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Old 07-19-2019, 09:40 AM   #1
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Alternator or Battery

Hey Everyone:

My port volt meter on both helms shows a much lower voltage while running than the starboard side. I also see the same thing on my GPS unit. When the motors are running, do these meters show the amount of power coming from the battery or the alternator on that engine? Either the battery or the alternator on the port side is bad, I'm trying to find out which one it is so I can replace it. The battery is only about a year old. When I first arrive to the boat and it's been on the trickle charger for days, it fires right up. After I've cruised around and anchored out somewhere for a few hours the port engine has trouble firing back up. Each engine has it's own starting battery.

Here is a pick of each volt meter while running around 1500RPMs
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Old 07-19-2019, 10:08 AM   #2
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The answer might be neither. You might have bad wiring. Take a multimeter and measure the battery at the battery terminals both with the engine on and off.

I should hav asked, are you sure your port and starboard batteries are isolated from each other?
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Old 07-19-2019, 10:30 AM   #3
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Thank you for the reply. They are isolated but can be joined with the battery switch. When seeing the readings on the pictures I posted they are not joined
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Old 07-19-2019, 10:37 AM   #4
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I would suspect alternator. If you combine the batteries and voltage comes up, start by having the alternator tested.
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Old 07-19-2019, 10:52 AM   #5
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Greetings,
Mr. fl. Just a WAG but I think with the ignition switch on and the engine off, the voltmeter will read the battery only. Once started, it will read the alternator output. I second/third a bad alternator OR bad wiring. I think you can check the output voltage of the alternator while the engine is running. Put VOM on B+. Should read north of 13.8 volts.
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Old 07-19-2019, 12:01 PM   #6
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If it actually is 11.6 volts while running would mean the alternator isn't working and the battery is almost dead. Does the engine still start ok off that battery? If it still does, I'd go with bad wiring or bad volt meter.

Check the voltage at the battery with a multimeter.
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Old 07-19-2019, 12:36 PM   #7
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Both volt meters show that reading as well as the volt meter on my gps/fishfinder. The engine will start but not nearly as strong as the other engine does
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