw on the FMTV A0 12V side of things with all the 12V accessories on?

I can't immediately come up with any reason why that setup wouldn't work, other than maybe potential trouble trying to find a converter of the appropriate current rating.
 

Suprman

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It will work fine. I have 100a surepowers in the classifieds that will do the trick. You would need to install a contactor relay to disconnect the 12v converter output when the truck is off.
 

Ronmar

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It should work, but you need a converter with a very low cutout voltage so it doesn't drop off line when you crank the engine.

They make these for commercial vehicles to provide a standalone 12V or to float the 12V center of a 24V battery string like the 24/12 alternator does.

Was working on getting accurate 12V and 24V loads but my current probe is broken. I calculated the lighting loads based on the bulb specs which should be the majority of the 12V loads. At night stepping on the brakes with turn signals on about 25A peak lighting load...

signal lights: 2.1A X 7 or 4.2A brakes, 4.2A signals(8.4A hazard) 2.1A Rev

Marker Lights: .56A X 13 or 7.28A

Headlights: 4.7A X 2 or 9.4A

Gauges/switches: .1A X 17 on a basic A0 truck?[FONT=&quot][FONT=&quot][/FONT][/FONT]
[FONT=&quot][FONT=&quot][/FONT][/FONT]
 

Suprman

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The surepowers divide. Your output will be half your input. I believe you can run it equalizing. Keep your 12v off the battery location and switch the converter 12v out to the batteries.
 

pontiac62

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Been looking at those 12v converters/ battery equalizers too. Haven’t looked at the electrical diagrams to see where all the voltages go.

Seems how the alternator and voltage regulator are pricey and near non existent on the regulator side of things.


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Ronmar

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I would get a straight 24 and one of Supermans 100A surepowers... There was quite a discussion on the quality of the double-alt not all that long ago.

In the end pulling 12V out of the middle of a 24V string and trying to charge 12 and 24 simultaneously is a problem prone way of doing a dual voltage setup. The best way is a pure 24V bank and a 24-12 converter. You also have a fallback if the converter fails. You can still tap 12V out of the middle of the 24V bank short term to get you home...
 

Suprman

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Simp has some cat ones in the classifieds that might bolt up. Have to measure. I get good used original
alts from time to time but they are pricey. Big demand for alts. With the converter you can be connected directly to the 12v from the batts. And use the converter/equalizer to keep the batts equalized. You would just need some contactor relays to disconnect the equalizer when the truck was not in use. The surepowers are very reliable. I believe they are basically just a big diode voltage divider. Your 12v output will be half of your 24v input.
 

SausageGuy

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OK found a surplus rebuilt factory alternator so I just got it going for now. But it did show some of my other problems were because of the alternator going out. I thought I had a dash ground issue because all of the gauges started jumping around including alternator gauge but it kept working its way lower slowly. Air brake buzzer was not working all the time. Fuel gauge would spike and then go to zero. That all stopped now. Keep an eye when driving. Looks like that may be a warning to get off the road to get it fixed. If I had realized sooner might have prevented some of the damage. Rebuild shop says diode plate and regulator were both bad. Might have been only one if I had checked it sooner.
 

DigDirt

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I'm also in the same boat with the N1506-1 going out and wanting to go 24v only. But I'm having trouble finding a civvie alt that will fit the existing bracket. SausageGuy, where did you find yours and what make and model was it?
 
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