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Bad alternator?

June 15, 2021, 03:38:47 pm

I am showing 12.25 volts at the battery with the engine running. I believe the problem is the alternator.  Is there anything else I should check before I replace the alternator? I await to get it fixed before I get away from a place large enough that I might be able to find what I need (Salt Lake City).

Dave & Kathy Bennett
2004 U270

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Re: Bad alternator?

Reply #1 – June 15, 2021, 03:41:47 pm

Check at the BATTERY ISOLATOR.  Could be the isolator, the alternator or even something as simple as the small sense wire coming off.

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Re: Bad alternator?

Reply #2 – June 15, 2021, 06:58:34 pm

Not that I'm an expert, but if it's your alternator make sure whoever replaces it knows about the Duvac system, and I would replace it with a Leece Neville alternator. I just went through this on a recent trip to nacogdoches, my alternator went out in Shreveport I went to a Love's truck stop, they got me in a new alternator and I got to FOT, but then I found out it was not the right alternator and was not charging my dual battery system. I got a very good education on alternators and the duvac system. I went by Love's on my way home and they returnrd my money 100%. Kudos to Love's truck stop.

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Re: Bad alternator?

Reply #3 – June 15, 2021, 07:41:23 pm

What is cost on the alternator with a core

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Re: Bad alternator?

Reply #4 – June 15, 2021, 08:20:49 pm

You can use something other than a LN DUVAC modified alternator.  These were set up connect to the original diode based isolators. Connected to the isolator the LN saw no voltage even with the engine running. So the LN has an Excite connection that provides 12v when the ignition is on so that the alternator can start producing power. Like many alternators it also has a Sense connection so that the alternator knows what the start battery voltage is.

And the LN output voltage was increased to accommodate the voltage loss across the diode based isolator.

There are zero voltage loss isolators and other ways to get the alternator to charge both sets of batteries and isolate the batteries from each other when appropriate.

Many have used DELCO Remy alternators with good success. I did.

Check voltage at the center post of the isolator and at each side post as Brett suggests.  Typical charge voltages are 13.8 to the low 14s.

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Re: Bad alternator?

Reply #5 – June 16, 2021, 01:02:47 am

Thanks to everyone for all the suggestions. I ended up pulling the alternator and taking it to a shop in Salt Lake City called Generator Exchange. They were very knowledgeable about the Leese Neville alternators and the DUVAC setup. They tested the alternator (bad) and had a rebuilt one in stock. I have it installed and it appears to be okay, although I am still in the Sam's parking lot and haven't gotten back on the road yet. I will get everything back together and hopefully be on the road again tomorrow morning.

Dave & Kathy Bennett
2004 U270

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