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Old 07-20-2019, 12:47 PM   #1
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Alternator on 2000 Harney coach 3126B cat

I just checked my alternator and it is putting out over 14 volts with nothing on, When I turn the headlights or the a/C on the volt meter on the dash drops to7 to 9 volts. The batteries are 21 months old and show 13.5 volts at rest.
I am not an electrician so any advice will be helpful. Is there something that is supposed to make the alternator increase the output when a load is placed on the batteries.

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Old 07-20-2019, 02:44 PM   #2
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Batteries only show 12.7 volts at rest. If your showing more, something is charging them or has been an hour or 2 before you tested them. Anything over 12.7 is a surface charge that will drop off.

14 volts right at the alternator output is good.

Your low reading on the dash volt meter makes me suspect a bad ground to the dash, if everything still works, while reading that low.

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Old 07-20-2019, 04:08 PM   #3
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We had a 1999 Harney Coach Renegade with the same 3126B CAT engine. We averaged 10.3mpg. We have a 2006 with a C-7 CAT and get 6.7mpg. We did travel more than130k miles though.
Safe travels.
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Old 07-20-2019, 05:32 PM   #4
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With no power to the coach, start engine, measure voltage at batteries.
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Old 07-20-2019, 06:41 PM   #5
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Thanks , did that. I think I have a bad ground for the dash instruments and will be checking that tomorrow.
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alternator, cat

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