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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
Attached Thumbnails
portvolt.jpg   starboardvolt.jpg  
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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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Old 10-01-2019, 09:56 AM   #8
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It appears that the self-exciting alternator didn’t excite. If you rev up that engine, say up to 1600rpm, does that get the voltage meter increasing? Is there a wire to the alternator for exciting? Maybe connected to an oil pressure switch?
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Old 10-01-2019, 10:21 AM   #9
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In addition to the above excellent suggestions --

Test both alternator amperage outputs with a clamp on meter. Then you can work your way downhill towards the panel volt meters for volts and amps. Also check your charger out put when on shore power. Are the tachs working ok? Often a fried alternator gives funny tach readings.
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Old 10-01-2019, 11:28 AM   #10
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Thanks for all the suggestions. I actually got it working, I went down to checkout the alternator and noticed the wire wasn't secured with a nut on the bolt. It was just hanging on there. Easy fix!
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Old 03-04-2020, 09:43 AM   #11
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i have a 1973 gulfstar with 2 perkins 90 hp diesels. one alternator either bad or wiring bad. can i wire the alternator directly to the battery?
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Old 03-06-2020, 07:38 AM   #12
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"one alternator either bad or wiring bad. can i wire the alternator directly to the battery? "

That depends on the alt.The one wire auto units yes , as the alt gets turned on by creating voltage the one wire vol regulator sees.

If you are going cruising you should upgrade the entire system ,especially lf your fridge is electric. A good system need not be expensive.

If there is a stand alone V regulator it should be powered by the engine key switch , or it will run down the batt when the engine is off.

What ever you do DO NOT disconnect an alt if it is charging the batts , you will blow the diodes in the unit.

Simplest might be a jumper cable from the batt being charged to the other batt ..
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Old 03-06-2020, 09:04 AM   #13
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very helpful. Thank you.this is a two battery to two alternator system that is isolated from each other. I didn't want to connect two batteries so if one of them went bad it would not drag the other one down. However your comment about the one wire alternator seems pertinent here. This alternator has one positive and one ground and that's all. what I make of your advice is that I can just leave the ground alone that comes from the alternator and as to the positive from the alternator I can connect that directly to the battery and it will work. I was wondering as a test of this proposition how this would go if I should have the positive wire from the alternator while the engine is running and just touch it to the positive on the battery to see if I get a decent small Spark.
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Old 03-07-2020, 10:32 PM   #14
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I suggest you do not do that.
--you could damage the diodes in the alternator meaning an alternator repair.
--the spark could ignite any free floating hydrogen which would likely burst a battery cell meaning an explosion. Don't ask how I know although mine was accidental.


Measure the battery voltage using a suitable range
Then with the engine OFF disconnect the battery cable you are mentioning. Connect a v meter between the battery terminal and the wire. Start the engine and read the meter voltage. It should go higher than the battery voltage climbing from the alternator. THe alternator won't damage the meter nor the reverse.
You may need to put the revs up a bit to get the alt. to produce enough power to start trying to charge.
Once you have confirmed the reading shut the engine down and remove the meter and reconnect the battery and wire.
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Old 03-07-2020, 11:07 PM   #15
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Quote:
Originally Posted by flboy View Post
Thanks for all the suggestions. I actually got it working, I went down to checkout the alternator and noticed the wire wasn't secured with a nut on the bolt. It was just hanging on there. Easy fix!
Nice when you find the problem! Congrats.
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Old 03-20-2020, 02:09 PM   #16
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thanks for all the suggestions. i did the voltmeter suggestion as a conduit between alt and batt and got next to nothing, so alternator bad despite being just rebuilt. wiring maze led me to a wrong connection that cooked it apparently. alternator now in shop, shop in limbo, we in limbo, waiting for virus outcomes. Blessings. Hammond
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