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Old 09-11-2009, 02:47 AM   #1
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Thomas Bus normal alternator output

My dash voltage reads 12.5 volts when running. I put a volt meter on the battery with the bus running...its right on 13.07. This seems low to me. Should my alternator but volting up to 14?

When i put the head lights on the dash reads 12 or just under.

Any ideas? Bus cranks but this does not seem right to me.

Thanks!!!
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Old 09-11-2009, 07:23 AM   #2
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Re: Thomas Bus normal alternator output

Obviously your dash voltmeter is reading low since your multimeter shows higher on the batteries. Did you check the voltage at the batteries at high idle, say 1500 RPM? Alternators need to spin to make power and it's not uncommon for vehicle to discharge when at idle. That's part of the reason why modern cars have PCM controlled idiot lights...they don't want the charge light coming on when you have your Grand Am stopped at a red light in gear with the heat on high, the stereo blasting, the rear defroster on, the headlights on, etc.

You could have a faulty alternator, but your symptoms sound a bit like dirty and corroded connections. Check the output of the alternator by going on the big positive stud on the back of it and touching the other meter lead to the alternator case. I bet it will read higher yet in which case I'd be looking at cleaning the battery posts and maybe replacing some cable ends.
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Old 09-15-2009, 05:04 PM   #3
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Re: Thomas Bus normal alternator output

Mine does the exact same thing. I haven't investigated it much since it starts with no effort everytime.
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Old 09-16-2009, 01:54 AM   #4
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Re: Thomas Bus normal alternator output

Mine seems to crank ok. I've not had a chance to test the alternator at 2000 rpms....but the battery with nothing else running is 13.07 on my digital multimeter.

My Dodge ram van b3500 runs 14 ish...so i'd expect this bohemoth to be close to that...but maybe not.

Marc
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